Energy—How Things Work — NCERT Solutions

Given: A magnifying glass is used to focus sunlight on a piece of paper.

Observation: The paper gets very hot and may start to burn or scorch (turn brown/black) at the point where the light is focused.

Reason: The magnifying glass concentrates (focuses) the Sun's rays onto a very small spot. This concentrated sunlight carries a large amount of heat energy, which is enough to heat the paper to the point of burning.

Safety Note: This activity must be done under adult supervision.

Conclusion: This shows that sunlight carries heat energy and when focused, it can be very powerful.

Given: A water wheel is made using a paper cup and spoons. Water is poured onto it.

Observation: Yes, the water makes the wheel move (spin).

Reason: Flowing water carries kinetic energy (energy of movement). When water falls on the spoons of the wheel, it pushes them and causes the wheel to rotate.

Conclusion: This activity shows that flowing water is a source of energy. This is the same principle used in hydroelectric power plants, where the energy of flowing water is used to generate electricity.

Answer:

Using the Sun:

1. Drying clothes and papads in sunlight.
2. Solar panels on rooftops to generate electricity.
3. Solar cookers to cook food.
4. Drying fish, grains, and pickles.

Using the Wind:

1. Wind turbines/windmills to generate electricity.
2. Sailing boats and ships using wind in their sails.
3. Flying kites.
4. Winnowing grains (separating grain from husk).

Using Flowing Water:

1. Hydroelectric dams to generate electricity.
2. Water mills to grind grain.
3. Irrigation of fields using rivers and canals.

Conclusion: The Sun, wind, and flowing water are natural, clean, and renewable sources of energy that we can use without causing pollution.

Answer:

If there is no electricity in the house for a day, the following things would happen:

1. Fans and air conditioners would stop working, making it hot and uncomfortable.
2. Lights and bulbs would not work, making it dark at night.
3. Television, computers, and mobile chargers would not work, so we could not watch TV or use the internet.
4. Refrigerators would stop, and food stored in them might spoil.
5. Electric stoves and microwaves would not work, making cooking difficult.

Conclusion: We depend on electricity for almost everything in our daily life. A day without electricity would be very difficult and would make us realise how important it is to save and use electricity wisely.

Answer:

It is better to use solar or wind energy instead of coal because:

1. No Pollution: Solar and wind energy do not produce smoke or harmful gases. Burning coal releases harmful gases that pollute the air and cause health problems.
2. Renewable: The Sun and wind are available naturally and will never run out. Coal is a non-renewable resource — once used, it cannot be replaced.
3. No Global Warming: Burning coal releases carbon dioxide, which causes global warming and climate change. Solar and wind energy do not cause this.
4. Safe for Health: Clean energy sources do not cause breathing problems or lung diseases, unlike coal smoke.

Conclusion: Solar and wind energy are clean, safe, and unlimited sources of energy that are much better for our health and the environment than coal.

Answer:

Here are two examples of energy being stored:

1. Battery: A battery stores electrical energy. When we put batteries in a torch or a remote control, the stored energy is used to produce light or operate the device.

2. Food: Food stores chemical energy. When we eat food, our body uses this stored energy to move, breathe, think, and do all activities.

Bonus Example: A dam stores the energy of water. The water held back by the dam has stored energy (potential energy) that is released when water flows down to run turbines and generate electricity.

Answer (Sample):

One thing I can do at home to save energy is: Switch off lights, fans, and other electrical appliances when they are not in use.

Other ways to save energy:

• Use natural sunlight during the day instead of switching on lights.
• Use energy-saving LED bulbs instead of regular bulbs.
• Do not keep the refrigerator door open for too long.
• Use a solar cooker or pressure cooker to save cooking fuel.

Conclusion: Saving energy is important because it reduces pollution, saves money, and ensures that energy resources last longer for future generations.

Answer:

Here is a sample comparison of different vehicles:

How to compare: We compare vehicles by looking at how many kilometres they travel on one litre of fuel. A vehicle that travels more kilometres per litre is more fuel-efficient (uses less fuel for the same distance).

Conclusion: Smaller vehicles like scooters and bikes are more fuel-efficient than large vehicles like buses and trucks. Using fuel-efficient vehicles and public transport helps save fuel and reduces pollution.

Answer:

Three examples to write in the answer:

1. Ceiling Fan → Energy Source: Electricity
2. Cooking Stove → Energy Source: LPG / Cooking Gas (Fuel)
3. Torch → Energy Source: Battery (Electricity)

Conclusion: Different objects around us use different sources of energy to do their work.

Answer:

Plan of a Clean Energy Home:

A clean energy home uses natural and pollution-free sources of energy. Here is a description of such a home:

1. Solar Panels on the rooftop → These capture sunlight and convert it into electricity to run lights, fans, and appliances.
2. Solar Water Heater on the roof → Uses sunlight to heat water for bathing and cooking.
3. Small Wind Turbine in the garden → Uses wind energy to generate additional electricity.
4. Solar Cooker in the backyard → Uses focused sunlight to cook food without any fuel.
5. Large Windows facing the Sun → Allow natural light and warmth inside the house, reducing the need for electric lights and heaters.
6. Rainwater Harvesting Tank → Collects rainwater for use in the garden and toilets.

Conclusion: A clean energy home uses the Sun, wind, and water to meet its energy needs without burning coal or other polluting fuels. It is good for health, saves money, and protects the environment.

(Students should draw a simple house diagram labelling the above features.)

Answer:

My Energy Diary — Sample for One Day

Total count for the day:

• Electricity used: 5 times
• Fuel (LPG) used: 2 times
• Diesel (vehicle): 1 time
• Food energy: 2 times (lunch and dinner)

Conclusion: We use energy in many forms throughout the day. Being aware of how much energy we use helps us find ways to save energy and use it more wisely.

(Students should fill in their own actual activities for the day.)