Types of energy

Types of energy

Q1. A. Choose the correct alternatives and rewrite the correct sentences.

  1. Energy ______________
    1. is the stored ability to do work.
    2. can be created or can be destroyed.
    3. cannot be converted from one form into another.
    4. exists only in mechanical form
  2. The energy stored in water in the reservoir at a height above the ground is __________ energy.
    1. potential energy
    2. kinetic energy
    3. mechanical energy
    4. electrical energy

  1. ____________ possesses kinetic energy.
    1. Running water
    2. water in the lake at sea level
    3. water in the dam
    4. stone at the foot of a hill

  1. The potential energy of a body at rest on the earth’s surface is equal to _________
    1. zero J
    2. 100 J
    3. 50 J
    4. 980 J

  1. The potential energy of your body is minimum when _______
    1. you are sitting in a chair
    2. you are sitting on the ground
    3. you lie down on the floor
    4. you stand on the floor.

  1. When you compress coil, potential energy __________________
    1. increases
    2. decreases
    3. remains unchanged
    4. becomes zero

  1. In SI system, the unit of work is _________
    1. potential
    2. kinetic
    3. mechanic
    4. heat.

  1. The SI unit of energy is _____________
    1. erg
    2. Newton
    3. joule
    4. kW – hr

  1. For an object released from some height _________
    1. PE is maximum when it reaches the ground.
    2. The sum of change in the kinetic energy and change in potential energy, is zero.
    3. The kinetic energy goes on decreasing.
    4. The mechanical energy goes on increasing till the object reaches the ground.

  1. Example of potential energy is ___________
    1. running water
    2. water in the lake at sea level
    3. water in the dam
    4. Stone at the foot of a hill.

Q1. B. Fill in the blanks.

  1. Energy is measured in terms of capacity to do work.
  2. Bullet firmed from the gun possesses kinetic energy.
  3. Kinetic energy and the potential energy together make up the total mechanical energy of an object.
  4. A ball thrown upwards will continue to go up till it has upward velocity.
  5. Friction is a resistive force.
  6. Electrical energy is converted into mechanical energy in electrical fans.

Q1. C. State whether the following statements are true or false.

  1. Total energy of an isolated system is constant. True.
  2. The potential energy of a body at rest on the earth’s surface is zero. True.
  3. S.I. unit of energy is erg.
False: - S.I. unit of energy is joule.
  1. In a telephone at the receiving end electrical signals are converted back into sound. True.
  2. Potential energy is always mechanical in nature.
False: - Potential energy may be mechanical or non – mechanical in nature.

Q2. Define the following

  1. Energy: - Energy is defined as the stored ability of a body to do work.
  2. Kinetic energy: - The energy possessed by a body due to its motion is called kinetic energy.
  3. Potential energy: - The energy possessed by a body due to its position or configuration is called potential energy.
  4. Mechanical energy: - The energy used to perform mechanical work is called mechanical energy.


Q3. Give scientific reasons.

1. The work done on an object by a conservative force is zero if it has come back to the same point from where it started.
Answer.
  1. Works is associated with displacement since W = F x S
  2. If an object comes back to the same point from where it started, then its displacement is zero. i.e. S = 0. Thus W = F x S = F x 0 = 0.
  3. Hence, if an object comes back to its original starting position, then the work done on the object is zero.

2. Potential energy of our body will be minimum when we lie down on the ground.
Answer.
  1. The gravitational potential energy is conventionally taken to be zero at the ground level.
  2. Thus if we lie down on the ground, the potential energy of our body will be minimum.

3. Water stored in dams is used to generate electricity.
Answer.
  1. Water stored in the reservoir of dam possesses gravitational potential energy since the reservoir is at a height above ground level.
  2. As the water falls, it rotates the blades of turbines and work is done due to this potential energy.
  3. These turbines are connected to generators that generate electricity.
  4. Hence, water stored in dams is used to generate electricity.

4. Electric fan blades rotate when we put the switch ‘ON’.
Answer.
  1. In the working of an electrical fan, electrical energy is converted into mechanical energy.
  2. When the switch is put ‘ON’, electricity reaches to the fan. This electrical energy is used to rotate the blades of the electric fan and the fan is set into motion.
  3. Hence, the electric fan blades rotate when we put the switch ‘ON’.

5. We use a pulley to draw water from the well.
Answer.
  1. A pulley is a device that simplifies the task of lifting heavy loads, pulley are used in cranes, building lifts, wells etc.
  2. When we use a pulley for removing water from a well, we pull one end of the rope in the downward direction, i.e. in the direction of gravity.
  3. This act of pulling the rope in the downward direction easily raises the bucket which is full of water.
  4. Hence, a pulley is used to draw water from a well.

6. While playing a child on a slide in a park he reaches the ground with maximum speed.
Answer.
  1. When a child climbs to the top of a slide, he gains potential energy due to height.
  2. As he starts sliding down, the height of the child from the ground reduces. Hence his PE decreases but KE increases.
  3. This is because the total energy of a system never changes i.e. PE + KE = constant.
  4. Thus as the KE of the child increases, the speed with which he slides too increases.
  5. Hence, the child reaches the ground with maximum speed.

7. A stone falling from a height makes a hole in the ground where it falls.
Answer.
  1. A stone at a height possesses potential energy.
  2. As it falls towards the ground, its potential energy gets converted into kinetic energy.
  3. As it reaches the ground, it uses its kinetic energy to make a hole at the place where it falls.

8. A ball falling from a height bounce on the ground.
Answer.
  1. A ball at a height possesses potential energy.
  2. As it falls towards the ground its potential energy gets converted into kinetic energy.
  3. Further as the ball is elastic it uses its kinetic energy to bounce back in air.

9. When stone is thrown up, it comes to rest momentarily after reaching certain height.
Answer.
  1. When a stone is thrown upwards it is acted upon acceleration due to gravity in the downward direction.
  2. The velocity of the stone decreases and finally become zero at the highest point.
  3. Thus, stone comes to rest momentarily after reaching a certain height.

10. For keeping some clocks working it is necessary to wind their springs from time to time.
Answer.
  1. When we wind the spring of a clock we do work on the spring and it gets compressed.
  2. This work is stored in the spring in the form of ‘elastic potential energy’.
  3. As the spring unwinds, its potential energy is converted into kinetic energy which is utilised for turning the hands of the clock.
  4. Hence, in order to keep some clock working, it is necessary to wind their springs from time to time.

11. Kinetic energy is never negative. OR
            Kinetic energy is always positive.
Answer..
1. The kinetic energy of a body is the energy possessed by the body on account of its motion. It is given by the formula
K.E. = 1/2 mv2
Where: - m = mass of the body
   v = velocity of the body
2. The mass of the body is always positive. However, the velocity can be negative but its square is always going to be positive.
Hence, the kinetic energy is never negative. 

12. The unit of energy is the same as the unit of work.
Answer.
  1. The energy of body or of a system of bodies is equal to the work they can perform.
  2. Thus energy is measured in terms of work done by the body.
  3. Hence, energy and work have the same units.



Q4. Distinguish between.

1. Kinetic energy and Potential energy.
Kinetic Energy
Potential Energy
  1. It is the energy possessed by a body due to its motion.
  2. It occurs only in one form and does not have to be converted into any other form in order to perform work.
  3. Kinetic energy is given by formula, KE=1/2mv2.
  4. E.g. Bullet fired from a gun, Flowing water etc.
  1. It is the energy possessed by a body due to its position or configuration.
  2. It occurs in various forms but to perform work, it has to be converted into kinetic energy.
  3. Potential energy is given by formula, PE=mgh.
  4. E.g. compressed spring, Water stored in a reservoir.

Q5. Answer the following questions.

1. What is gravitational potential energy? What are the other forms of potential energy.
Answer.
  1. The energy possessed by a body due to its height above the ground level is called ‘Gravitational Potential Energy’. It is denoted by the symbol ‘U’.
  2. The other forms of potential energy are:
    1. Elastic potential energy (Mechanical form)
    2. Electric potential energy (Non – mechanical form)

2. What is energy? State the S.I. and CGS units of energy.
Answer.
  1. Energy is defined as the stored ability of a body to do work.
  2. The S.I. and C.G.S. units of energy are Joule and erg respectively.

3. What is mechanical energy? At what condition it remains constant?
Answer.
  1. The energy used to perform mechanical work is called mechanical energy.
  2. Mechanical energy remains constant if
    1. There are no external forces working on the body.
    2. The internal forces of the body are conservative.
    3. There are no sudden changes in the motion of the body..

4. Give two examples of conversion of chemical energy into mechanical energy.
Answer.
  1. The two examples of conversion of chemical energy into mechanical energy are.
    1. When a bomb explodes, its chemical energy is converted into mechanical energy (kinetic energy) of the constituents.
    2. In a steam engine, the chemical energy of fossil fuels (coal) is converted into mechanical energy (kinetic energy).

5. State four examples of conversion of electrical energy into other forms.
Answer.
  1. The four examples of conversion of electrical energy into other forms are:
    1. Electrical energy to heat energy.
                                                              i.      Electric heater.
    1. Electrical energy to light energy.
                                                              i.      Electric bulb.
    1. Electrical energy to sound energy.
                                                              i.      Telephone receiver.
    1. Electrical energy to chemical energy.
                                                              i.      Charging battery.

6. State two applications of conversion of electrical energy into mechanical energy.
Answer.
  1. The two applications of conversion of electrical energy into mechanical energy are.
    1. Working of electric fan.
    2. Raising of water to the overhead tank of a building.

7. Work done is related to the amount of energy transformed from one form to another.
Explain the statement with suitable examples.
Answer.
  1. Energy is the cause of work. When we do the work of lifting or throwing an object, ther energy stored within our body is converted into the “kinetic energy” of the object.
  2. For example, when we use a pulley to draw water from a well. We do the work of pulling the rope. The energy stored in our body is used to raise the bucket full of water through a height.
  3. Hence our energy is transformed into the potential energy of the bucket.

8. State the various uses of solar energy.
Answer.
  1. The various uses of solar energy are
    1. Heating of land and water.
    2. During photosynthesis in plants.

9. Television is common example that uses conversion of energy from one form to another. Explain.
Answer.
  1. In a television, the audio (sound energy) and pictures (light energy) are first converted into electrical signals. These signals are transmitted by the broadcasting stations.
  2. Our television sets receive these electrical signals and once again re-converts them into audio(sound energy) and pictures(light energy).
Thus television is common example that uses conversion of energy from one form to another.

10. Classify the following examples into potential energy and kinetic energy.

  1. A book kept on a table.
  1. A stretched catapult.
  1. A running train
  1. Water stored in a dam
  1. Wind
  1. The explosive mixture in a bomb.
  1. A bullet fired from a gun
  1. A stretched bow
  1. An arrow released from a stretched bow
  1. A compressed spring
  1. Waterfall
  1. Rotating hands of a clock
  1. A raised hammer
  1. Running water
  1. Sea waves
  1. A rotating top
  1. A wound spring in a watch. 
  1. A running boy




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11. When the potential energy of a body is lost, an exactly equal amount of kinetic energy is gained by the body – Explain the statements with an example.
Answer.
  1. The total energy of an isolated system at any instance is the sum of its kinetic energy and potential energy and it is always constant. i.e. K.E + P.E. = E (Where ‘E’ is the constant) 
  2. When a body of mass ‘m’ is at a height ‘h’, then its P.E. at that height is ‘mgh’. it has no KE
  3. When the body falls towards the earth under the force of gravity and reaches the ground, then its KE at the ground level is . It has no P.E. 
  4. From eq(2) and eq(3)

Hence, when potential energy is lost, and exactly equal amount of kinetic energy is obtained.



Q6. Answer briefly.

1. Many appliances is daily use are based on inter – changeability of different forms of energy explain.
Answer.
  1. According to the law of conservation of energy, energy can neither be created nor destroyed. But it can be transformed from one form to another.
  2. Many appliances/devices work on this principle. Some examples of inter-changeability of energy from one form to another are as follows.
No.
Example
From (Energy)
To (Energy)
1.
2.
3.
4.
5.
6.
7.
Charging of battery
Using a battery
Broadcasting stations
Radio
Electric fan
Electric iron
Hydro – electric power station.
Electrical
Chemical
Sound
Electrical
Electrical
Electrical
Mechanical
Chemical
Electrical
Electrical
Sound
Mechanical
Heat
Electrical

2. State and explain law of conservation of energy using an example.
Answer.
  1. The law of conservation of energy states that “Energy can neither be created nor destroyed. It may be converted from one form to another. The total amount of energy in the universe always remains constant.”
  2. According to this law, the sum of various energies in the universe remains constant. Hence, if energy is lost in one form, then an exactly equal amount of energy will be obtained in another form.
  3. Foe instance, KE and PE together make up the total mechanical energy of an object. The sum of KE and PE does not change with time. i.e. the total energy of an isolated system is constant. Hence any decrease in the P.E. of an object will result in an equal increase in the KE of the object and vice versa.
  4. For example if a stone is kept at a height, all its energy is PE while it has no KE. When this stone falls towards the ground under gravity, then at the instance when it touches the ground, all its energy is KE. The magnitude of this KE is equal to the magnitude of the PE of the stone at the height.

3. Give two examples of inter changeability of different forms of energy.
Answer. Energy in one form can be converted into another form. During such transformation, whenever energy in one form disappears, and exactly equal amount of energy reappears in another form. The examples of energy transformation are given below.
1.      While charging a battery, electrical energy is converted into chemical energy, which is stored in the battery. When the battery is connected into an electric circuit the chemical energy is converted into electrical energy.
2.      The explosive mixture in a bomb possesses chemical energy. When the bomb explodes, this chemical energy is converted into various forms such as kinetic energy, heat, light, sound, etc.
3.      In thermal poser station, heat energy is converted into electrical energy.
In hydro electric power station, potential energy is converted into electrical energy.
In nuclear power station, nuclear energy is converted into electrical energy.
The electrical energy can be converted into different forms such as
a.       Kinetic energy for e.g. electric trains, electric fans etc.
b.      Potential energy for e.g. water stored by electric pump at a height.
c.       Heat energy for e.g. water heater, electric iron, etc.
d.      Light energy for e.g. electric bulbs, fluorescent tubes etc.
e.       Sound energy for e.g. electric bell, radio, etc.
f.       Chemical energy for e.g. electrolysis, electroplating.

4. Explain how the total energy of the stone is conserved when it is dropped from a height ‘h’ from the ground. (OR) Explain how potential energy is converted into Kinetic Energy?      (OR)    Prove that potential energy of a body is lost are exactly equal to kinetic energy of a body obtained.
Answer. A stone at rest on top of a hill possesses potential energy. If it rolls down the hill, its potential energy goes on decreasing while its kinetic energy goes on increasing. When the stone reaches the bottom of the hill, all its energy is kinetic. The potential energy of the stone when it is on the top of the hill gets converted into kinetic energy when it reaches the bottom.

For Eg.
  1. When a body (say stone) of mass (m) (say some kg.)  is raised to a height (h) from the ground. The work is done against the gravitational force. This work is stored in the body in the form of potential energy. Hence, the potential energy of the body at the point A is mgh.
  2. If the body is released from the point A its initial potential energy = mgh. Since, the body is initially at rest, its kinetic energy is zero.
But, we know that,
Total energy of the body = Potential energy + Kinetic energy
  1. When the body is released it falls down due to gravitational force. Its velocity goes on increasing. Let V be the velocity of the body, Now

5. The energy gets converted from usable form to unusable form – Explain the statement with example.
Answer.
  1. When we use fossil fuels like coal or oil or natural gas, chemical energy is converted into heat energy.
  2. But only a small portion of this heat energy is used by us drive to our vehicles or to generate electricity at a thermal power station (usable form).
  3. However a major portion of the heat energy is gained by the earth and the atmosphere and it is stored as the internal energy of the earth.
  4. The earth later radiates this energy into space (unusable form).
  5. Hence the energy gets converted from usable form to unusable form.

6. “Energy can neither be created nor destroyed.” But still we say that fossil fuels will soon be exhausted. Why?
Answer.
  1. When fossil fuels are burnt, their chemical energy is converted into heat energy. This heat energy is utilised for driving our vehicles generating electricity etc.
  2. This heat energy causes a rise in the temperature of the earth and is stored as the internal energy of the earth. later, the earth radiates this energy into space and it is lost forever by us.
  3. Thus when fossil fuels are burnt, the energy does not get destroyed. Only it is converted from usable from to unusable form.

7. Explain how electricity is generate and in a hydroelectric power station.
Answer.
  1. In a hydro – electric power station, water is stored in a dam in large reservoirs. These reservoirs are at a considerable height above sea level.
  2. As the water from the dam is allowed to flow under gravity, it rotates the blades of large turbines.
  3. These turbines are connected to electric generators that generate electricity.
  4. Hence, in a hydro – electric power station the potential energy of the water stored in a dam is used to generate electricity.

Q7. Solve the following numerical.

Ex. 7.1

  1. Energy of 2J is used to lift a block of 0.5 kg. How high will it rise?
  2. A rocket of mass 100 tonnes is propelled with a vertical velocity 1km/s. Calculate its kinetic energy.
  3. An object of mass 10kg is lying 10m above the ground. Calculate the potential energy possessed by the object.
  4. Calculate the power of a crane which lifts a load of mass 1000 kg to a height of 30m in one minute.
  5. An electric pump is used to lift water to a height of 60m. If the power of the pump is 3.92kw. Find the mass of water raised in 2 minutes.
A body of 4 kg mass falls freely from the top of the tower. If it reaches the ground after 5 sec. find the K.E.