Energy and Power | Science Class 9 PDF Download

Energy

Car Engine Work

• When a car operates, its engine creates a force that moves the car. This means that the car is doing work, which is powered by fuel. Fuel provides the necessary energy for the car to function. Thus, without an energy source, no work can be accomplished.
• Example: Consider a lift transporting people from the ground floor to the second floor. The lift is performing work, which is driven by an electrical motor. Therefore, electrical energy is responsible for its operation. If there is no electricity, the lift cannot function. Again, without a source of energy, no work can be done.
• This principle applies universally, not just to the lift example. Consequently, energy is understood as the ability to perform work.
• Energy is defined as the ability to perform work.
• Energy is a scalar quantity.
• The unit of energy is the same as that of work, which is the joule (J). 1 J represents the energy required to perform 1 joule of work.
• Larger units include 1 kJ = 1000 J, and 1 MJ = 10⁶ J.
• The C.G.S. unit of energy is the erg.
• Energy exists in various forms, such as kinetic energy, potential energy, heat energy, and chemical energy.

Memorise: Energy is the capacity to perform work. The more energy available, the more work can be done, and vice versa.

Forms of Energy

Nature has been very kind to us in providing us energy in various forms. These forms of energy are as follows.

1. Solar energy. The energy radiated by the Sun is called solar energy. Plants collect and store this energy to make food through photosynthesis.

2. Heat energy. It is the energy released when coal, oil, gas or wood burn and it produces in us the sensation of warmth.

3. Light energy. It is the form of energy which produces in us the sensation of light. Sun is the natural source of light.

4. Chemical energy. It is the energy possessed by fossil fuels (coal; petroleum and natural gas) and is also called the fuel energy. The food that we eat has chemical energy stored in it.

5. Hydro energy. The energy possessed by water flowing in rivers and streams is called hydro energy. This energy is used to generate electricity in hydroelectric power plants.

6. Wind energy. The energy possessed by moving air is called wind energy.

7. Ocean thermal energy (OTE). Solar energy stored in the oceans in the form of heat is called ocean thermal energy.

8. Geothermal energy. It is the heat energy of the Earth and is found within rock formations and the fluids held within those formations.

9. Biomass energy. It is the energy obtained from biomass (i.e., living matter or its residues).

10. Tidal energy. It is the energy derived from the rising and falling ocean tides.

11. Sound energy. It is the energy possessed by vibrating objects and it produces in us the sensation of hearing.

12. Mechanical energy. It is the energy possessed by a body due to its position (or configuration) or motion. The energy possessed due to position or configuration is called potential energy and that due to motion is called kinetic energy. The sum of these two energies is called the mechanical energy.

13. Electric energy. The energy possessed by charges (either at rest or in motion) is called electric energy.

14. Magnetic energy. It is the energy possessed by magnetised bodies e.g. a magnet.

15. Electromagnetic energy. It is the general name for electric and magnetic energies.

16. Nuclear energy. The energy produced in the processes of fission and fusion is called nuclear energy.

 According to the law of conservation of energy, energy can only change from one form to another; it cannot be created or destroyed.

Are Various Energy Forms Interconvertible?

We have discussed various forms of energy available to us. We cam convert energy from one form to another. Given below are some examples.

1. Conversion of mechanical energy into electrical energy. The potential energy of water stored in a dam is changed to kinetic energy when it falls from a height. This kinetic energy rotates a turbine to produce electric energy.

2. Conversion of electrical energy into mechanical energy. An electric motor uses electrical energy to run various electrical appliances, e.g., a train, a fan, washing machine, mixer, grinder etc.

3. Conversion of electrical energy into heat energy. In an electric heater, a geyser, a toaster, an oven etc., electric energy is changed to heat energy.

4. Conversion of heat energy into mechanical energy. In heat engines (e.g., a steam engine), heat energy changes to mechanical energy.

5. Conversion of electrical energy into light energy. In an electric bulb, a fluorescent tube, a flood light etc., electrical energy is changed to light energy.

6. Conversion of electric energy into sound energy. An electric bell, a stereo, a loudspeaker etc., change electric energy into sound energy.

7. Conversion of chemical energy into heat energy. When fuels are burnt, chemical energy gets converted into heat energy.

8. Conversion of electrical energy into chemical energy. When a battery is charged, electrical energy changes into chemical energy. An inverter in our home does the same thing.

9. Conversion of sound energy to electrical energy. A microphone converts sound energy into electrical energy.

10. Conversion of chemical energy to electrical energy. An electric cell converts chemical energy into electrical energy.

11. Conversion of light energy into electric energy. A solar cell converts light energy into electrical energy.

12. Conversion of chemical energy into mechanical energy. In automobiles, chemical energy of petrol, diesel or CNG (compressed natural gas) is converted into mechanical energy.

13. Conversion of light energy into chemical energy. In photosynthesis, light energy .from the Sun is absorbed by green plants and is converted to chemical energy.

14. Conversion of nuclear energy into electrical energy. Nuclear power. plants are used to generate electrical energy from nuclear energy.

Some man-made devices that convert one form of energy into another are is given as follows. 

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Power (Rate of Doing Work)

• Power is the speed at which work is done or energy is transferred.
• Power is calculated as work divided by time: Power = work/time.
• Power is a scalar quantity.
• The unit of power is the watt (W), named after James Watt (1736-1819). One watt is equal to the power used when one joule of work is done in one second.

If you lift a block of mass 1 kg through a distance of 1 m in 2 seconds, what is the work done?

W = F x s = mg x h = 1 x 9.8 x 1 = 9.8 J

If you lift the same block through the same height in 1 minute, what is the work done? 

The answer comes out to be the same 9.8 J.

What is the work done if the time taken is 5 minutes? 

The work done is again 9.8 J.

But we are generally interested in time-oriented work, i.e., work should be completed in a particular amount of time. The physical quantity which takes care of 'how fast is the work done' is power.

Note: If we want to find rate of work done by a man then the word 'machine' can be replaced by 'man'.

Unit of Power

• The SI unit of power is watt (W), named after James Watt (1736-1819), symbolised as W.
• 1 kW = 1000 W
• 1 MW (Megawatt) = 10⁶ W
• Another unit of power is horse power (HP).
• 1 GW (Gigawatt) = 10⁹ W
• 1 HP = 746 W

Definition of Watt

• When t = 1 s, W = 1 J, then P = 1 W.
• If a machine does 1 joule of work in 1 second, its power is 1 watt.
• 1 watt represents the power of an agent working at the rate of 1 joule per second.

Power in terms of Energy

• Since work and energy can change into each other,
• Power. Energy/Time or P = E/t
• E = P x t
• Also, W = F x s
• When movement is in the direction of the force,
• P = W/t = F x s/t
• → P = F x v
• This shows power in terms of force and velocity.
• The power of an agent can change over time. Thus, average power is useful.
• Average power is calculated by dividing the total energy used by the total time taken.

Average Power

• Average power is the rate at which work is done or energy is transferred.
• It is calculated by dividing the total energy consumed by the total time taken.
• Average Power = (total energy consumed / total time taken)

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Commercial Unit of Energy

Kilowatt-hour (kWh)

• One kilowatt-hour is the amount of energy used by an agent over one hour at a constant rate of one kilowatt.
• 1 kilowatt = 1000 watts.

Is kWh a unit of power or energy?

The answer is energy.

We can write 1 kWh as 1 kW x 1h.

Now, since P = E/t

 ∴ E = P x t

If power is in kW (kilowatt) and time in hour, then the unit of energy is kWh.

The unit kWh is important because this is a commercial unit of energy used by electricity boards. If you enquire from your parents what was the last electricity bill? If the answer is 600 units, it means that you have used

600 kWh of energy during the duration of bill. Thus, you pay for the electrical energy that you use.

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Relation between kWh and Joule

Now, 1 W = 1 Js^-1 and 1 h = 60 x 60 s = 3600 s

∴ 1kWh = 1000 Js^-1 x 3600 s 

= 3600000 J 

= 3.6 x 10⁶ J

= 3.6 MJ

∴ 1 kWh = 3.6 MJ

Note: An energy of 1 kWh is equivalent to using a bulb of 1 kW for 1 hour.

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What can you conclude from the relationship, 

we can conclude as follows :

When the velocity of a body is kept constant, the kinetic energy is directly proportional to the mass of the body, K.E. ∝ m

Thus,

• If the mass of a body is doubled (v remaining constant), the kinetic energy of the body also gets doubled.
• If the mass of the body is reduced to half (v remaining constant), the kinetic energy of the body also gets halved.

The kinetic energy of a body is directly proprotional to the square of its velocity (or speed) i.e., K.E. ∝ v²

So, (∵ m is constant)

• If the velocity of a body is doubled, then its kinetic energy increases four times.
• If the velocity of a body is reduced to half, then its kinetic energy gets to one-fourth.

How is the kinetic energy of a body related to its momentum

Let us consider a body of mass m having a velocity v. Then

Momentum of the body p = Mass x velocity = m x v

This gives, v = P/m .........(1)

From definition,

Kinetic energy (K.E.) of the body   ... (2)

Substituting the value of v from Equation (1) into Equation (2) we can write,

Then, we can write, p² = 2m x K.E.

Thus, Momentum