CBSE Class 7

NCERT Solutions for Class 7 English

The Tiny Teacher NCERT Solutions for Class 7 English An Alien Hand Chapter 1

The Tiny Teacher NCERT Text Book Questions and Answers

The Tiny Teacher Comprehension check – I

Question 1.
The story of an ant’s life sounds almost untrue.
The italicized phrase means
i. highly exaggerated.
ii. too remarkable to be true.
iii. not based on facts.
Answer:
ii. too remarkable to be true.

Question 2.
Complete the following sentences.
Answer:

• An ant is the smallest, but the wisest insect.
• We know a number of facts about an ant s life because people have kept ants as pets, and have watched their daily behaviour closely.

Question 3.
In what ways is an ant s life peaceful?
Answer:
An ant’s life is very peaceful because each ant in a colony does its share of work intelligently and bravely, and never fights with other members of the group.

The Tiny Teacher Comprehension check -II

Question 1.
How long does it take for a grub to become a complete ant?
Answer:
It takes around five to six weeks for a grub to become a complete ant.

Question 2.
Why do the worker ants carry the grubs about?
Answer:
The worker ants carry the grubs about for airing, exercise and sunshine.

Question 3.
What jobs are new ants trained for?
Answer:
New ants are trained to become workers, soldiers, builders and cleaners.

Question 4.
Name some other creatures that live in anthills.
Answer:
Some other creatures that live in anthills include beetles, some lesser breeds of ants and the greenfly.

Question 5.
Mention three things we can learn from the ‘tiny teacher Give reasons for choosing these items.
Answer:
There are many qualities that one can learn from ants. These include hard work, sense of duty and discipline, cleanliness, care for the young ones, and, above all, a firm loyalty to the land where they live.

For example, the ants work very hard to collect and store their food. Similarly, each of them does their bit of work peacefully and does not complain.

The way they care for their young ones by carrying the grub on their backs for airing and sunshine shows how caring they are. The soldiers also protect the eggs till they hatch.

The Tiny Teacher Exercises Question and Answer

Discuss the following topics in groups.

Question 1.
i. What problems are you likely to face ifyou keep ants as pets?
Answer:
There are many people who keep ant colonies as pets to study their behaviour, or to understand these creatures better. They often keep them in artificial containers called formicariums. The problems one might face as a result could be one of maintaining moisture inside the formicarium, or preventing mold in the ant habitats.

ii. When a group of bees finds nectar, it informs other bees of its location, quantity, etc. through dancing. Can you guess what ants communicate to their fellow ants by touching one another’s feelers?
Answer:
Ants communicate with each other by touching one another’s feelers. They often greet each other using their feelers, which also contain their touch and smell organs. This means that they can recognize each other through the touch and prevent intruders from entering their colony (because each ant colony has its own distinct smell).

The ants are also able to smell the food on each other so they both know the food that the other has found. As ants cannot see, speak or hear, they use the touch to alert the other of dangers, or to tell them to follow a particular trail. In fact, their feelers can help them detect if the other ant is hungry. In this case, they may also feed them with the food stored in their ‘social stomach’.

Question 2.
Complete the following poem with words from the box below. Then recite the poem.With answers
Answer:
Soldiers live in barracks
And birds in nests,
Much like a snake that rests
In a hole. No horse is able
To sleep except in a stable.
And a dog lives well,
Mind you, only in a kennel.
To say ‘hi’ to an ant, if you will,
You may have to climb an anthill.
(Encourage the students to recite the poem aloud together.)

These NCERT Solutions for Class 7 Science Chapter 18 Wastewater Story Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Wastewater Story NCERT Solutions for Class 7 Science Chapter 18

Class 7 Science Chapter 18 Wastewater Story Textbook Exercise Questions and Answers

Question 1.
Fill in the blanks:
a. Cleaning of water is a process of removing ……………….
b. Wastewater released by houses is called ……………….
c. Dried ………………. is used as manure.
d. Drains get blocked by ………………. and ……………….
Answer:
a. pollutants,
b. sewage,
c. sludge,
d. oils, fats

Question 2.
What is sewage? Explain why it is harmful to discharge untreated sewage into rivers or seas.
Answer:
Water containing liquid wastes disposed from domestic household activities, industrial activities and agricultural activities is termed as sewage.

The untreated sewage can harm the rivers and seas as the contaminated water pollutes the whole source of water. As the sewage water may contain harmful substances and disease causing organisms, it is dangerous to release the untreated sewage into the water bodies.

Question 3.
Why should oils and fats be not released in the drain? Explain.
Answer:
Oils and fats can block the drains. In open drains, they block the pores in the soil and thus hamper the filtration by soil. Hence, oils and fats should not be released in the drains.

Question 4.
Describe the steps involved in getting clarified water from wastewater.
Answer:
The following steps are involved in clarifying wastewater:

• In the first step, all the physical impurities like stones, plastic bags, cans, etc., are removed by passing the water through bar screens.
• Now, water is subjected to sedimentation where impurities are removed by the force of gravity.
• Solids settled at the bottom of the sedimentation tank are removed. This is called sludge.
• An aerator cleanses the water from other impurities.
• Chlorination is done to remove all disease causing bacteria.

Question 5.
What is sludge? Explain how it is treated.
Answer:
The solid waste which settles at the bottom of the sedimentation tank is called sludge. The sludge is further treated by the process of aeration. This helps in growth of bacteria which eat away the human waste. Activated sludge is used as manure.

Question 6.
Untreated human excreta is a health hazard. Explain.
Answer:
Untreated human excreta contains many microbes which can cause deadly diseases like cholera, diarrhoea, jaundice and typhoid. The germs of these diseases can easily reach human population through insects and through contaminated food and water. Thus, untreated human excreta is a health hazard.

Question 7.
Name two chemicals used to disinfect water.
Answer:
Bleaching powder and chlorine tablets.

Question 8.
Explain the function of bar screens in a wastewater treatment plant.
Answer:
Bar screens are large filters which are placed in the form of vertical screens. Large solid wastes like sticks, pebbles, sand, cans, plastic bottles, plastic bags, etc., are filtered out when the wastewater moves through the bar screens.

Question 9.
Explain the relationship between sanitation and disease.
Answer:
Proper sanitation should be maintained in order to avoid diseases. They are related to each other. By sanitation, proper disposal of sewage and refuse from household and public places is carried out. Implementation of proper sanitation results in a state where we are free from any diseases. Lack of it may lead to various diseases.

Question 10.
Outline your role as an active citizen in relation to sanitation.
Answer:
As an active citizen, we should take care of our personal environmental sanitation. For this, we should follow these steps:

• Throwing garbage at designated places and in garbage bins.
• Preventing clogging of drains by not releasing oils and fats and solid wastes into the drains.
• Using a dustbin to throw trash at public places.
• By not spitting at public places.

Question 11.
Here is a crossword puzzle. Good luck!

Across:
3. Liquid waste products
4. Solid waste extracted in sewage treatment
6. A word related to hygiene
8. Waste matter discharged from humans
Down:
1. Used water
2. A pipe carrying sewage
5. Micro-organisms which cause cholera
7. A chemical to disinfect water
Answer:
1. Wastewater
2. Sewer
3. Sewage
4. Sludge
5. Bacteria
6. Sanitation
7. Ozone
8. Excreta

Question 12.
Study the following statements about ozone:
(a) It is essential for breathing of living organisms.
(b) It is used to disinfect water.
(c) It absorbs ultraviolet rays.
(d) Its proportion in air is about 3%. Which of these statements are correct?
(i) (a), (b) and (c)
(ii) (b) and (c)
(iii) (a) and (d)
(iv) All four
Answer:
(ii) (b) and (c)

NCERT Extended Learning Activities and Projects

Question 1.
Then and now: Talk to your grandparents and other elderly people in the neighbourhood. Find out the sewage disposal systems available to them. You can also write letters to people living in far off places to get more information. Prepare a brief report on the information you collected.
Hint:
Do it yourself

Question 2.
Visit a sewage treatment plant.
It could be as exciting and enriching as a visit to a zoo, a museum or a park. To guide your observation here are a few suggestions.
Record in your notepad:
Place …………..
Date …………..
Time …………..
Name of the official at the plant ………….. Guide/
Teacher …………..
a. The location of the sewage plant.
b. Treatment capacity.
c. The purpose of screening as the initial process.
d. How is air bubbled through the aeration tank?
e. How safe is the water at the end of the treatment? How is it tested?
f. Where is the water discharged after treatment?
g. What happens to the plant during heavy rains?
h. Is biogas consumed within the plant or sold to the other consumers?
i. What happens to treated sludge?
j. Is there any special effort to protect nearby houses from the plant?
k. Other observations.
Hint:
Do it yourself with the help of your teacher.

Activity 1

Objective: To locate an open drain near your home, school or on the road side and inspect water flowing through it.
Procedure: Visit the nearby drain and observe it carefully.
Observations:

Conclusion: The open drain water is dirty and has a foul smell. Many organic impurities are found there such as human faeces, animal wastes, fruit and vegetable wastes, etc.

Wastewater Treatment Plant (WWTP) or Sewage Treatment Plant: These are large plants where wastewater is cleaned before being sent to the nearest water bodies or being reused. The sewage treatment involves physical, chemical and biological processes to remove impurities from the wastewater. This is done in the following steps:

i. Wastewater is passed through bar screens to remove large objects such as rags, cans, plastic packets, polythene, etc.

ii. Sand and Grit removal tank: The speed of incoming wastewater in this tank is decreased to allow sand, grit, pebbles, etc., to sediment and settle down.

iii. The water then goes into a large tank sloping in the middle where solids, like faeces, settle at the bottom and are removed with a scraper. This solid is called the sludge.
A skimmer removes the floatable solids like oil and grease. Water so cleared is called the clarified water.

iv. Anaerobic bacteria: Bacteria which can grow and multiply in absence of free oxygen are called anaerobic bacteria. The sludge is transferred to a separate tank for decomposition by anaerobic bacteria.

The gas produced by the decomposition of sludge by the anaerobic bacteria is known as biogas. The biogas produced during decomposition can be used as fuel or used to produce electricity.

v. Role of aerobic bacteria: The clarified water also needs to be cleaned further. Hence, it is moved to an aerator that pumps air into it. This allows aerobic bacteria to grow in this water. Bacteria which require free oxygen for their growth and multiplication are called aerobic bacteria. The aerobic bacteria consume organic wastes, soaps, food waste and other elements that remain in the clarified water.

vi. Activated sludge: After several hours of decomposition in the tank, the microbes settle down at the bottom as activated sludge. It contains about 97% water. The water is then removed from the top by sand drying beds or machines. The treated water with very low amount of suspended matter is discharged into water bodies where it is further cleaned by nature. Water can also be disinfected with chemicals like chlorine UV radiations and ozone before releasing it into the distribution system.

Activity 2

Objective: To treat a sample of polluted water and make it clean.
Materials Required: Funnel, a glass jar, filter paper, funnel stand, soil, grass pieces, orange peels, detergent, ink, strainer and beaker.
Procedure:

• Fill a glass jar three-fourths with tap water. Add a little of soil, some dirty organic matter such as grass pieces and orange peels, a small amount of detergent and a few drops of ink to it. Cap the glass jar, shake it well and keep it in the sun for at least two days.
• Shake the contents of the glass jar well and pour the content over a strainer kept over a beaker. The large pieces of grass and orange peels will not pass through the holes of strainer and hence get removed. The remaining part of the polluted water will collect in the beaker kept below the strainer.
• Allow the polluted water to stand in the beaker for some time. The solid fine soil present in the polluted water will settle down at the bottom of the beaker.
• Transfer the polluted water from this beaker into another beaker by decantation. Pass air into this polluted water for several hours by using an aerator.
• When the aeration is complete, we will find that the foul smell from polluted water has almost disappeared. Filter the aerated water through the layer of sand, fine gravel and medium gravel arranged on a filter paper in a funnel. Collect the clean water as a filtrate in a beaker kept below the funnel.
• Add a small piece of chlorine tablet to the clean water collected in the beaker. Mix well until the water is clear and colourless. This is the clean and clear safe water obtained by the treatment of polluted water.

Observation:

Conclusion: With this treatment, we can clean polluted water. This treated water does not give foul smell and it is safe to use.

Sanitation: Sanitation refers to the health and hygienic conditions of people related to the disposal of sewage, human excreta and provision of clean drinking water. Poor sanitation and polluted drinking water can lead to many diseases. Hence, a sanitation system aims to provide a clean environment for us so that we can stay away from diseases. Improper sanitation can give rise to different diseases that arise from contaminated water such as typhoid, dysentery, hepatitis, polio, cholera and meningitis. The human excreta is a health hazard and can lead to soil and water pollution. It can pollute the surface water and groundwater. Hence, people should never defecate in open areas.

Alternative Arrangement for Sewage Disposal:
i. In the absence of a sewerage system, arrangements for onsite sewage disposal can be made. For example, septic tanks are built in which human excreta are collected. In due course of time, the human excreta gets decomposed into compost.
ii. Composting pits can be made to dump waste and to make manure from them. Sewage can be collected into biogas plants to produce useful biogas.

Chemical toilets are new discoveries. They do not require much water for the disposal of human excreta and are environment friendly. Such toilets are ideal for the trains.

Sanitation at Public Places: There are numerous places in our country which are very busy such as airports, railway stations and bus depots. Many times, large gatherings such as fairs and festivals are also organised where a large number of people visit. This results in the generation of large amounts of waste and if the waste is not treated properly, it can lead to several diseases and even an epidemic. Hence, we should all become aware and contribute to maintaining sanitation not only in our homes but also at public places. If we adopt certain practices at public places we can help in maintaining their sanitation, for instance:

• We should not throw garbage in public places and always use the dustbins.
• We should not spit around in a public place.
• We should wash our hands thoroughly before eating anything at a public place.

Class 7 Science Chapter 18 Wastewater Story Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Which day is celebrated as World Water Day?
Answer:
22 March.

Question 2.
What is sewerage?
Answer:
It is like a transport system that carries sewage from the point of being produced to the point of disposal, i.e., treatment plant.

Question 3.
What are the harmful effects of drinking contaminated water?
Answer:
Contaminated drinking water is the cause of a large number of diseases such as cholera, typhoid, polio, meningitis, hepatitis and dysentery.

Question 4.
What is cleaning of water?
Answer:
Cleaning of water is a process of removing pollutants before it enters a water body or is reused. This process of wastewater treatment is commonly known as ‘Sewage Treatment”.

Question 5.
Why chemicals should not be thrown down the drain?
Answer:
Chemicals should not be thrown down the drain because chemicals like paints, solvents, insecticides, motor oil and medicines may kill microbes that help purify water.

Question 6.
Name two nutrients present in the sewage?
Answer:
Nitrogen and phosphorus.

Question 7.
Name the solid waste extracted during sewage treatment.
Answer:
Sludge.

Question 8.
Name any two things that can be made from the sludge extracted during treatment of sewage.
Answer:
Biogas and manure.

Question 9.
Write the examples of on site sewage disposal systems.
Answer:
Septic tanks, chemical toilets, composting pits, etc.

Question 10.
Name the toilet in which human excreta is treated by earthworms.
Answer:
Vermi-processing toilet.

Question 11.
What is the use of dried sludge?
Answer:
Dried sludge is used as manure, thus returning organic matter and nutrients to the soil.

Question 12.
How can we improve the sanitation?
Answer:
To improve sanitation, low cost onsite sewage disposal systems must be encouraged.

Question 13.
Why are ozone and chlorine used in water purification?
Answer:
Ozone and chlorine are used to kill the bacteria, etc., present in the clarified water.

Question 14.
How are floatable solids removed from the wastewater?
Answer:
A skimmer is used to remove floatable impurities.

Question 15.
Which microorganism decomposes the sludge?
Answer:
Anaerobic bacteria decompose the sludge.

Short Answer Type Questions

Question 1.
Define community hygiene? How can it be effectively maintained?
Answer:
Community hygiene means maintenance, protection and improvement of health of the whole community. It can be maintained by:

• Provision of clean and safe drinking water.
• Maintaining green open space.
• Maintaining proper sanitation.
• Maintaining clean air.

Question 2.
What are the problems arising due to open drains and other unsanitary conditions?
Answer:
Open drains and unsanitary conditions produce bad smell. It becomes an idle place of breeding for mosquitoes, flies and other harmful insects. These insects spread many harmful diseases and other health hazards.

Question 3.
How kitchen waste blocks the drain?
Answer:
Cooking oils that drain from kitchen into the pipes, hardens and blocks the pipes. The oil in open drains clogs the soil pores thereby preventing the filtration process and its effectiveness. It further hampers the degradation process.

Question 4.
How bar screen and grit and sand removal tank help in clarification of water?
Answer:
When wastewater is passed through bar screens, it separates big and large objects like rags, sticks, cans, plastic packets, napkins, etc. In grit and sand removal tank, other solid impurities like pebbles, sand, etc., are removed.

Question 5.
What is vermi-processing toilet?
Answer:
Vermi-processing toilet is a new toilet system developed in India. In this system, human excreta is treated by earthworms for safe processing of human waste. The operation of this toilet is very simple and hygienic. It has the potential to reduce freshwater use and improve sanitation levels.

Question 6.
What is the need to chlorinate any water supply? Name two other methods which help to do the same.
Answer:
For making water safe for drinking, chlorination can be done. In this process, chlorine tablets, bleaching powder or alum is added to water to prevent it from diseases. Filtration or sedimentation can also be done in order to make the water safe for drinking.

Question 7.
Why is air pumped into the clarified water?
Answer:
Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soaps and other unwanted matter still remaining in the clarified water.

Question 8.
Why should eucalyptus trees be planted along sewage ponds?
Answer:
It has been suggested that we should plant eucalyptus trees all along the sewage ponds. These trees absorb all surplus wastewater rapidly and release pure water vapour into the atmosphere.

Question 9.
Why are bacteria important in sewage treatment?
Answer:
Bacteria are important in sewage treatment because, they consume human waste, food waste, soaps and other unwanted matter still remaining in clarified water. Also, they help in the decomposition of solid sludge to make useful things like biogas.

Question 10.
Suggest some alternative arrangements for sewage disposal.
Answer:
To improve sanitation, low cost onsite sewage disposal systems are being encouraged. Examples are septic tanks, chemical toilets, composting pits. Septic tanks are suitable for places where there is no sewerage system, for hospitals, isolated buildings or a cluster of 4 to 5 houses. Some organisations offer hygienic on-site human waste disposal technology. These toilets do not require scavenging. Excreta from the toilet seats flow through covered drains into a biogas plant. The biogas produced is used as a source of energy.

Long Answer Type Questions

Question 1.
What are the different types of inorganic and organic impurities generally present in sewage?
Answer:
Wastewater is composed primarily of natural organic substances which are by-products of human, animal and plant processes. The primary inorganic elements in domestic wastewater are nitrogen, phosphorous, ammonia and carbon. These elements are released with the growth of bacteria in standing water. The bacteria consume the oxygen present in water and as a result, living organisms in the water start to die.

We know that sewage is a complex mixture containing suspended solids, organic and inorganic impurities, nutrients, saprotrophic and disease causing bacteria and other microbes.

Question 2.
List some ways to minimise or eliminate waste and pollutants at their source.
Answer:
One of me ways to minimise or eliminate waste and pollutants at their source is to see what you are releasing down the drain. Other ways may include:

• Cooking oil and fats should not be thrown down the drain. They can harden and block the pipes. In an open drain, the fats clog the soil pores reducing its effectiveness in filtering water. Throw oil and fats in the dustbin.
• Chemicals like paints, solvents, insecticides, motor oil and medicines may kill the microbes that help purify water. So we should not throw them into the drain.
• Used tea-leaves, solid food remains, soft toys, cotton, sanitary towels, etc., should also be thrown in the dustbin as these wastes choke the drains. They do not allow free flow of oxygen which hampers the degradation process.

Question 3.
Write some consequences of the malfunctioning of sewage.
Answer:

• Rupture or, blockage or leakage of sewerage lines.
• Sewerage may overflow on the roads and streets, creating poor sanitary conditions.
• It may upset the ecological balance in lakes and rivers.
• Water may become unfit for drinking and other domestic uses.
• The place may stink and become breeding ground for mosquitoes, flies, etc.

Question 4.
What is a septic tank? How does it function?
Answer:
Septic tank is a low cost on-site sewage disposal system. Septic tanks are suitable where there is no sewerage system for hospitals, isolated buildings or a cluster of four to five houses. Septic tank needs cleaning every four to six months.

Septic tanks serve three functions:
a. Removal of solids: As sewage enters the septic tank, its rate of flow is reduced so that the larger solids sink to the bottom and soaps, grease and smaller solids rise to the surface. These solids are retained in the tank and the clarified effluent with suspended and dissolved solids is discharged.

b. Bacterial Action: The solids and the liquids in the tank are partially decomposed by bacteria and other natural processes. These bacteria are called anaerobic because they thrive in the absence of free oxygen. This decomposition of sewage under anaerobic conditions is termed “septic,” hence the name of the system (and the cause of the odour).

c. Sludge and scum storage: Sludge is the accumulation of solids at the bottom of the tank, while scum is a partially submerged mat of floating solids that may form at or near the surface. Space must exist in the tank to store these residues during the intervals between pumping. Otherwise, the sludge and scum will eventually be scoured from the tank and will clog the leaching field and receiving soil.

The final Stage of Disposal:
The treated effluent from the septic tank is discharged to the leach field where it percolates through suitable “septic stone” and finally into the subsoil for further purification.

Picture-Based Questions

Question 1.
Observe the diagram shown below and answer the following questions.

a. What does it depict?
b. Which layers are used in the funnel?
Answer:
a. It depicts purification of polluted water by filtration.
b. Layers of sand, silt and gravel are used in the funnel.

Question 2.
Identify the diagram shown below.

Answer:
Sewage treatment plant

NCERT Solutions for Class 7 Science

• Class 7 Science Chapter 1 Nutrition in Plants
• Class 7 Science Chapter 2 Nutrition in Animals
• Class 7 Science Chapter 3 Fibre to Fabric
• Class 7 Science Chapter 4 Heat
• Class 7 Science Chapter 5 Acids, Bases and Salts
• Class 7 Science Chapter 6 Physical and Chemical Changes
• Class 7 Science Chapter 7 Weather, Climate and Adaptations of Animals of Climate
• Class 7 Science Chapter 8 Winds, Storms and Cyclones
• Class 7 Science Chapter 9 Soil
• Class 7 Science Chapter 10 Respiration in Organisms
• Class 7 Science Chapter 11 Transportation in Animals and Plants
• Class 7 Science Chapter 12 Reproduction in Plants
• Class 7 Science Chapter 13 Motion and Time
• Class 7 Science Chapter 14 Electric Current and its Effects
• Class 7 Science Chapter 15 Light
• Class 7 Science Chapter 16 Water: A Precious Resource
• Class 7 Science Chapter 17 Forests: Our Lifeline
• Class 7 Science Chapter 18 Wastewater Story

These NCERT Solutions for Class 7 Science Chapter 17 Forests: Our Lifeline Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Forests: Our Lifeline NCERT Solutions for Class 7 Science Chapter 17

Class 7 Science Chapter 17 Forests: Our Lifeline Textbook Exercise Questions and Answers

Question 1.
Explain how animals dwelling in the forest help it grow and regenerate.
Answer:
Animals which live in the forest help it grow and regenerate in following ways:

• By eating vegetation, they clear the land for fresh growth.
• Animals help flowering plants in pollination and seed dispersal.
• Dead remains of animals make humus after decomposition. This increases soil fertility.

Question 2.
Explain how forests prevent floods.
Answer:
Forest acts as a natural absorber of rainwater and allows it to seep underground. It helps in controlling the flow of water and slows it down which helps in preventing floods. Also, the trees present in the forest prevents the rain from directly hitting the ground and bind the soil together which helps in absorption of the rainwater and thus prevent flood.

Question 3.
What are decomposers? Name any two of them. What do they do in the forest?
Answer:
The organisms which decompose the dead remains of plants and animals are called decomposers. Fungi and bacteria are examples of decomposers. They clear the debris of dead remains and convert them into humus which enriches the topsoil.

Question 4.
Explain the role of forest in maintaining the balance between oxygen and carbon dioxide in the atmosphere.
Answer:
Forests are called the green lungs. This is because plants in forests release oxygen through the process of photosynthesis and help in providing oxygen to animals for respiration. Plants consume carbon dioxide released by the animals. In this way, plants help in maintaining a balance of oxygen and carbon dioxide in the atmosphere.

Question 5.
Explain why there is no waste in a forest.
Answer:
Forest works like a self sustaining system. Whatever is produced in the forest is utilised by different components of the forest. Even the waste is converted into beneficial substances in a forest. Due to this, there is no waste in a forest.

Question 6.
List five products we get from forests.
Answer:

• We get medicines from forests.
• We get gum from forests.
• We get wood which is used for many purposes like making furniture, paper, etc.
• We get food for animals from forests.
• We get sealing wax from forests.

Question 7.
Fill in the blanks:
a. The insects, butterflies, honeybees and birds help flowering plants in ………………..
b. A forest is a purifier of ……………….. and ………………..
c. Herbs form the ……………….. layer in the forest.
d. The decaying leaves and animal droppings in a forest enrich the ………………..
Answer:
(a) pollination,
(b) water and air,
(c) lowest,
(d) topsoil

Question 8.
Why should we worry about the conditions and issues related to forests far from us?
Answer:
We should worry about the conditions and issues related to forests far from us because, more or less, we are dependent on them because of the following reasons:

• The amount of carbon dioxide in air will increase if forests will disappear and thus less oxygen will be there in the atmosphere.
• There will be more soil erosion and thus increase in floods as soil will not be able to hold water.
• Increase in the percentage of carbon dioxide will also leads to global warming.
• Deforestation will endanger our life and environment and also there will be no shelter for animals.
• There will also be an imbalance in the nature and thus causing climate changes and less rainfall.

Question 9.
Explain why there is a need of variety of animals and plants in a forest.
Answer:
There is a need of variety of animals and plants in a forest as each of them contribute for maintaining the ecosystem of the forest. They also help forests in regeneration and growth. Herbivores are needed to eat green plants and to provide food for the carnivores and carnivores are needed to eat herbivores and check their population. Also, decomposers are needed to maintain the supply of nutrition to the soil and to the growing plants.

Question 10.
In fig. 17.15 (NCERT textbook), the artist has forgotten to put the labels and directions on the arrows. Mark the directions on the arrows and label the diagram using the following labels: clouds, rain, atmosphere, carbon dioxide, oxygen, plants, animals, soil, roots, water table.

Answer:

Question 11.
Which of the following is not a forest product?
a. Gum
b. Plywood
c. Sealing wax
d. Kerosene
Answer:
d. Kerosene

Question 12.
Which of the following statements is not correct?
a. Forests protect the soil from erosion.
b. Plants and animals in a forest are not dependent on one another.
c. Forests influence the climate and water cycle.
d. Soil helps forests to grow and regenerate.
Answer:
b. Plants and animals in a forest are not dependent on one another.

Question 13.
Micro-organisms act upon the dead plants to produce
a. sand
b. mushrooms
c. humus
d. wood
Answer:
c. humus

NCERT Extended Learning Activities and Projects

Question 1.
The Department of Environment is to decide whether some portion of a forest in your area could be cleared for a housing complex. Write a letter to the department explaining your point of view as a concerned citizen.
Hint:
To
The Director
The Department of Environment
New Delhi
Subject: Deforestation in ……… area.
Sir/Mam
As an aware resident of our society, I have come to know that your department has granted the permission to clear some forest area of our residence. I want to draw your attention towards some points, i.e., the after-effects of your decision.

• If you cut the forest area, then the wild animals will move towards the residential area, which is very dangerous to people and animals also.
• Due to deforestation, the climate will be affected the most. There will be more carbon dioxide and other dangerous gases in the environment.
• Loss of trees and other vegetation may cause soil erosion.
• Due to deforestation, the people won’t get the fresh air and health problems will increase day by day.

These are the points which may cause danger to our lives and environment due to deforestation. 1 request to reconsider your decision in the public interest.
Thanking you
Yours faithfully
A concerned citizen

Question 2.
Visit a forest. Here is a list of points that would make your visit more fruitful.
a. Make sure that you have permission to go into the forest.
b. Make sure that you can find your way around. Get a map and go along with someone who is familiar with the area.
c. Keep a record of the things you see and do. Observations make the visit interesting. Sketches and photographs are useful.
d. You may record bird calls.
e. Collect different kinds of seeds or hard fruits like nuts.
f. Try to recognise various types of trees, shrubs, herbs, etc. Make lists of plants from different places in the forest and of different layers. You may not be able to name all the plants, but it is worth recording and seeing where they grow. Make a record of approximate height of plants, crown shape, bark texture, leaf size and flower colour.
g. Learn to recognise the animal’s droppings.
h. Interview the forest officials and the people of surrounding villages and other visitors.
Hint:
Do it yourself.

Activity 1

Objective: To verify the fact that the decaying organic matter is always moist and warm.
Materials Required: Khurpi, leaves, water, etc.
Procedure:

• Dig a small pit with the help of a khurpi.
• Fill the vegetable wastes, dry leaves, etc., in this pit and cover it with soil. Pour some water over it.
• After 3-4 days, remove the upper layer of soil and observe the decaying matter.

Observation: We observe that the decaying matter is moist and warm.
Conclusion: The decaying organic matter is always moist and warm.

Forests are Green Lungs: Plants take up carbon dioxide and release oxygen during photosynthesis. The plants keep balance of carbon dioxide and oxygen in the atmosphere. This is a reason why forests are called green lungs of the earth.

Forests as Dynamic Living Entity: By harbouring a greater variety of plants, the forests provide greater opportunities for food and habitat for herbivores. Larger number of herbivores means increased availability of food for a variety of carnivores. The wide variety of animals helps the forest to generate and grow. Decomposers help in maintaining the supply of nutrients to the growing plants in the forest. Therefore, the forest is a called ‘dynamic living entity’ which means ‘full of life and vitality’.

Soil Erosion: Washing away of the top fertile layer of soil by water or wind is called soil erosion. Soil helps the forest to grow and regenerate. Forests protect the soil and avoid soil erosion.

Deforestation: Large scale cutting of trees to clear land for human use is called deforestation. Deforestation may lead to floods, global warming, danger to environment and our life.

Afforestation: It refers to the large scale planting of trees. It helps in increasing the forest land and thus helps in improving the environment of the earth. It also leads to several benefits that the forests provide us.

Significance of Forests: Forest plays an important role in preserving environment. Green plants take solar energy and convert it into chemical energy while making food during photosynthesis. Thus, solar energy is transferred to other living beings through the green plants. Green plants utilise carbon dioxide during photosynthesis and release oxygen. That is how the green plants maintain the balance of carbon dioxide and oxygen in the environment. It also protects the soil and increase underground water table. It is a home to a wide variety of fauna.

Class 7 Science Chapter 17 Forests: Our Lifeline Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Define deforestation.
Answer:
Cutting and destroying of the forest is called deforestation.

Question 2.
What is meant by afforestation?
Answer:
Planting more and more trees by which we can preserve our forest wealth is called afforestation.

Question 3.
What are producers?
Answer:
Green plants are called producers.

Question 4.
What are herbivores?
Answer:
The organisms which directly depend on plants for their food are called herbivores.

Question 5.
What are carnivores?
Answer:
The organisms which are flesh-eaters and depend on other animals for their food are called carnivores.

Question 6.
Draw any simple food chain going on in the forests?
Answer:
Grass → Insects → Frogs → Snake → Eagle

Question 7.
What happens if an animal dies in the forest?
Answer:
Dead animals become food for vultures, crows, jackals and insects.

Question 8.
What is known as the crown of the tree?
Answer:
Branchy part of a tree above the stem is known as the crown of the tree.

Question 9.
What is canopy?
Answer:
Branches of the tall trees look like a roof over the other plants in the forest. This is called a canopy.

Question 10.
What is humus?
Answer:
Humus is a dark-coloured substance which is formed from the decomposition of dead bodies of plants and animals.

Question 11.
What are known as understroyes in a forest?
Answer:
Trees have crowns of different types and sizes. These create different horizontal layers in the forest. These are known as understroyes.

Question 12.
What is the effect of deforestation on soil?
Answer:
Roots of trees normally bind the soil together, but in their absence, the soil is washed away or eroded.

Question 13.
In which layer of the soil would you find humus? What is its importance to the soil?
Answer:
Humus is found in the top, organic layer of soil. Humus provides many useful nutrients to the soil.

Question 14.
Would you see similar kinds of trees and animals in every forest?
Answer:
No, we don’t see similar kinds of trees and animals in every forest.

Question 15.
What do you mean by food chain?
Answer:
The organisms which take their food from plants get eaten by other organisms and so on. This chain of eating and being eaten is called food chain.

Question 16.
Name some factors responsible for clearing of forests.
Answer:
Construction of roads and buildings, industrial development, increasing demands of wood, etc.

Question 17.
What role does a decomposer play in forest?
Answer:
Decomposers convert dead leaves and dead animals into humus.

Question 18.
How will water pollution be affected with the depletion of forests?
Answer:
Without trees, there will be no roots holding the soil in a forest. Soil will run into the river and will cause water pollution.

Question 19.
How do animals help forest to grow?
Answer:
The animals disperse the seeds of certain plants and help the forest to grow and regenerate.

Question 20.
How can we recognise the presence of an animal in the forest?
Answer:
By its droppings and footprints, we can recognise the presence of an animal in the forest.

Short Answer Type Questions

Question 1.
Why does the forest floor seem to be dark-coloured?
Answer:
The forest floor seems to be dark-coloured as it is covered with a layer of dead and decaying leaves, fruits, seeds, twigs and small herbs. The decaying matter is moist and warm.

Question 2.
How are the trees planted in a forest?
Answer:
In nature, trees produce enough seeds. The forest floor provides favourable conditions for them to germinate and develop into seedlings and saplings. Some grow up into trees.

Question 3.
Why even after heavy rainfall, water does not stagnate in the forest?
Answer:
Forest acts as a natural absorber of rainwater and allows it to seep. It helps maintain the water table throughout the year. Forests not only help in controlling floods but also help maintain the flow of water in the streams so that we get a steady supply of water.

Question 4.
Why are forests called the green lungs?
Answer:
Plants release oxygen through the process of photosynthesis. The plants help to provide oxygen for animal respiration. They also maintain the balance of oxygen and carbon dioxide in the atmosphere. That is why forests are called the green lungs.

Question 5.
Why raindrops do not hit the forest floor directly?
Answer:
The uppermost layer of the forest canopy intercept the flow of raindrops and most of the water come down through the branches and the stems of the trees. From the leaves it drips slowly over branches of the shrubs and herbs. Thus, raindrops do not hit the forest floor directly.

Question 6.
Why are forests disappearing?
Answer:
Forests are disappearing due to:

• construction of roads, buildings, industrial development and increasing demand of wood.
• overgrazing of animals and indiscriminate felling of trees for agricultural land.

Question 7.
Explain the importance of forests.
Answer:
Forests are very important to us. They provide us with oxygen. They protect soil and provide habitat to a large number of animals. They help in bringing good rainfall in neighbouring areas. They are a source of medicinal plants, timber and many other useful products.

Question 8.
Explain how forest affects food chain.
Answer:
All animals, whether herbivores or carnivores, depend ultimately on plants for food. Organisms which feed on plants often get eaten by other organisms, and so on. For example, grass is eaten by insects, which in turn, is taken by the frog. The frog is consumed by snakes. This is said to form a food chain: Grass → insects → frog → snake → eagle. Many food chains can be found in the forest. All food chains are linked. If any one food chain is disturbed, it affects other food chains. Every part of the forest is dependent on the other parts.

Question 9.
Define deforestation. Write its causes and consequences also.
Answer:
The excessive cutting of forests and trees is called deforestation.
Causes of deforestation:

• Trees are used for charcoal, fuel, timber, etc.
• The cleared land is used as pasture for livestock, plantations of commodities, and settlements.

Consequences of deforestation:

• The removal of trees resulted in damage to habitat, biodiversity loss and aridity.
• It has adverse impacts on bio-sequestration of atmospheric carbon dioxide.
• Deforested regions may degrade into wasteland due to soil erosion.

Question 10.
How do forests maintain the water cycle?
Answer:
The role of rainforests in the water cycle is to add water to the atmosphere through the process of transpiration (in which plants release water from their leaves during photosynthesis). This moisture contributes to the formation of rain clouds which release the water back onto the rainforests. This maintains the water cycle and water level in the environment.

Question 11.
How are the nutrients of the soil’ related to decomposers?
Answer:
Different animals and plants live in forests. When they die, nobody cremates their dead bodies. They keep on lying there. Scavengers like vultures, eagle, etc., eat their dead bodies. They eat flesh and other soft organs. The rest of the dead bodies are broken down by various microorganisms called decomposers. They decompose the dead bodies of plants and animals into a dark substance called humus which maintains the nutrient level of the soil.

Long Answer Type Questions

Question 1.
Explain why forest is called “the dynamic living entity”?
Answer:
The wide variety of animals helps the forest to regenerate and grow. In forest, plants produce food. All animals, whether herbivores or carnivores, depend ultimately on plants for food. Organisms which feed on plants often get eaten by other organisms, and so on. This is said to form a food chain.

Decomposers feed upon the dead plant and animal tissues and convert them into a dark coloured substance called humus. By harbouring greater variety of plants, the forest provides good opportunities for food and habitat for the herbivores. Larger number of herbivores means increased availability of food for a variety of carnivores. Thus forest is the dynamic living entity, that is, full of life and vitality.

Question 2.
What would happen if forests disappear?
Answer:
The conditions and issues we have to face when forests disappear are:

• Soil erosion: Trees in a forest hold the soil with their roots. If trees are gone, soil will be washed away.
• Landslides: Without trees, land will be very unstable and easily move down the slope.
• Water pollution: Without tree roots holding the soil in a forest, soil will run into the river by flowing water and wind.
• Loss of biodiversity: Plants and animals will lose their food and shelter.
• Floods: Bald hills and mountains can no longer hold back sudden flows of water, which may cause floods.
• Increase in carbon dioxide: Depletion of forests results in increase in carbon dioxide which will cause global warming.

Question 3.
Why is replenishment of forests necessary?
Answer:
Forests need to be replenished because of the following reasons:

• Rainfall: During transpiration, trees give out enormous amount of water vapour. This water vapour helps in the formation of rain clouds. So, if trees are cut and not replenished, the rainfall in the area will reduce.
• Natural rate of tree growth: Forests cannot be regrown in a few days or months as trees take many years to grow fully. Thus, it becomes necessary to replenish the forests periodically.
• Soil erosion: If a large number of trees are cut, the soil becomes naked. The topsoil, which is rich in organic matter will be washed away by water or carried away by wind. Trees help in binding the soil.
• Carbon dioxide-oxygen balance: Forests have a very large number of trees which give out O₂ and take in CO₂ by photosynthesis. In this way, they help in maintaining the carbon dioxide-oxygen balance in the atmosphere.
• Timber and fuel: Forests are the best suppliers of timber for furniture and fuel. So, for their constant supply, forests need to be replenished.

Question 4.
What are the various steps, we can take to conserve our forests?
Answer:

• We should mindfully celebrate the Van Mahotsav Day.
• We should lay emphasis on tree plantation.
• We should discourage the use of wooden furniture.
• We should impose strict environmental laws.
• Educating and sensitising people regarding the role of forests in maintaining stable ecosystem should be done.

Picture-Based Questions

Question 1.
Draw a diagram of different types of crowns of trees.
Answer:

Question 2.
Draw a diagram to show canopy and understorey.
Answer:

Question 3.
Observe the following figure and label its various components.

Answer:
1. Carbon dioxide
2. Oxygen
3. Photosynthesis
4. Nutrients
5. Decomposers
6. Soil
7. Water

NCERT Solutions for Class 7 Science

• Class 7 Science Chapter 1 Nutrition in Plants
• Class 7 Science Chapter 2 Nutrition in Animals
• Class 7 Science Chapter 3 Fibre to Fabric
• Class 7 Science Chapter 4 Heat
• Class 7 Science Chapter 5 Acids, Bases and Salts
• Class 7 Science Chapter 6 Physical and Chemical Changes
• Class 7 Science Chapter 7 Weather, Climate and Adaptations of Animals of Climate
• Class 7 Science Chapter 8 Winds, Storms and Cyclones
• Class 7 Science Chapter 9 Soil
• Class 7 Science Chapter 10 Respiration in Organisms
• Class 7 Science Chapter 11 Transportation in Animals and Plants
• Class 7 Science Chapter 12 Reproduction in Plants
• Class 7 Science Chapter 13 Motion and Time
• Class 7 Science Chapter 14 Electric Current and its Effects
• Class 7 Science Chapter 15 Light
• Class 7 Science Chapter 16 Water: A Precious Resource
• Class 7 Science Chapter 17 Forests: Our Lifeline
• Class 7 Science Chapter 18 Wastewater Story

These NCERT Solutions for Class 7 Science Chapter 16 Water: A Precious Resource Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Water: A Precious Resource NCERT Solutions for Class 7 Science Chapter 16

Class 7 Science Chapter 16 Water: A Precious Resource Textbook Exercise Questions and Answers

Question 1.
Mark ‘T’ if the statement is true and ‘F’ if it is false:
a. The freshwater stored in the ground is much more than that present in the rivers and lakes of the world.
b. Water shortage is a problem faced only by people living in rural areas.
c. Water from rivers is the only source for irrigation in the fields.
d. Rain is the ultimate source of water.
Answer:
a. T
b. F
c. F
d. T

Question 2.
Explain how groundwater is recharged?
Answer:
The rainwater and water from other sources, such as rivers and ponds, seeps through the soil and fills the empty spaces and cracks deep below the ground. The process of seeping of water into the ground is called infiltration. The groundwater thus gets recharged by this process.

Question 3.
There are ten tube wells in a lane of fifty houses. What could be the long term impact on the water table?
Answer:
Ten tube-wells in a lane of fifty houses can lead people to mindlessly exploit the groundwater. This can lead to depletion of the water table in the long run.

Question 4.
You have been asked to maintain a garden. How will you minimise the use of water?
Answer:
Drip irrigation can be implemented to reduce the wastage of water. By this process, water is delivered to the base of the plants directly. Pipes should be checked for leaks. Arrangements should be made for collecting rainwater in small pits. This water can be used later for various purposes.

Question 5.
Explain the factors responsible for the depletion of water table.
Answer:
Here are a few factors which are responsible for the depletion of water table:
a. Increased population: As our population increases, correspondingly the demand for water also increases.

b. Increasing industries: The number of industries is on the rise ever since the human population began multiplying exponentially. These industries need water for various processes.

c. Lack of water conservation techniques: Rain is the main source of water for both the surface and the underground water bodies. Upon proper conservation, the ground water can be increased. This has not yet been achieved due to lack of implementation of such water conservative techniques.

d. Agricultural activities: Our country depends vastly on its agricultural roots and the land that is used for the cultivation has increased. Hence, water consumption has also increased. Groundwater is exploited as there is an irregularity in the rainfall. This results in the depletion of groundwater.

Question 6.
Fill in the blanks with the appropriate answers:
a. People obtain groundwater through ……………….. and ………………..
b. Three forms of water are ……………….., ……………….. and ………………..
c. The water bearing layer of the earth is ………………..
d. The process of water seepage into the ground is called ………………..
Answer:
a. wells and tube-wells, b. solid, liquid and gas, c. aquifer, d. infiltration

Question 7.
Which one of the following is not responsible for water shortage?
a. Rapid growth of industries
b. Increasing population
c. Heavy rainfall
d. Mismanagement of water resources
Answer:
c. Heavy rainfall

Question 8.
Choose the correct option. The total water
a. in the lakes and rivers of the world remains constant.
b. under the ground remains constant.
c. in the seas and oceans of the world remains constant.
d. of the world remains constant,
Answer:
d. of the world remains constant.

Question 9.
Make a sketch showing ground-water and water table. Label it.
Answer:

NCERT Extended Learning Activities and Projects

Question 1.
Role play: You are a water detective in your school. You have a team of six members. Survey the campus and make a note of the following.
a. Total number of taps
b. Number of taps leaking
c. Amount of water wasted due to leaking
d. Reasons of leakage
e. Corrective measures taken
Hint:
Do it yourself.

Question 2.
Groundwater pumped out: Try to find out if there are any hand pumps in your neighbourhood. Go to the owner or the users of a few of these and find out the depth at which they struck water. If there are any differences, think of the probable reason. Write a brief report and discuss it in your class. If possible, visit a place where boring is going on to install a hand pump. Watch the process carefully and find out the depth of the water table at that place.
Hint:
There is a difference in depth of water table in difference places. According to the depth of the water table, the pumps are bored to different, depths at different places.

Question 3.
Catching rainwater-Traditional methods: Form groups of 4 to 5 students in the class and prepare a report on the various traditional ways of water harvesting. If possible, use the following web link: www.rainwaterharvesting.org.
Hint:
Do it yourself.

Question 4.
Conservation of water: Carry out a campaign to conserve water at home and in the school. Design posters to remind others of the importance of water resources.
Hint:
Do it yourself.

Question 5.
Create a Logo: Hold a competition to create a logo or a symbol depicting water scarcity.
Hint:
Do it yourself.

Activity 1

Objective: To collect information (articles and pictures) on water shortage from the newspapers and magazines.
Materials Required: Old newspapers and magazines, scrap book.
Procedure:

• Collect clipping from newspapers and magazines on the news items, articles, pictures related to water shortage.
• Paste them in your scrapbook.

Observation:
Preserve water bodies in the country

Conclusion: The water is very precious for every living being but nowadays there is a shortage of water everywhere around the world.

Forms of Water: in nature, water exists in three forms:

1. As a solid, it exists as ice-caps at the poles, snow-covered mountains and in glaciers.
2. As a liquid, it is in the form of water in oceans, lake and rivers, and the underground water.
3. Its gaseous form is the water vapour in the air around us.

Water Cycle: The water cycle is a natural process that continuously happens on earth. The water cycle maintains the amount of water on the earth. It can be divided into four phases:
i. Evaporation: It is a process when the water from the earth’s surface gets converted into water vapour due to the heat of the sun. The water vapour then goes into the air.
ii. Condensation: The water vapour rises up in the atmosphere and gets accumulated above, condenses and forms water droplets. These droplets collect together and form clouds.
iii. Precipitation: When the volume of droplets increases in the atmosphere, they fall on the earth as rainfall, also called as precipitation. Sometimes it also occurs as snowfall in some areas.

iv. Collection: As the water falls back on the earth, it travels along the surface and gets accumulated at different places like lakes, rivers, seas, oceans and under the ground. That’s how the amount of water is regulated on the earth.

• Infiltration: The process of seeping of water into the ground to recharge underground water is known as infiltration.
• Aquifer: Sometimes, ground water accumulates between layers of hard rocks. This is known as an aquifer. Water in aquifers can be drawn with tube-wells and hand pumps.
• Groundwater: The space between particles of soil and gaps between rocks are filled with groundwater.
• Water table: The level below which the ground is saturated with water is called water table. The water table varies from place to place. It may even change at a given place with time.

Depletion of Water Table: As long as the water drawn from the ground is replenished by seepage of rain water, the water table remains unaffected. The problem starts when we take more water from the ground than is replenished by natural means. Then the water table goes down and it is said to have been depleted.

Reasons for the depletion of water table:
The depletion of water table is taking place due to some causes. These causes are:

• Increasing population that creates more demand for water.
• More wells are dug and water is drawn from them.
• Industrial activities are increasing the demand for water. For example, the construction industry uses tube-wells to draw groundwater.
• An increase in agricultural activities demands more water. In areas where there are no streams and lakes, groundwater is used for irrigation.

Water Management: It is the continuous matching of water resources with the water requirements of a place. Water management essentially involves activities that identify sources of water, prevent wastage of water, and implement recycling of water. It may also include treatment of water to make it suitable for human consumption. It may include the following exercises:

• Prevent wastage of water.
• Prevent water leaking from pipes.
• Fixing leaking taps.
• Prevent water over-flowing from buckets while clothes are being washed alongside.
• Using water wisely while brushing the teeth, shaving, bathing, washing and during many other activities,
• Practicing rainwater harvesting.

Rainwater Harvesting: Instead of letting rainwater runoff into the sea, it can b>e used to recharge groundwater. This is known as rainwater harvesting. Rainwater harvesting can be used to raise the water table in arid areas. It can also be used to create water storage areas.

Bawris: This is an old method of rainwater storage and recharge. In olden days people built deep step wells into the ground. These deep step wells are called bawris. During rainy season these wells are filled with water. The water is stored in these wells for longer time because they are very deep and the evaporation of water is less. During shortage, people use this water.

Drip irrigation: It can be used by farmers to save water. It is a technique of watering plants by making use of narrow tubing, which delivers water directly at the base of plant.

Effects of Water Scarcity on Plants: Plants need water to absorb nutrients from the soil and make their food. Without water, plants would die and greenery would be lost. This, in turn, would mean the end of all life on the earth. Because without plants, there would have no food, oxygen or rainfall. There would also be many other problems.

Every individual should use water judiciously. Overdrawing of underground water should be avoided and efforts should be made to recharge underground water. To avoid wastage of water in the fields, drip irrigation may be adopted for watering the plants. One should also promote afforestation.

Class 7 Science Chapter 16 Water: A Precious Resource Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
Which day is celebrated as the world water day?
Answer:
22 March is celebrated as the world water day.

Question 2.
What is groundwater?
Answer:
The water found below the water table is called groundwater.

Question 3.
What is water table?
Answer:
The upper limit of underground water where all the space between the particles of soil and gaps between rocks are filled with water is called water table.

Question 4.
What are the three forms of water?
Answer:

1. Solid form: ice, snow
2. Liquid form: water
3. Gaseous form: steam or water vapour

Question 5.
Define aquifer.
Answer:
The groundwater stored between the layers of hard rocks below the water table is known as an aquifer.

Question 6.
What is infiltration?
Answer:
The process of seeping of water into the ground is called infiltration.

Question 7.
Why is water cycle important?
Answer:
The water cycle is important because this process provides the earth with the natural, continual water supply that all living things need in order to survive.

Question 8.
What is the source of groundwater?
Answer:
The rainwater and water from other sources, such as rivers and ponds, seeps through the soil and fills the empty spaces and cracks deep below the ground.

Question 9.
What percentage of earth’s surface is covered with water?
Answer:
71%.

Question 10.
Why do certain regions on the earth become desert?
Answer:
Certain regions on the earth become desert because they get very little rain.

Question 11.
Name the available water resource on the earth.
Answer:
Oceans, seas, lakes, rivers, ice, groundwater and moisture in the air.

Question 12.
What is water management?
Answer:
Water management refers to the activities that ensure the continuous matching of water resources with the water requirements of a place.

Question 13.
What do you mean by rainwater harvesting?
Answer:
Instead of letting rain water run-off into the sea, it can be used to recharge groundwater. This is known as rainwater harvesting.

Question 14.
How water in an aquifer can be pumped out?
Answer:
With the help of tube wells or hand pumps.

Question 15.
In which state can the water be found when it circulates through the water cycle?
Answer:
When water circulates through the water cycle it can be found in all the three forms, i.e., solid, liquid and gas (vapour).

Short Answer Type Questions

Question 1.
State various uses of water?
Answer:
Water is used for various activities such as agriculture, industries, cooking, cleaning utensils, bathing, washing clothes and most importantly for drinking.

Question 2.
Why do we need water cycle?
Answer:
The earth is covered by water, however, almost 97% is salt water found in the oceans. We cannot drink salt water or use it for crops because of the salt content. We can remove salt from ocean water but the process is very expensive. Hence, water cycle is required to maintain the flow of fresh usable water on the earth.

Question 3.
What do you mean by drip irrigation?
Answer:
Drip irrigation is a method through which maximum number of plants can be irrigated with minimum use of water. For this, pipelines are laid throughout the rows of plants. Pipes are pierced at strategic points to release water in droplets. This helps in saving the water which is an important but scarce resource.

Question 4.
What is rainwater harvesting?
Answer:
Collecting and conserving the rainwater from the rooftops of the buildings is called rainwater harvesting. In this technique water from the rooftops of the houses, industries, etc., is collected. It is used directly or allowed to seep into the soil instead of allowing it to flow down. It helps in raising the water table.

Question 5.
What are bawris?
Answer:
Bawris was the traditional way of collecting rainwater. Bawris were recharged time to time. With time, they fell into discuss and garbage started piling in these reservoirs. Nowadays, the bawris are being revived.

Question 6.
What will be the consequences if water is not available to the plants?
Answer:
Plants need water to get nutrients from the soil to prepare their food. If water is not available to plants, the green character of the planet shall be lost. This may mean the end of all life, for a world without plants shall mean no food, no oxygen, not enough rain and innumerable other problems.

Question 7.
How increasing industries are responsible for depletion of water table?
Answer:
Water is used by all the industries. Almost everything that we use needs water somewhere in its production process. The number of industries is increasing continuously. Water used by most of the industries is drawn from the ground.

Question 8.
Why is distribution of water uneven in our country?
Answer:
India is a vast country and the rainfall is not the same everywhere. Some regions have excessive rains while some others have very little rainfall. Excessive rains cause floods, whereas the absence of rains results in droughts. Therefore, some regions in our country may have floods while others may suffer from droughts at the same time.

Question 9.
Define water management.
Answer:
Preventing the wastage of precious water is called water management. A couple of techniques can be employed to save precious water.

• Construction of Bawris.
• Performing rainwater harvesting.
• Drip irrigation

Long Answer Type Questions

Question 1.
How is increasing population responsible for depletion of water table?
Answer:
Increasing population creates demand for construction of houses, shops, offices, roads and pavements. This decreases the open areas like parks and playgrounds. This, in turn, decreases the seepage of rainwater into the ground. Moreover, a huge amount of water is required for construction work. Often, groundwater is used for this purpose. So, on one hand we are consuming more groundwater and on the other hand we are allowing lesser water to seep into the ground. This results in the depletion of water table.

Question 2.
What is meant by the water cycle? How water cycles in the nature? Why this cycle is not as straight forward and simple as we discuss in the definition?
Answer:
The whole process in which water evaporates and falls back on the land as rain and later flows back into the sea via rivers is known as the water cycle. It is the circulation of water between the atmosphere and the surface water.

Water evaporates continuously from the surface of seas, rivers, lakes, oceans and vegetation on land into the atmosphere due to the heat of the sun. Water vapours, being lighter, rise upwards and join clouds in the sky. The clouds are carried in the air through long distances. When cooled sufficiently, water vapours condense into water droplets which come down as rain, snow or hail. The rain water finds its way back into the rivers and sea thereby completing the water cycle.

This cycle is not as straight forward and simple as the statement seems to imply. All of the water that falls on the land does not immediately flow back into the sea. Some of it seeps into the soil and becomes part of the underground reservoir of freshwater. Some of this underground water finds its way to the surface through springs. We bring it back to the surface for our use through wells or tube-wells. Water is also used by terrestrial animals and plants for various life-processes.

Question 3.
Discuss some of the steps involved in the conservation of water.
Answer:
The steps involved in the conservation of water are:

• Always keep the taps closed while brushing or washing the face to prevent water from flowing away unnecessarily. One should use it only when needed.
• Fix any water taps that are leaking, immediately.
• Instead of taking a shower, use a bucket to take bath.
• Use water left from washing clothes for mopping the floor and washing cars instead of running water.
• Water your plants in the morning or evening times so that the water does not immediately get evaporated.
• Do not waste food as it takes a lot of water to irrigate the fruits and vegetables.
• Do not throw away water unnecessarily. Use water in a judicious manner.

Question 4.
Water is very precious for all living organisms. What will happen in future, if we do not save water now?
Answer:
Water is needed for various life activities. If we continue the misuse and wastage of water and do not conserve it, the life on the earth will get affected. This may lead to consequences such as:

• Excessive pumping results in lowering groundwater table.
• Groundwater and surface water are connected. When groundwater is over-used, lakes, streams and rivers connected to ground water also get little supply.
• Plants need water to prepare their food. If there will be scarcity of water on the earth, the plant will be unable to the prepare their food. This will lead to end of the life because world without plants means no food and no oxygen.

Picture-Based Questions

Question 1.
a. Observe the picture shown below and identify it.

b. Identify all the processes numbered 1-7.
Answer:
a. It shows water cycle.
b.

1. Source of water
2. Evaporation
3. Condensation
4. Clouds
5. Transpiration
6. Infiltration
7. Rainfall

Question 2.
Observe the given picture and identify it. What is the importance of this process?

Answer:
The given picture shows the drip irrigation. It is an allocation technique used by farmers to water their fields. In this, water is allowed to slowly drip into the plants so that the water directly gets into the roots of the plants. This minimises evaporation, saves water and allows the nutrients to reach the plants easily. Narrow tubes are used to make water reach the roots of the plants in drip irrigation.

NCERT Solutions for Class 7 Science

• Class 7 Science Chapter 1 Nutrition in Plants
• Class 7 Science Chapter 2 Nutrition in Animals
• Class 7 Science Chapter 3 Fibre to Fabric
• Class 7 Science Chapter 4 Heat
• Class 7 Science Chapter 5 Acids, Bases and Salts
• Class 7 Science Chapter 6 Physical and Chemical Changes
• Class 7 Science Chapter 7 Weather, Climate and Adaptations of Animals of Climate
• Class 7 Science Chapter 8 Winds, Storms and Cyclones
• Class 7 Science Chapter 9 Soil
• Class 7 Science Chapter 10 Respiration in Organisms
• Class 7 Science Chapter 11 Transportation in Animals and Plants
• Class 7 Science Chapter 12 Reproduction in Plants
• Class 7 Science Chapter 13 Motion and Time
• Class 7 Science Chapter 14 Electric Current and its Effects
• Class 7 Science Chapter 15 Light
• Class 7 Science Chapter 16 Water: A Precious Resource
• Class 7 Science Chapter 17 Forests: Our Lifeline
• Class 7 Science Chapter 18 Wastewater Story

These NCERT Solutions for Class 7 Science Chapter 15 Light Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Light NCERT Solutions for Class 7 Science Chapter 15

Class 7 Science Chapter 15 Light Textbook Exercise Questions and Answers

Question 1.
Fill in the blanks:
a. An image that cannot be obtained on a screen is called ………………..
b. Image formed by a convex ……………….. is always virtual and smaller in size.
c. An image formed by a ……………….. mirror is always of the same size as that of the object.
d. An image which can be obtained on a screen is called a ……………….. image.
e. An image formed by a concave ……………….. cannot be obtained on a screen.
Answer:
a. virtual
b. mirror
c. plane
d. real
e. lens

Question 2.
Mark “T” if the statement is true and “F” if it is false.
a. We can obtain an enlarged and erect image by a convex mirror.
b. A concave lens always forms a virtual image.
c. We can obtain a real, enlarged and inverted image by a concave mirror.
d. A real image cannot be obtained on screen.
e. A concave mirror always forms a real image.
Answer:
a. False
b. True
c. True
d. False
e. False

Question 3.
Match the items given in Column I with one or more item of Column II.

Answer:
1. (e),
2. (b)
3. (a),
4. (c),
5. (f)

Question 4.
State the characteristics of the image formed by a plane mirror.
Answer:
Characteristics of the image formed by a plane mirror are:

• The image formed is virtual.
• The image is laterally inverted.
• It is of the same size as the object.
• The image is situated at the same distance from the mirror as the object.
• The image is erect.

Question 5.
Find out the letters of English alphabet or any other language known to you in which the image formed in plane mirror appears exactly like the letter itself. Discuss your findings.
Answer:
A, H, I, M, O, T, U, V, W, X, Y are the letters of English alphabet in which the image formed in a plane mirror appears exactly like the letter itself.

Question 6.
What is a virtual image? Give one situation where a virtual image is formed.
Answer:
A virtual image is an image that cannot be obtained or formed on a screen. The image formed in plane mirror cannot be obtained on a screen because this is a virtual image.

Question 7.
State two differences between a convex and a concave lens.
Answer:
Difference between convex lens and a concave lens are:

Question 8.
Give one use each of a concave and a convex mirror.
Answer:
Convex mirrors are used in roads and large buildings to allow people to see what is around the bend. Concave mirrors are used in reflecting telescopes and also as a magnification tool.

Question 9.
Which type of mirror can form a real image?
Answer:
A concave mirror can form a real image of an object. The nature of the image depends on the distance of the object from the concave mirror.

Question 10.
Which type of lens forms always a virtual image?
Answer:
A concave lens always forms a virtual image of an object.

Choose the correct option in questions 11-13.

Question 11.
A virtual image larger than the object can be produced by a
a. concave lens
b. concave mirror
c. convex mirror
d. plane mirror
Answer:
c. concave mirror.

Question 12.
David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
a. 3 m
b. 5 m
c. 6 m
d. 8 m
Answer:
c. 6 m

Question 13.
The rearview mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rearview mirror, the image of a truck parked behind the car. The speed at which the image of the truck appears to approach the driver will be
a. 1 m/s
b. 2 m/s
c. 4 m/s
d. 8 m/s
Answer:
c. 4 m/s

NCERT Extended Learning Activities and Projects

Question 1.
Play with a mirror. Write your name with a sketch pen on a thin sheet of paper, polythene or glass. Read your name on the sheet while standing in front of a plane mirror. Now look at your image in the mirror.
Hint:
Do it yourself.

Question 2.
A burning candle in water: Take a shoebox, open on one side. Place a small lighted candle in it. Place a clear glass sheet (roughly 25 cm x 25 cm) in front of this candle. Try to locate the image of the candle behind the glass sheet. Place a glass of water at its position. Ask your friends to look at the image of the candle through the sheet of glass. Ensure that candle is not visible to your friends. Your friends will be surprised to see the candle burning in water. Try to explain the reason.

Hint:
When we put a glass filled with water in that position, then the image of the burning candle appears to be in it. This is all because plane glass sheet is transparent so the image of burning candle is fonned behind the glass sheet.

Question 3.
Make a rainbow. Try to make your own rainbow: You can try this project in the morning or in the evening. Stand with your back towards the sun. Take a hosepipe or a water pipe used in the garden. Make a fine spray in front of you. You can see different colours of rainbow in the spray.
Hint:
Do it yourself.

Question 4.
Visit a laughing gallery in some science centre or a science park or a village mela. You will find some large mirrors there. You can see your distorted and funny images in these mirrors. Try to find out the kind of mirror used there.
Hint:
Spherical mirrors are used to form funny images. This is because normal plane mirrors produce a perfect image whereas we need distorted, funny images which can be only produced by curved mirrors.

Question 5.
Visit a nearby hospital. You can also visit the clinic of an ENT specialist or a dentist. Request the doctor to show you the mirror used for examination of ear, nose, throat and teeth. Can you recognise the kind of mirror used in these instruments?
Hint:
Concave mirrors are used by dentists because at a short range, the concave mirror produces magnified, upright images of face. By these mirrors, dentists can examine the patient more easily.

Question 6.
Roleplay: Here is a game that a group of children can play. One child will be chosen to act as object and other will act as the image of the object. The object and the image will sit opposite to each other. The object will make movements, such as raising a hand, touching an ear, etc. The image will have to make the correct movement following the movement of the object. The rest of the group will watch the movements of the image. If the image fails to make the correct movement, she/he will be retired. Another child will take her/ his place and the game will continue. A scoring scheme can be introduced. The group that scores the maximum will be declared the winner.
Hint:
Do it yourself.

Activity 1

Objective: To show that the image formed in a plane mirror is erect and virtual.
Materials Required: A large-sized plane mirror and a candle.
Procedure:

• Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if similar candle is placed behind the mirror.
• The candle, which appears behind the mirror, is the image of the candle formed by the mirror. The candle itself is the object.
• Now move the candle to different positions in front of the mirror. Observe the image in each case.
• Now place a vertical screen behind the mirror. Try to obtain the image of the candle on this screen. The image of the candle cannot be obtained on the screen.
• Now place the screen in front of the mirror. The image of the candle cannot be obtained on the screen in this case also.

Observations: The image of candle cannot be obtained on a screen in either case.
Conclusion: This happened because the image formed in a plane mirror is erect and virtual.

Activity 2

Objective: To show that the image formed in a plane mirror is laterally inverted.
Materials Required: A large size plane mirror.
Procedure:

• Stand in front of a big plane mirror and observe your image in the mirror.
• Lift your right hand, then the image lifts its left hand.
• Now lift your left hand and then the image appears to lift its right hand.

Left hand appears on the right side in the image

Observations: When you see yourself in the plane mirror, your right side part of the body becomes left and left side part of the body become right.
Conclusion: When an object is placed in front of a plane mirror, the lateral sides of the object are changed and this is called lateral inversion.

Spherical Mirrors: When the mirror is a part of a sphere having a curved reflecting surface, it is called a spherical mirror. Spherical mirrors are classified based on the shape of their reflecting surface in two types- concave and convex. When a part of sphere is cut, the inside part is called the concave side, while the outside part is called the convex side. A common example is a stainless steel spoon whose inside part behaves as a concave mirror and outer part behaves as a convex mirror.

i. Concave Mirror: It is a spherical mirror whose reflecting surface is curved inwards. In case of concave mirror, the image is usually inverted, real and smaller than the object. When the object is kept too close to the concave mirror, the image is erect, virtual and larger than the object.

Uses of Concave Mirror:

• Concave mirror is used by dentists to focus a beam of light to see inside a patient’s mouth or ears.
• Concave mirror is used in solar furnaces, because rays of sunlight converge at a point once they reflect from the concave mirror, generating heat.
• Concave mirror is used as barber’s mirror because it shows a larger image when object is too close to the mirror. It helps in getting a closer shave.
• It is also used in reflectors of torches and headlights of vehicles.

Activity 3

Objective: To show that a concave mirror forms a real image of the sun.
Materials Required: A concave mirror and a sheet of paper.
Procedure:

• Take a concave mirror.
• Hold it facing the sun.
• Try to get the light reflected by the mirror on a sheet of paper.
• Adjust the distance of the paper until you get a sharp bright spot on it.
• Hold the mirror and the sheet of paper steady for a few minutes.

Observation: While doing this activity, the paper starts burning because the real image of sun is formed by the concave mirror.
Conclusion: Concave mirror forms real images.

ii. Convex Mirror: It is a spherical mirror whose reflecting surface is curved outwards. In case of convex mirror, the image is always erect, virtual and smaller than the object.

Uses of Convex Mirror:

• Convex mirror is used in rear view mirrors of vehicles, as it shows smaller images from a bigger field of view.
• Convex mirror is also used on hairpin bends of roads to see the vehicles coming from the other side of the bend.

Activity 4

Objective: To study the images formed by a convex mirror.
Materials Required: Convex mirror, mirror stand, candle and white paper screen.
Procedure:

• Mount a convex mirror on a mirror stand and place it on a table.
• Place a screen made from white paper at a distance in front of it.
• Place a lighted candle in between the mirror and the screen.
• Move the screen to see if any image is formed on it. No image is seen on the screen.
• Now look into the convex mirror. An erect and smaller image is seen in the mirror.
• Slowly move the candle away from the mirror and keep observing the nature of the image formed in the mirror.

Observation: In convex mirror, the images of candle are smaller and erect at all distances.
Conclusion: Convex mirror always forms erect and small-sized images.

Lens: A lens is a part of a reflecting material like glass or plastic but transparent from both sides. Lenses are unlike mirrors that have a reflecting surface only on one side. Depending upon its shape, a lens can be categorised as follows:
i. Convex lens: This lens is thinner at the edges and thicker in the centre. It is a converging lens as it converges light rays. It can form real, inverted and diminished image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified.

A magnifying glass is a convex lens which is used to make small things look bigger by producing their magnified (enlarged) images.

ii. Concave lens: It is the lens that is thinner at the centre than at the edges. It is a diverging lens as it diverges light rays. Image formed is always virtual, erect and diminished.

Refraction of light ray through lenses
(a) Diverging action of a concave lens
(b) Converging action of a convex lens

Uses of Lenses: Lenses are used in microscopes, telescopes and cameras. They are used in astronomical telescopes. They are also used in reading glasses.

Components of White Light: The white light or sunlight is composed of seven colours. These colours in order are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). When white light passes through a glass prism, it breaks down into its component colours. This is the reason why rainbows show all the colours of the white light. Several phenomena explain that sunlight or white light is composed of seven colours.

Activity 5

Objective: To show that the sunlight consists of seven colours.
Materials Required: A glass prism and a white sheet of paper.
Procedure:

• Take a glass prism
• Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism.
• Let the light coming out of the other face of the prism fall on a white sheet of paper.

Observation: When the light coming out of the other face of the prism falls on a white sheet then a band of seven colours appear on the screen.
Conclusion: When a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen. Hence, sunlight is made up of seven colours.

i. Dispersion of light: The splitting up of white light into seven colours on passing through a transparent medium, like a glass prism, is called dispersion of light. The formation of spectrum of seven colours on the other side of the prism shows that white light is a mixture of seven colours.

ii. Rainbow: The rainbow is an arch of seven colours in the sky. The rainbow is produced by the dispersion of sunlight by tiny rain drops suspended in the atmosphere that behave like minute prisms.

iii. Newton’s Disc: It is a circular disc on which seven colours of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows how the seven colours combine to make the white light.

Class 7 Science Chapter 15 Light Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is light?
Answer:
Light is a form of energy which helps us in getting the vision.

Question 2.
How many colours are there in the visible spectrum?
Answer:
Seven.

Question 3.
Which types of images can be formed on a cinema screen?
Answer:
Real images.

Question 4.
Is the image of an object in a plane mirror virtual or real?
Answer:
Virtual.

Question 5.
Define the image of an object.
Answer:
Due to the reflection of light, the impression of an object formed in a mirror is called the image of the object.

Question 6.
What is virtual image?
Answer:
The image that cannot be captured on a screen is called a virtual image.

Question 7.
What is real image?
Answer:
Image that can be captured on a screen is known as real image.

Question 8.
Give an example of real image.
Answer:
In a camera, images are real and can be captured on the negative, which acts as a screen.

Question 9.
What happens when light rays are incident on a concave lens?
Answer:
When light rays are incident on a concave lens, they bend outwards from the lens or diverge.

Question 10.
How is a rainbow formed in the sky?
Answer:
A rainbow is formed by the dispersion of the sun’s rays through raindrops into seven colours.

Question 11.
What is VIBGYOR?
Answer:
It represents the order of seven colours in the rainbow, i.e., violet, indigo, blue, green, yellow, orange and red.

Question 12.
What happens when light falls on a polished or a shiny surface?
Answer:
When light falls on a polished or a shiny surface, it gets reflected.

Question 13.
What makes things visible to us?
Answer:
Objects are visible only when light reflected from them reaches our eyes.

Question 14.
What is reflection of light?
Answer:
Bouncing back of a light ray after hitting any surface is known as reflection of light.

Question 15.
Why we are not able to see the candle flame through a bent pipe?
Answer:
We are not able to see the candle flame through a bent pipe because light travels along straight lines.

Question 16.
How can we change the path of light?
Answer:
We can change the path of light by keeping any shiny or polished or reflecting material in the path of the light beam.

Question 17.
What type of image does the outer side of a spoon show?
Answer:
The outer surface of a spoon acts like a convex mirror.

Short Answer Type Questions

Question 1.
What do you mean by lateral inversion? Give a suitable example.
Answer:
When an object is placed in front of a plane mirror, then the right side of the object appears to be the left side of the image and the left side of the object appears to be the right side of the image. This change of sides of an ‘object’ and its ‘mirror image’ is called lateral inversion. For example, the word ‘AMBULANCE’ on an ambulance is written in mirror writing

Question 2.
State any two uses each of concave mirror and convex mirrors.
Answer:
Concave mirrors:

1. Concave mirrors are used as reflectors in torches, headlights of vehicles.
2. Concave mirrors are used by the dentist to see an enlarged image of teeth.

Convex mirrors:

1. Convex mirrors are used as rearview mirrors or side-view mirrors in automobiles such as cars, scooter, trucks, buses, etc., to see the traffic coming from behind.
2. Convex mirrors are used as staircase mirrors on the double-decker buses.

Question 3.
Why is a concave mirror called a converging mirror and a convex mirror called a diverging mirror?
Answer:
Concave mirror is called a converging mirror because it converges the parallel rays of light that fall on the mirror at a point called focus. Convex mirror is called a diverging mirror because parallel rays of light falling on it diverge after reflection.

Question 4.
Distinguish between concave mirror and convex mirror.
Answer:

Question 5.
Why is convex mirror used as side mirror in scooters?
Answer:
We can recognise that the mirrors used as side mirror in scooters are convex mirrors. Convex mirror can form erect images of object spread over large area, so helps the drivers to see the traffic behind them.

Question 6.
What is the difference between a virtual image and a real image?
Answer:
The following are the main differences between a real image and a virtual image.

• A real image can be obtained on a screen whereas a virtual image cannot be obtained on a screen.
• A real image is always inverted whereas a virtual image is always erect.
• A real image is formed when the rays of light, after reflection, actually meet at some point whereas a virtual image is formed when the rays of light, after reflection, appear to meet at a point but do not actually meet.

Question 7.
Why do we need a shiny surface for reflection?
Answer:
The extent of reflection depends upon the shine and smoothness of the surface. More the shine and smoothness of the surface, more will be the reflection. That is why, mirrors reflect most of the light falling on them. Hence, for reflection, shiny surfaces are required.

Question 8.
Give the characteristics of images formed by convex and concave lenses.
Answer:

1. Convex lens: A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified.
2. Concave lens: A concave lens always forms erect, virtual and smaller image than the object.

Question 9.
What are spherical mirrors? Give their types.
Answer:
Mirrors having curved surfaces are known as spherical mirrors. Their name is so, because they are considered as a part of a hollow sphere. Mirrors are mainly of two types: concave mirrors and convex mirrors.

a. Concave mirror: Its reflecting surface is curved inwards.
b. Convex mirrors: Its reflecting surface is bulged out.

Long Answer Type Questions

Question 1.
a. What is a lens? Name the two types of lenses. Name any five things which use lenses.
b. What kind of lens is used as a magnifying glass?
c. Which type of reflector is used in car headlights for producing a parallel beam of light?
Answer:
a. A lens is a piece of any transparent material bound by two curved surfaces or by one curved and one plane surface. When light rays pass through a lens, they bend and change their direction. The two types of lenses are:

• convex lens, and
• concave lens.

The five things which use lenses are: Spectacles, cameras, microscopes, telescopes and film projectors.
b. Convex lens is used as a magnifying glass.
c. Concave mirrors are used in vehicle headlights to send parallel rays because it allows the light rays to be focused as a single beam and give more power to the light that makes it more efficient for seeing and to be seen by others.

Question 2.
What is Newton’s disc? Why does it appear white when rotated?
Answer:
A circular disc constructed by Newton which is divided into seven parts painted with seven colours is known as Newton’s disc. The colours painted on disc are those of the spectrum of visible light. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. This appearance of white light on rotating disc suggests that the white light consists of seven colours.

Question 3.
Show that the image is at the same distance behind the plane mirror as the object is in front of it.
Answer:
Take a chessboard. Draw a thick line in the middle of it. Fix a plane mirror vertically on this line. Place a pencil sharpener; at the boundary of the third square counting from the mirror. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. We will find that the image is at the same distance behind the mirror as the object is in front of it.

Image in plane mirror is at the same distance as the object in front of it

Question 4.
Show how light gets reflected from a plane mirror with the help of an activity.
Answer:
Take a torch. Cover its glass with a chart paper which has three slits. Spread a sheet of chart paper on a smooth wooden board. Fix a plane mirror strip vertically on the chart paper. Now direct the beam of light on the mirror from the torch with slits. Place the torch in such a way that its light is seen along the chart paper on the board. Now adjust its position so that the light from the torch strikes the plane mirror at an angle. We will see that the mirror changes the direction of light that falls on it. Now move the torch slightly to either side. We will see change in the direction of reflected light too. Look into the mirror along the direction of the reflected light. We see the slits in the mirror. This is the image of the slits. This activity shows how light gets reflected from a plane mirror.

Question 5.
Write an experiment to show that the sunlight consists of seven colours.
Answer:
Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall. We see colours similar to those in a rainbow. This shows that the sunlight consists of seven colours.

Picture-Based Questions

Question 1.
Draw a diagram to show converging nature of convex lens and diverging nature of a concave lens. What do you mean by converging and diverging nature?
Answer:

Refraction of light ray through lenses
(a) Diverging action of a concave lens
(b) Converging action of a convex lens
A convex lens is a converging lens because it converges the light rays falling on it.
A concave lens is a divergent lens because it diverges the light rays falling on it.

Question 2.
Draw a diagram to show the image formation by a plane mirror. What type of image is formed by a plane mirror?
Answer:
The image formed by a plane mirror is erect and virtual. It is of same size as the object and is laterally inverted.

NCERT Solutions for Class 7 Science

• Class 7 Science Chapter 1 Nutrition in Plants
• Class 7 Science Chapter 2 Nutrition in Animals
• Class 7 Science Chapter 3 Fibre to Fabric
• Class 7 Science Chapter 4 Heat
• Class 7 Science Chapter 5 Acids, Bases and Salts
• Class 7 Science Chapter 6 Physical and Chemical Changes
• Class 7 Science Chapter 7 Weather, Climate and Adaptations of Animals of Climate
• Class 7 Science Chapter 8 Winds, Storms and Cyclones
• Class 7 Science Chapter 9 Soil
• Class 7 Science Chapter 10 Respiration in Organisms
• Class 7 Science Chapter 11 Transportation in Animals and Plants
• Class 7 Science Chapter 12 Reproduction in Plants
• Class 7 Science Chapter 13 Motion and Time
• Class 7 Science Chapter 14 Electric Current and its Effects
• Class 7 Science Chapter 15 Light
• Class 7 Science Chapter 16 Water: A Precious Resource
• Class 7 Science Chapter 17 Forests: Our Lifeline
• Class 7 Science Chapter 18 Wastewater Story

These NCERT Solutions for Class 7 Science Chapter 14 Electric Current and its Effects Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

Electric Current and its Effects NCERT Solutions for Class 7 Science Chapter 14

Class 7 Science Chapter 14 Electric Current and its Effects Textbook Exercise Questions and Answers

Question 1.
Draw in your notebook the symbols to represent the following components of electrical circuits: connecting wires, switch in the ‘OFF’ position, bulb, cell, switch in the ‘ON’ position, and battery.
Answer:

Question 2.
Draw the circuit diagram to represent the circuit shown in figure given below.

Answer:

Question 3.
The given figure shows four cells fixed on a board. Draw lines to indicate how you will connect their terminals with wires to make a battery of four cells.

Answer:

Question 4.
The bulb in the circuit shown in given figure does not glow. Can you identify the problem? Make necessary changes in the circuit to make the bulb glow.

Answer:
The positive terminals of both the cells are together and that is why the bulb is not glowing. The cells should be kept in a way that the positive terminal of one cell is near the negative terminal of another cell, to make the bulb glow.

Question 5.
Name any two effects of electric current.
Answer:
The effects of electric current are:

1. Heating effect
2. Magnetic effect

Question 6.
When the current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position. Explain.
Answer:
The needle gets deflected when it is kept near a current carrying wire due to the magnetic effect of the current. The needle of the compass is made up of a thin magnet and when it comes in contact with the current carrying wire, which also acts like a magnet, then the like poles repel each other and opposite poles attract each other. Therefore the needle gets deflected.

Question 7.
Will the compass needle show deflection when the switch in the circuit shown below is closed?

Answer:
The compass needle does not get deflected as there is no battery or any source of current.

Question 8.
Fill in the blanks:
a. Longer line in the symbol for a cell represents it’s ………………… terminal.
b. The combination of two or more cells is called a …………………
c. When current is switched ‘on’ in a room heater, it …………………
d. The safety device based on the heating effect of electric current is called a …………………
Answer:
a. positive,
b. battery,
c. becomes red hot and emits heat
d. fuse

Question 9.
Mark ‘T’ if the statement is true and ‘F’ if it is false:
a. To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (T/F)
b. When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T/F)
c. An electromagnet does not attract a piece of iron. (T/F)
d. An electric bell has an electromagnet. (T/F)
Answer:
a. F,
b. T,
c. F,
d. T

Question 10.
Do you think an electromagnet can be used for separating plastic bags from a garbage heap? Explain.
Answer:
Electromagnets can only attract magnetic materials. Plastic bag is a non-magnetic material and will not be attracted by an electromagnet. Hence, an electromagnet cannot be used for separating plastic bags from a garbage heap.

Question 11.
An electrician is carrying out some repairs in your house. He wants to replace a fuse by a piece of wire. Would you agree? Give reasons for your response.
Answer:
No, we would not agree to replace the fuse by a wire. Wires in the fuses are of specific melting points. So we should always use ISI marked fuses in our houses to prevent short circuits.

Question 12.
Zubeda made an electric circuit using a cell holder shown in the given figure, a switch and a bulb. When she put the switch in the ‘ON’ position, the bulb did not glow. Help Zubeda in identifying the possible defects in the circuit.

Answer:
The cells should be connected properly. The positive terminal of one cell should be in series with the negative terminal of the other cell. The switch should be closed properly and a bulb which is fused should not be used. Then, the bulb will certainly glow.

Question 13.
In the circuit shown in the figure given below

a. Would any of the bulbs glow when the switch is in the ‘OFF’ position?
b. What will be the order in which the bulbs A, B and C will glow when the switch is moved to the ‘ON’ position?
Answer:
a. No, when the switch is in ‘OFF’ position, the current does not flow through the circuit. Hence, none of the bulbs will glow,
b. Bulbs will glow simultaneously. When the switch is moved to the ‘ON’ position, then all the bulbs will glow at once. This is because they all are connected to the same battery and switch.

NCERT Extended Learning Activities and Projects

Question 1.
Set up the circuit shown in fig 14.17 (of NCERT textbook) again. Move the key to ‘ON’ position and watch carefully in which direction the compass needle gets deflected. Switch ‘OFF’ the current. Now keeping rest of the circuit intact, reverse the connections at the terminals of the cell. Again switch ‘ON’ the current. Note the direction in which the needle gets deflected. Think of an explanation.
Hint:
The direction of magnetic poles get reversed in a conductor, when the direction of current is reversed. Hence, the needle will show deflection in opposite direction in the second case.

Question 2.
Make four electromagnets with 20, 40, 60 and 80 turns. Connect them one by one to a battery of 2 cells. Bring the electromagnet near a box of pins. Count the number of pins attracted by it. Compare the strengths of the electromagnets.
Hint:
The electromagnet with 80 turns will attract the maximum number of pins, followed by that of 60 turns, then 40 turns and finally 20 turns which will attract the least number of pins. This is because the strength of an electromagnet increases by increasing the number of turns in the coil.

Question 3.
Using an electromagnet, you can make a working model of a railway signal as shown in figure given below.

Hint:
To make a model railway signal, you will need an iron nail, a thread, two pieces of plywood, a paper clip, a battery, a pencil and a length of copper wire. Fix the two pieces of plywood to each other. They . should be perpendicular to each other. Tie the nail through the thread and fix the thread to the vertical plyboard. Take a card and cut out a signal arm and fix it to the upper end of the jaw with the help of paper clip. Next, take the pencil and wind some copper wire around it, making a hollow coil. There should be extra copper wire at both the ends. Place the coil on the horizontal plywood and connect it to the battery. Insert an iron nail inside the coil loosely. Taking another drawing pin and a paper clip, create a circuit. When you complete the circuit, the nail will be attracted inwards into the coil. This is because it creates a magnetic field. You will find the signal arm moving up and down lifting the signal.

Question 4.
Visit an electric shop. Request a mechanic to show you the various types of fuses and MCB and to explain how they work.
Hint:
Do it yourself.

Activity 1

Objective: To demonstrate the heating effect of electric current.
Materials Required: Electric cell (1.5 volts), small torch bulb, switch, connecting wires with the insulation removed from its ends and adhesive tape.
Procedure:

• Connect the electric cell to the torch bulb along with the switch with the help of connecting wires and tapes.
• Keep the switch in the OFF position.
• Touch the bulb.
• Now put the switch in the on position.
• Allow the bulb to glow for a while.
• Touch the bulb carefully.

Observation: When the switch is in OFF position, the bulb is not warm but when it is in ON position, the bulb feel warm.
Conclusion: When the electric current flows in the wires, the bulb becomes warm. Hence, current has heating effects.

Electric bulb: When electric current passes through the filament of an electric bulb, it gets heated up to high temperatures due to heating effect of current. The heat generated is so high that the filament of the bulb starts glowing, giving off light. Hence, filaments are made of tungsten that has very high melting point.

CFLs and LEDs: Incandescent bulbs give off heat along with light which leads to wastage of electricity in the form of heat. Hence, fluorescent tube lights or Compact Fluorescent Lamps (CFLs) are used which are better electricity efficient. Nowadays, light emitting diode (LED) bulbs are used because they consume much less electricity the CFLs and incandescent bulbs for the same intensity of light. Also, CFLs contain mercury vapour which is toxic and needs to be disposed off carefully.

ISI mark: Bureau of Indian Standards, New Delhi assign a standard mark, called the ISI mark on electrical products which is an assurance of conformity of the specifications given on the product and its safety. We should hence use products with ISI mark.

Electric Fuse: Electric fuse is a safety device which is used in household wirings and in many appliances. Electric fuse has a body made up of ceramic and two points for attaching the fuse wire. The fuse wire is made of a special material which easily melts whenever there is a large current in the wiring. This breaks the circuit and helps in preventing damage to costly appliances and to the wiring.

Activity 2

Objective: To demonstrate the working of a fuse.
Materials Required: Two nails, a sheet of thermocol/a wooden block, steel wool, a four-celled battery, switch, 6 volt bulb and connecting wires.
Procedure:

• Fix two nails about 5 to 10 cm apart on a sheet of thermocol or a wooden block.
• Tie a thin strand of steel wool between the two nails.
• Connect the nails to the two terminals of a four-celled battery through a switch and a 6 volt bulb by using connecting wires.
• Now, pass the electric current through the circuit by moving the switch to ‘ON’ position.

Observations: After a few seconds, the thin strand of steel wool melts and breaks and the bulb goes off. Actually, thin strand of steel wool is like a fuse wire here.
Conclusion: When a large current passes through the thin strand of steel wool, a lot of heat is produced in it due to the heating effect of current. This heat melts the thin strand of steel wool and breaks it, cutting off the flow of current in the circuit. This is how a fuse works in the household electric wiring.

MCBs (Miniature Circuit Breaker): MCBs have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built-in mechanism by which the electric supply is automatically resumed.

Magnetic Effect of Electric Current: We know that magnets can attract iron pieces. A compass needle is a small magnet that points in North-South, direction. This needle gets deflected whenever an magnet is brought close to it. Hans Christian Oersted first observed that when a compass needle is kept close to a current carrying wire, the needle gets deflected. This shows that current passing through a conductor produces magnetic effects.

Activity 3

Objective: To demonstrate the magnetic effect of current.
Materials Required: Insulted copper wire, a compass needle, electric cell, switch and bar magnet.
Procedure:

• Take a thick, insulated copper wire ‘AB’ and fix it on a table in such a way that this wire is in the North-South direction.
• A compass is placed under the wire AB.
• The two ends of the wire AB are connected to a cell through a switch by using connective wires.
• Now close the switch so that an electric current passes through the wire AB.

Observation: When we closed the switch then the current passes through the wire AB and the needle of the compass moves from its original direction.
Conclusion: When an electric current flows through a wire, it behaves like a magnet.

Electromagnets: An electromagnet is a temporary magnet, which behaves like a magnet only when electric current is passed through it. It consists of an insulated wire wound around a magnetic material like iron. Both ends of the wires behave like both poles of the magnet.
Uses of electromagnet: Electromagnets are used in telephones, telegraphs, electric motors, electric bells, generators, etc.

Activity 4

Objective: To make an electromagnet and to show that it attracts iron.
Materials Required: Long piece of copper wire, iron nail, electric cell and iron pins.
Procedure:

• Wind the copper wire around the iron nail.
• Connect the two ends of the wire to the two terminals of an electrical cell to form an electromagnet.
• Bring a small iron pin close to it and observe.
• Disconnect the wire ends from the two terminals of the electric cell and observe what happens.

Observation: When the two ends of the wire are connected to the cell, the current is flowing through the coil. As a result, the pin gets attracted to the electromagnet. When we disconnect the wire, the pin falls off.
Conclusion: The iron nail behaves like a magnet which attracts the pins towards it. It will happen when the current flows through the wire, but if we disconnect the circuit, the iron losses its magnetic property.

Electric Bell: Electric bell is composed of two rods of cast iron. Coil is wound around the iron rods. A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end. Another end of the strip is connected to the circuit. A gong is placed in a position so that it can be hit by the hammer.

When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the contact screw which has joined it to the circuit and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.

Class 7 Science Chapter 14 Electric Current and its Effects Additional Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What are the components of a simple electric circuit?
Answer:
Battery, wire, key and bulb are the components of a simple electric circuit.

Question 2.
Give the symbol of a battery.
Answer:

Question 3.
Does a bulb glow when the circuit is open?
Answer:
No, the bulb does not glow when the circuit is open.

Question 4.
What are the effects of electricity?
Answer:
The flow of electric current causes the following effects in a conductor:

• Heating effect
• Chemical effect
• Magnetic effect

Question 5.
On what factors does the heat produced in a wire depend?
Answer:
It depends on the material, length and thickness of the wire.

Question 6.
Differentiate between electric current and electric circuit.
Answer:
Electric current is the flow of charges. But an electric circuit is the closed path in which electric current flows.

Question 7.
What is an electric cell?
Answer:
It is device which produces electric charge because of some chemical reactions.

Question 8.
The electric bulb works on which property of electric current?
Answer:
Heating effect of electric current.

Question 9.
What is the full form of MCB?
Answer:
Miniature Circuit Breaker.

Question 10.
Write full form of CFLs.
Answer:
Compact Fluorescent Lamps.

Question 11.
What do you mean by open circuit?
Answer:
Open circuit means the switch is in ‘on’ position.

Question 12.
What do you mean by a closed circuit?
Answer:
Closed circuit means switch is ‘in’ on position.

Question 13.
Which effect of electric current is utilised in an electric bell?
Answer:
Magnetic effect.

Question 14.
Which effect of electric current is utilised in the working of an electric fuse?
Answer:
An electric fuse works on the heating effect of current.

Question 15.
Name the alloy which is used to make the heating elements of electrical heating appliances like an electric iron.
Answer:
Nichrome.

Question 16.
Write the name of the scientist who discovered the magnetic effect of electric current.
Answer:
Hans Christian Oersted (H.C Oersted).

Question 17.
How can we reduce the wastage of electricity?
Answer:
The wastage of electricity can be reduced by using CFLs and LEDs.

Question 18.
What is meant by a battery?
Answer:
When two or more cells are joined together in such a way that the positive terminal of one cell is kept in contact with the negative terminal of other, we get a battery of cells.

Question 19.
Name two devices which are based on the magnetic effect of current.
Answer:

1. Compass needle
2. Electric bell

Question 20.
What is a circuit diagram?
Answer:
A circuit diagram is used to symbolically represent an electric circuit.

Short Answer Type Questions

Question 1.
Define an electric circuit and explain it with the help of a diagram.
Answer:
Any closed path along which electric current can flow is called an electric circuit. An electric circuit generally consists of the following components:

1. A source of electric current (cell or battery).
2. A conducting wire (say copper wire).
3. An electrical appliance (like a bulb).
4. A switch/key.

Question 2.
Name and define any two effects of electric current.
Answer:
a. Heating Effect: When an electric current flows through a wire, the wire gets heated. It is the heating effect of current due to the resistance of wire to the flow of current. This effect has many applications like electric heater, light bulb, etc.

b. Magnetic effect: When an electric current flows through a wire, it produces magnetic effect around it that is detected by a magnetic compass. For examples: electric bell, motor, fan, etc., are based on this effect.

Question 3.
What are filaments of a bulb and a heater made up of?
Answer:
In a bulb, there is a thin wire called the filament made up of tungsten. When
current passes through it, it gets heated up and glows. In a heater, there is a coil of wire called element which is made up of nichrome. When current passes through the element, it becomes red hot and gives out heat.

Question 4.
What is a switch?
Answer:
Switch is used to close and open a circuit. When switch is ‘ON’, circuit is complete and current flows through it and when it is ‘OFF’, circuit breaks (air is an insulator) and flow of current stops.

Question 5.
What are the reasons for excessive currents in electrical circuits?
Answer:
One reason for excessive currents in electrical circuits is the direct touching of wires. This may happen if the insulation on the wires has come off due to wear and tear. This may cause a short circuit. Another reason for excessive current can be the connection of many devices to a single socket. This may cause overload in the circuit.

Question 6.
What is the use of electrical fuse?
Answer:
In all buildings, fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident, the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off by melting away and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires.

Question 7.
Why should we not touch the lighted bulb with bare hands?
Answer:
We should never touch a lighted electric bulb connected to the mains with bare hands. It may be very hot and our hand may get burnt badly. In the same way, we should not experiment with the electric supply from the mains or a generator or an inverter. It may cause an electric shock.

Question 8.
What are MCBs? How do they work?
Answer:
MCBs (Miniature Circuit Breaker) have been replacing electric fuse from wirings at most of the places. The electric fuse has a big practical problem. Whenever the wire fuses, one needs to replace the wire to resume electric supply. More often than not, this proves to be a cumbersome task. Miniature circuit breakers break the circuit automatically. One just needs to switch it on to resume the electric supply. Many models of MCBs have a built-in mechanism by which the electric supply is automatically resumed.

Question 9.
Write three differences between a bar magnet and an electromagnet.
Answer:

Question 10.
Differentiate between an open and a closed circuit.
Answer:
Closed circuit: The electric path which starts from the positive terminal of a cell or a battery ends at its negative terminal, without any break, is called a closed electric circuit. Current flows in this circuit and switch is in ‘on’ position.

Open circuit: The electric path which starts from the positive terminal of a cell or battery but is broken at some point, is called an open electric circuit. Current does not flow from such a circuit and the switch is in ‘off’ position.

Question 11.
Of what substance is the fuse wire made? Why?
Answer:
The fuse wire is usually made from tin-plated copper wire. The tin-plated copper wire has low melting point due to which it can easily melt by the heating caused by the excessive flow of current in the circuit.

Question 12.
Why are CFLs preferred over incandescent bulbs?
Answer:
An incandescent bulb is used for lighting but it also gives heat. This is not desirable. This results in the wastage of electricity. This wastage can be reduced by using compact fluorescent lights (CFLs) in place of the bulbs. CFLs are also used to reduce wastage of electricity. These can be fixed in ordinary bulb holders.

Long Answer Type Questions

Question 1.
Explain with the help of a diagram, the construction and the working of an electric bell.
Answer:
Electric bell is composed of two rods of cast iron. Coil is wound around the iron rods. A metallic strip is placed parallel to the coils. The metallic strip is fitted with a hammer at one end. Another end of the strip is connected to the circuit. A gong is placed in a position so that it can be hit by the hammer. When current flows in the circuit, the cast iron rods become electromagnet and attract the metallic strip. The metallic hammer hits the gong because of that. Once the metallic strip is pulled towards the electromagnet, it gets disconnected from the contact screw that has attached it to the circuit and there is a break in the circuit. A spring pulls the metallic strip back because no magnetism is left in the electromagnet. This cycle continues and the electric bell produces a ringing sound.

Question 2.
Define an electromagnet. Give various uses of electromagnets.
Answer:
A core of magnetic material (such as iron) surrounded by a coil of wire behaves like a magnet when electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism. Such coils are called electromagnets. Magnetic effect of electric current has been used in making powerful electromagnets. More number of turns makes more powerful magnet.

Uses of Electromagnets:
Electromagnets are used in telephones, telegraphs, electric motors, electric bells, generators, etc.

• Powerful electromagnets are used in cranes.
• Bullet trains use the principle of electromagnetism.
• Audio and video tapes use the principle of magnetism.
• The most popular use of the electromagnets is in electric motors.

Question 3.
Define: a. overloading, and b. heating effect of current.
Answer:
a. Overloading: All electrical appliances and wires used in a circuit have definite rating. If the appliances connected to the circuit draw more current than rated, then it is called overloading.

b. Heating effect: When current is passed through a wire the current starts to flow from the positive to the negative terminal. This current produces heat energy. It is called heating effect of current.

Question 4.
What are the differences between an electromagnet and a permanent magnet?
Answer:

Question 5.
How does an electric room heater or an electric iron work?
Answer:
The wires get hot when an electric current passes through them. An electric room heater or immersion heater, geysers, electric kettles, hair dryers, etc., contain a coil of wire. This coil of wire is called an element. When these appliances are switched ON after connecting to the electrical supply, their elements become red hot and give out heat. These appliances work on the heating effects of current, i.e., the resistance offered by the metal alloy to the flow of current which produces the heating effect.

Picture-Based Questions

Question 1.
Draw diagrams of: (a) closed circuit, (b) open circuit. In which of these does the bulb glow?
Answer:
The bulb glows when the circuit is closed or switched ‘ON’.

Question 2.
a. Draw a labelled diagram to show how an electromagnet is made.
b. What is the purpose of soft iron core used in making an electromagnet?
Answer:
a.

b. Soft iron core is used to increase the strength of the electromagnet.

Question 3.
Observe the given figure and identify it. On which principle does it work?

Answer:
This diagram is of an electric bell. An electric bell works on the principle of magnetic effect of electricity.

NCERT Solutions for Class 7 Science

• Class 7 Science Chapter 1 Nutrition in Plants
• Class 7 Science Chapter 2 Nutrition in Animals
• Class 7 Science Chapter 3 Fibre to Fabric
• Class 7 Science Chapter 4 Heat
• Class 7 Science Chapter 5 Acids, Bases and Salts
• Class 7 Science Chapter 6 Physical and Chemical Changes
• Class 7 Science Chapter 7 Weather, Climate and Adaptations of Animals of Climate
• Class 7 Science Chapter 8 Winds, Storms and Cyclones
• Class 7 Science Chapter 9 Soil
• Class 7 Science Chapter 10 Respiration in Organisms
• Class 7 Science Chapter 11 Transportation in Animals and Plants
• Class 7 Science Chapter 12 Reproduction in Plants
• Class 7 Science Chapter 13 Motion and Time
• Class 7 Science Chapter 14 Electric Current and its Effects
• Class 7 Science Chapter 15 Light
• Class 7 Science Chapter 16 Water: A Precious Resource
• Class 7 Science Chapter 17 Forests: Our Lifeline
• Class 7 Science Chapter 18 Wastewater Story