Selina Concise Class 7 Physics Solution Chapter No. 3- ‘Energy’ For ICSE Board Students.
A)
1.) a) A man going up has potential energy and kinetic energy both.
Answer: True, The man has potential energy because of his height and kinetic energy because of his velocity.
b) A gum bottle lying on a table has no energy.
Answer: False, The gum bottle possesses potential energy.
c) In an electric fan, electrical energy changes into mechanical energy.
Answer: True, electric fan required electric energy.
d) Potential energy changes into kinetic energy when it is put to use.
Answer: True, potential energy possesses in the object because of it’s specific position. Thus potential energy changes into kinetic energy when it is put to use.
e) One form of energy cannot be converted into another form.
Answer: False. According to law of conservation of energy, energy always converted into one form to another.
f) There is always some loss of energy in conversion from one form of energy to another form, so the total energy is not conserved.
Answer: False. Energy always conserved.
g) The energy of flowing water can be converted into electric energy (electricity).
Answer: True, energy of flowing water can be converted into electric energy by using dynamo.
2.)
Answer:
a) An electric fan converts electrical energy into mechanical energy.
b) Cooking gas converts chemical energy into heat energy.
c) Energy possessed by a compressed spring is potential energy.
d) The ability to do work is called energy.
e) The energy possessed by a body due to its position is called potential energy
f) The energy possessed by a body due to its motion is called kinetic energy.
g) Green plants convert light energy into chemical energy.
h) The S.l. unit of energy is Joule.
i) An object falling freely from the roof of a multistorey building has kinetic and potential energy when halfway down the building.
3.) Match the following columns :
Answer:
Column A Column B
(a) Running water (iv) kinetic energy
(b) Burning (i) heat energy
(c) Energy (v) joule
(d) Sound energy(ii) vibrations
(e) Nuclear energy (iii) atom bomb
4.) a) When we rub our hands
(i) kinetic energy changes into potential energy.
(ii) mechanical energy changes into heat energy.
(iii) potential energy changes into kinetic energy.
(iv) heat energy changes into mechanical energy.
Answer: (ii) Mechanical energy changes into heat energy.
When we rub hand on each other then heat will be produced.
b) A ball rolling on the ground possesses
(i) kinetic energy
(ii) potential energy
(iii) no energy
(iv) heat energy.
Answer:
(i) kinetic energy
The ball possesses kinetic energy due to it’s motion.
c) The energy stored in an electric cell is
(i) chemical energy
(ii) electrical energy
(iii) heat energy
(iv) mechanical energy.
Answer:
(i) chemical energy
Ionization takes places in electric cell.
d) When a bulb lights up on passing current, the change of energy is
(i) from electrical energy to heat energy
(ii) from electrical energy to light energy
(iii) from electrical energy to heat and light energy
(iv) from electrical energy to mechanical energy.
Answer:
(iii) from electrical energy to heat and light energy
When a bulb light up then the bulb heats up and produces light.
e) The correct statement is
(i) Both work and energy have the same units
(ii) Potential energy of a body is due to its motion
(iii) Kinetic energy of a body is due to its position or state
(iv) Kinetic energy can change into potential energy. but potential energy cannot change into kinetic energy
Answer:
(i) Both work and energy have the same units
Unit of work and energy is Joule.
f) According to law of conservation of energy. energy changes from one form to another form, but the total energy of that system
(i) increases
(ii) decreases
(ii) alternates
(iv) remains the same
Answer:
(iv) remains the same
According to law of conservation of energy, total energy of the system always remains constant.
B.) Short/Long Answer Questions:
1) Define the term energy.
Answer: Energy is necessary for doing work. It is also easy to calculate energy in the form of work done. Thus energy is defined as capacity to do work
2) State the unit of energy and define it.
Answer:
SI unit of energy is Joule. It is also measured in kg m²/s², calories, kilo calories etc.
One joule of energy is defined as when an object displaced one meter due to 1 Newton force applied on it.
3) Name five different forms of energy.
Answer:
Different forms of energy are
- Heat energy
- Light energy
- Sound energy
- Chemical energy
- Atomic energy
- Electric energy.
- Mechanical energy.
4) What are the two kinds of mechanical energy
Answer:
Mechanical energy is divided into two types and these types are
- Kinetic energy.
- Potential energy.
5) What is potential energy ? State its unit.
Answer:
Potential energy:
Potential energy is related to position of the object and it’s certain shape.
The energy possesses in object due to its position and certain shape is called potential energy.
SI unit of potential energy is Joule or kg m²/s².
6) Give one example of a body that has energy, in each of the following :
(i) due to position,
(ii) due to its state.
Answer:
Example of each of the following terms-
(i) due to position- A ball is put on certain height.
(ii) due to its state- a stretched rubber band.
7) State two factors on which the potential energy of a body at a certain height above the depends.
Answer:
Potential energy can be calculated by,
Potential energy = mgh
Where,
m – mass of the object,
g – acceleration due to gravity = 9.8 m/s².
h – height of the object .
Therefore potential energy will depend on mass of the object and height where it put.
8) Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above ground. Which of the two has greater potential energy ?
Answer:
We know that, potential energy depends on mass of the object also. If the object has greater mass then it possesses greater potential energy.
The object B has greater mass so it possesses greater potential energy.
9) A bucket full of water is on the first floor your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy ?
Answer:
We know that, potential energy depends on height of the object also. If the object put on greater height then it possesses greater potential energy.
The bucket kept on second floor possesses greater potential energy.
10) Define the term kinetic energy. Give one example of a body which possesses kinetic energy.
Answer: Kinetic energy of the object depends on velocity of the object and mass of that object.
Definition- The energy possesses in the object due to it’s motion is called kinetic energy.
example- flowing wind, a ball in motion etc.
11) State two factors on which the kinetic energy of a moving body depends.
Answer:
As we know the formula to calculate Kinetic energy,
Kinetic energy = ½ mv²,
Where,
m – mass of the object,
v – velocity of the object.
Thus, kinetic energy depends on mass and velocity of the object.
12) 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:
As we know that, Kinetic energy depends on mass of the object also. If the object has greater mass then it possesses greater kinetic energy. Toy A has greater mass than toy B so toy A has greater kinetic energy than toy B.
13) A cyclist doubles his speed. How will his kinetic energy change : increases, decreases remains the same ?
Answer: As we know that, Kinetic energy depends on velocity of the object also. If the object has greater velocity then it possesses greater kinetic energy.
If a cyclist doubles his speed then it’d kinetic energy Also increases.
14.) Name the form of energy which a wound watch spring possesses.
Answer:
Wound watch spring possesses potential energy because of it’s shape.
15.) Can a body possess energy even when it is in motion ? Explain your answer with an example.
Answer:
Yes. As we know that, when the object is in motion then it possesses kinetic energy.
Example- When we throws a ball then the ball possesses kinetic energy. This ball has ability to do work so we can say that the ball contained energy .
16) Name the type of energy (kinetic or potential ) possessed by the following :
(i) A moving cricket ball.
(ii) A stone at rest on the top of a building.
(iii) A compressed spring.
(iv) A moving bus.
(v) A bullet fired from a gun.
(vi) Water flowing in a river.
(vii)A stretched rubber band.
Answer:
(i) A moving cricket ball- kinetic energy (energy due to it’s motion)
(ii) A stone at rest on the top of a building- potential energy. (energy due to it’s position)
(iii) A compressed spring- potential energy. (energy due to it’s shape)
(iv) A moving bus – kinetic energy (energy due to it’s motion)
(v) A bullet fired from a gun – kinetic energy (energy due to it’s motion)
(vi) Water flowing in a river- kinetic energy (energy due to it’s motion)
(vii)A stretched rubber band- potential energy. (energy due to it’s shape).
17) Give one example to show the conversion of potential energy to kinetic energy when put in use.
Answer:
Potential energy must convert into kinetic energy for practical uses. We understand this with an example.
Water flow from second floor to first floor through pipes. There is no need of external energy to do this work. Simply potential energy stored in water at second floor do this work.
18) State the energy changes that occur in the following :
(i) The unwinding of a watch spring.
(ii) Burning coal while operating a steam engine.
(iii) Lighting of a torch bulb.
(iv) An electric generator (or dynamo).
Answer:
According to law of conservation of energy, energy can be converted into one form to another.
(i) The unwinding of a watch spring- potential energy stored in unwinding of a watch is converted into kinetic energy.
(ii) Burning coal while operating a steam engine–Chemical energy stored in coal is converted into heat energy while operating a steam.
(iii) Lighting of a torch bulb- Chemical energy is converted into light energy and heat energy
(iv) An electric generator (or dynamo)- Chemical energy in a fuel is converted into mechanical energy and mechanical energy again converted into electric energy.
19) 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:
According to law of conservation of energy, energy can be converted into one form to another.
Example of conversion of energy from one form to another.
- Electric energy into light energy- electric bulb.
- Light energy into electric energy- Solar cell.
- Electric energy into mechanical energy- electrical fan.
20) Give one relevant example for each of the following transformations of energy :
(i) Electrical energy to heat energy.
(ii) Electrical energy to mechanical energy. (iii) Electrical energy to light energy.
(iv) Chemical energy to heat energy.
(v) Chemical energy to light energy.
Answer:
(i) Electrical energy to heat energy- electric heater, electric induction.
(ii) Electrical energy to mechanical energy- electric fan, DC motor.
(iii) Electrical energy to light energy- electric bulb, LED.
(iv) Chemical energy to heat energy- exothermic chemical reaction.
(v) Chemical energy to light energy- bulb connected to cell.
21) What do you mean by conservation of mechanical energy ? State the condition when it holds.
Answer:
Sum of kinetic energy and potential energy is equal to mechanical energy of the system. If the sun of kinetic energy and potential energy remains constant then we say that the mechanical energy is conserved.
Condition for conservation of mechanical energy.
The system does not share energy with it’s surrounding, means it does not produces heat because of friction.
22) Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.
Answer:
According to law of conservation of energy, total energy of the system will remains conserved. We illustrate this with a simple example.
When we drop a stone from certain height. At every point potential energy converts into kinetic energy if we ignore friction.
23) 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
(b) just in the middle
(c) at the ground.
Answer:
When a ball fall freely from a height then energy related to each point are as
(a) at the highest point – maximum potential energy and zero kinetic energy.
(b) just in the middle – half kinetic energy and half potential energy .
(c) at the ground- maximum kinetic energy and zero potential energy.
24) State the changes in form of energy while producing hydro electricity.
Answer:
Multiple times changes in form of energy while producing hydro electricity-
Potential energy stored in water > kinetic energy > mechanical energy (to rotate dynamo) > electrical energy.
25) A truck starts from rest on a plane road. What are the possible energy changes taking place while the truck is in motion ?
Answer: When a truck starts from rest on a plane road then changes in chemical energy stored in fuel are as
1.) Kinetic energy of truck
2.) Sound energy.
3.) Heat energy due to friction.