Class 7 physics Chapter 3 Energy
SHORT TYPE QUESTIONS
1. Define the term energy.
Answer: Energy is the ability or capacity to do work.
2. State the unit of energy and define it.
Answer: The unit of energy is the joule (J). One joule is defined as the energy exerted when a force of one newton is applied over a distance of one metre.
3. Name five different forms of energy.
Answer:
1. Kinetic Energy (the energy of motion)
2. Potential Energy (stored energy)
3. Thermal Energy (the energy of heat)
4. Electrical Energy (the energy of moving charges)
5. Chemical Energy (the energy stored in chemical bonds)
4. What are the two kinds of mechanical energy?
Answer:
1. Kinetic Energy (the energy of motion)
2. Potential Energy (stored energy, such as gravitational or elastic energy)
- due to state - compressed spring.
7. Two bodies A and B are at the same height above the ground. Which of the two has greater potential energy?
Answer: They have the same potential energy, since potential energy depends on height and both are at the same height.
8. A bucket full of water is on the first floor of your house and another identical bucket with the same quantity of water is kept on the second floor. Which of the two has greater potential energy?
Answer: The bucket on the second floor has greater potential energy, since it is at a higher height.
9. Define the term kinetic energy. Give one example of a body which possesses kinetic energy.
Answer: Kinetic energy is the energy of motion. Example: A rolling ball possesses kinetic energy.
10. State two factors on which the kinetic energy of a moving body depends.
Answer: The kinetic energy of a moving body depends on:
1. Mass of the body
2. Velocity (speed) of the body
11. Two toy-cars A and B of masses 500 g and 200 g respectively are moving with the same speed. Which of the two has greater kinetic energy?
Answer: Toy-car. A has greater kinetic energy, since it has a larger mass.
12. A cyclist doubles his speed. How will his kinetic energy change?
Answer: His kinetic energy will increase four times, since kinetic energy depends on the square of the speed.
13. Name the form of energy which a wound up watch spring possesses.
Answer: Potential energy (elastic potential energy)
14. Name the type of energy (kinetic or potential) possessed by the following:
(i) A moving cricket ball - Kinetic energy
(ii) A stone at rest on the top of a building - Potential
energy
(iii) A compressed spring - Potential energy
(iv) A moving bus - Kinetic energy
(v) A bullet fired from a gun - Kinetic energy
(vi) Water flowing in a river - Kinetic energy
(vii) A stretched rubber band - Potential energy
15. State the energy changes that occur in the following:
(i) The unwinding of a watch spring.
Answer: Potential energy (elastic) → Kinetic energy
(ii) Burning coal while operating a steam engine.
Answer: Chemical energy (coal) → Thermal energy (heat) → Kinetic energy (steam engine)
(iii) Lighting of a torch bulb.
Answer: Electrical energy → Light energy + Heat energy
(iv) An electric generator (or dynamo).
Answer: Kinetic energy (rotating coils) → Electrical energy
16. A ball is made to fall freely from a height. State the kind/kinds of energy possessed by the ball when it is:
(a) at the highest point
Answer: Potential energy
(b) just in the middle
Answer: Both potential energy and kinetic energy
(c) at the ground
Answer: Kinetic energy (just before impact) and then heat energy (after impact)
LONG ANSWER TYPE
1. State two factors on which the potential energy of a body at a certain height above the ground depends.
Answer:- Height of the body above the earth surface .
- Mass of the body.
2. Can a body possess energy even when it is not in motion? Explain your answer with an example.
Answer: Yes, a body can possess energy even when it is not in motion. Example: A stone at the top of a building possesses potential energy, even though it is not moving.
3. Give one example to show the conversion of potential energy to kinetic energy when put in use.
Answer: A roller coaster at the top of a hill has potential energy, which converts to kinetic energy as it rolls down
4. Energy can exist in several forms and may change from one form to another. Give two examples to show the conversion of energy from one form to another.
Answer:
- Water stored behind a dam (potential energy) → Kinetic energy (water flowing through turbines) → Electrical energy (generated power)
- Chemical energy (gasoline) → Kinetic energy (moving car) → Thermal energy (heat generated by friction)
5.Give one relevant example for each of the following transformations of energy:
(i) Electrical energy to heat energy.
Answer: Electric heater (electrical energy → Heat energy)
(ii) Electrical energy to mechanical energy.
Answer: Electric motor (electrical energy → Kinetic energy)
(iii) Electrical energy to light energy.
Answer: Light bulb (electrical energy → Light energy)
(iv) Chemical energy to heat energy.
Answer: Burning of natural gas in a stove (chemical energy → Heat energy).
(v) Chemical energy to light energy.
Answer: Fireworks (chemical energy → Light energy)
6. What do you mean by conservation of mechanical energy? State the condition when it holds.
Answer: Conservation of mechanical energy means that the sum of potential energy and kinetic energy remains constant. This holds when friction is ignored or negligible.
7. Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.
Answer:
A ball falling vertically downwards from a height is an example of conservation of mechanical energy. In Figure, when the ball is stationary at point A at a height h above the ground, potential energy (P.E.) is maximum = E (say) which is equal to the work done in lifting the ball from the ground to the point A, while the kinetic energy (K.E.) is zero.
As the ball starts falling down, its potential energy changes to kinetic energy, At the point B (at height h/2 above the ground), half of the potential energy converts into the kinetic energy. So at B, potential energy is E/2 and kinetic energy is also E/2.
On reaching the point C on ground, whole of the potential energy has converted into kinetic energy. So at C, potential energy is zero and kinetic energy is E.
Thus, we notice that during the vertical fall of a ball, if friction due to air is neglected, the total sum of potential energy and kinetic energy at each point of its path remains same.
8. State the changes in form of energy while producing hydro electricity.
Answer: Potential energy (water in dam) → Kinetic energy (water flowing through turbines) → Electrical energy (generated power)
9.A truck starts from rest on a plane road. What are the possible energy changes taking place while the truck is in motion?
Answer: Chemical energy (fuel) → Kinetic energy (moving truck) → Heat energy (friction and engine heat)
