Energy

Energy is the capacity or ability of a body to do work. It is a scalar quantity. Its SI unit is the same as that of work i.e. joule (J). In CGS system, energy is measured in calorie (4.2 joule= 1 calorie).

There are various forms of energy. Some of them are as follows:

• Mechanical energy
• Electrical energy
• Heat energy
• Light energy
• Sound energy
• Chemical energy
• Electrical energy
• Magnetic energy
• Nuclear energy

Mechanical energy

The energy possessed by a body by the virtue of its motion or its position or its configuration is called mechanical energy. It is two types: Kinetic energy and potential energy.

Kinetic energy

The energy possessed by a body by the virtue of its motion is called kinetic energy. Examples: A flying bird, a moving car, running water etc.

The Kinetic energy of a moving body is determined by the formula:

Kinetic energy (KE)=1/2mv²
Here, m = mass of the body
V =velocity of the body

Derivation of KE = $\frac{1}{2}$mv²

Let us consider a body of mass 'm' is at rest on a smooth surface whose initial velocity is 0. If a constant force 'F' is applied on it then it displaces the body by displacement 'd' in its own position. If 'v' be the final velocity then the workdone by the force is given by

W = Fd
Or, W = mas...........(i) (since,F=ma and d=s)
We also have,
a = $\frac{v - u}{t}$ and s = $\frac{ u + v}{2} \times t$

substuting these values on equation (i)
Or, W = m$\frac{ v - u}{t}$ $\times$ $\frac{u +v}{2} \times t$

Or, W = m $\frac{v^2 - u^2}{2}$
Or, W = $\frac{1}{2}$mv² (since, initial velocity u=0)

The workdone becomes the kinetic energy of the body. Thus, the kinetic energy of a body of mass 'm' moving with the velocity equal to 'v' is$\frac{1}{2}$mv^2.

Potential energy

The energy possessed by a body by the virtue of its position or its configuration is called potential energy.

The potential energy (PE) of a body at a height is determined by the formula:

P.E. = mgh
Here, m = mass
g = acceleration due to gravity
h = Height

Work done is stored in the body as potential energy which is called gravitational potential energy.

The potential energy possessed by a stretched rubber, a compressed spring etc. is called elastic potential energy.

Derivation of PE = mgh

Let us consider a body of mass 'm' . The weight 'mg' of the body acts vertically downwards, where 'g' is the acceleration due to gravity. Let 'h' be the height upto which body is lifted. In order to lift the body, force 'mg' should be applied in upward direction.
Now, the workdone is given by
W = F.d
i.e. W =mgh (Since F= mg and d =h)
This work done is stored in the body as potential energy which is called gravitational potential energy.
$\therefore$ Gravitational potential energy (P.E.) = mgh.

Electrical energy

Electrical energy is the energy possessed by the continuous flow electrons or by the change in a number of electrons in bodies. Electronic devices like computers, telephones, televisions, radios, calculators etc. use electrical energy.

Heat energy

A substance is made of tiny particles called molecules. The sum total of the kinetic energy of all constituent molecules of the body is heat energy. Sun is the main source of heat energy. Heater is an example of heat energy.

Light energy

Light is a form of energy which produces the sensation of sight. The main source of heat and light energy in the earth is the sun. Some examples of light energy are lightbulb, torch light, tube light, etc.

Sound energy

A sound is a form of energy which is produced by the vibration of sounding bodies. Sound requires material medium to for its propagation. Speakers, radio, television etc are some examples of sound energy

Magnetic energy

The energy possessed by the magnet or a current carrying conductor that effect on magnetic bodies is called magnetic energy. Magnets are used in many electronic devices such as radio, telegraph. Loudspeaker, etc.

Chemical energy

The energy stored in a body, which is released from it, when it undergoes chemical change is called chemical energy. Generally, food, fuel and cells store chemical energy.

Nuclear energy

The energy possessed from a nuclear fission or nuclear fusion is called nuclear energy. Nuclear fission is a process of splitting up of a heavy nucleus into light nuclei with the release of energy. Nuclear fusion is a process in which lighter nuclear fuse to form heavier nuclei.

Transformation of Energy

Conversion of energy from one form to another form is called transformation of energy.

When a candle burns, it changes chemical energy into light and heat energy. Similarly, when a lamp glows, it changes electrical energy into light and heat energy. These examples show that one type of energy can be changed into another type.

Examples:

• A steam engine changes heat energy into mechanical energy.
• Electric fan converts electric energy into mechanical energy.
• A microphone converts electrical energy into sound energy.
• Bulb converts electrical energy into light energy.
• Solar panels convert light energy into electrical energy.

Conservation of energy

Energy can neither be created nor destroyed but can only be changed from one form to other. This is called the principle of conservation of energy.