ICSE Class 8 Physics Chapter 09 More About Energy

More About Energy

More About Energy

You have already learnt in your earlier classes that a body capable of doing work is said to possess energy. Energy is required to do any type of work. Your body needs energy for movement and physiological functions like respiration, circulation of blood, digestion, etc. Your body gets this energy from the food you eat. Thus, it can be concluded that energy of a body is its capacity to do work. It is measured in terms of the amount of work that the body can do. Energy has the same unit as that of work. The S.I. unit of energy is joule (denoted by J). 1 J is the energy required to do 1 Joule of work. Sometimes a large unit of energy called kilo joule (kJ) is used.

\[1 \text{ kJ} = 1000 \text{ J}\]

Energy is used in various forms to do work, to provide warmth and to move people and goods from one place to another place. Electrical energy operates vacuum cleaners, washing machines and other appliances. Heat energy cooks food and heats your room. Chemical energy of fuel is used to move vehicles.

Different Forms Of Energy

Energy exists in various forms in nature. These forms are, mechanical energy, heat energy, light energy, chemical energy, sound energy, magnetic energy, electrical energy and atomic energy.

Mechanical Energy

The energy possessed by a body when the body is in a state of rest or motion is called the mechanical energy.

A body can move horizontally or vertically. It can also move in both the directions e.g.

1. A ball rolling on the ground (horizontal).

2. A ball thrown vertically upwards (vertical).

3. A javelin thrown in air (horizontal and vertical both).

The energy possessed by a body by virtue of its state of rest or the state of motion (mechanical state) is called its mechanical energy.

Mechanical energy is of two types :

(a) Kinetic Energy (KE)

(b) Potential Energy (PE)

Kinetic Energy

The energy possessed by a body by virtue of its motion is known as its kinetic energy. Energy possessed by strong blowing winds, flowing water, a bullet fired, a fast moving ball are some of the examples of energy due to motion.

Consider a body of mass 'm' moving with velocity v. Its kinetic energy will be given by the formula K.E. = \(\frac{1}{2} mv^2\).

It is clear from the formula, given above, that kinetic energy is directly proportional to the mass of the body and the square of the velocity with which it moves.

Example 1

A body of mass 'm' is moving with a velocity 'v'. What would be its change in kinetic energy if the velocity is doubled?

Solution

Mass of the body = m

Initial velocity = v

Initial kinetic energy, \(KE_i = \frac{1}{2} mv^2\)

And, final kinetic energy, \(KE_f = \frac{1}{2} m(2v)^2\)

\[= \frac{1}{2} m \left(4v^2\right)\]

\[\therefore \frac{KE_f}{KE_i} = \frac{\frac{1}{2} m \left(4v^2\right)}{\frac{1}{2}mv^2} = \frac{KE_f}{KE_i} = 4\]

or

\[KE_f = 4 \times KE_i\]

Therefore, final kinetic energy is four times the initial kinetic energy.

Teacher's Note

When you kick a moving soccer ball, you increase its kinetic energy, making it travel faster - this is why professional players can make the ball move significantly further with a stronger kick.

Potential Energy

The energy possessed by a body due to its position or change in shape is known as potential energy.

A stretched rubber band, a stretched bow, water in a dam, etc., are some of the examples where the body possess potential energy.

Consider a body of mass 'm' placed at a vertical height of h above the earth's surface. The potential energy of the body is given by the formula,

\[PE = m \times g \times h\]

The potential energy of an object at a height depends on the ground level or the zero level you choose. An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with respect to some other level.

Example 2

Calculate the potential energy gained by a body of mass 15 kg lifted to a height of 10 metres (Take g = 10 m/s²)

Solution

Given : m = 15 kg, h = 10 m, g = 10 m/s²

PE = ?.

\[PE = mgh\]

\[= 15kg \times 10m/s^2 \times 10m\]

\[= 1500 \text{ Joules}\]

Example 3

Calculate the height through which a body of mass 30 kg can be lifted when 500 J of work is done on it (Take g = 10 m/s²).

Solution

Given : m = 30 kg, g = 10 m/s² and potential energy of the body = work done on it = 500 J. Then h = ?.

\[PE = mgh\]

\[\Rightarrow h = \frac{PE}{mg}\]

\[= \frac{500 \text{ J}}{30 \text{ kg} \times 10 \text{ m/s}^2}\]

\[= \frac{5}{3} \text{ m} = 1.67 \text{ m}\]

From the formula PE = mgh, it is clear that :

1. Potential energy is directly proportional to mass of the body.

2. Potential energy is directly proportional to the acceleration due to gravity.

3. Potential energy is directly proportional to the vertical height upto which the body is lifted.

Teacher's Note

A book resting on a high shelf has more potential energy than the same book on a low shelf - which is why it causes more damage if it falls on your foot from above.

Heat Energy

The energy released during the burning of coal, oil, wood, gas, etc., is called the heat energy. Steam possesses heat energy and is capable to do work. We see in our day-to-day life that when we boil water in a container with a lid on it, the lid starts moving (vibrating) as soon as the water starts boiling. This is the heat energy obtained from the steam which makes the lid move. The steam engine, invented by James Watt in the year 1765, works on the theory of heat energy.

Light Energy

Light is a form of energy in the presence of which, different objects become visible. We cannot see the energy given out by light but we can feel its effects. The energy of light can be used to emit electrons from a metal surface (photoelectric effect). We can see an object only when light energy coming from the object enters into our eyes.

Chemical Energy

The energy possessed by fuels such as coal, oil, gas, etc., often gets released as heat in a chemical reaction (oxidation or burning) and is called chemical energy. The chemical energy given out by petrol or diesel can move a car or a truck. The food that we eat possesses the chemical energy which helps us to do work.

Sound Energy

Sound energy is possessed by a vibrating body. When sound is produced by a vibrating body, it reaches our ear membrane and makes it vibrate. Thus, we hear the sound of the vibrating body.

Magnetic Energy

Magnetic energy is the energy given out by a magnet. A magnet attracts iron nails from a distance and make them move. An electric motor used to run a fan or other appliances gets energy from the magnet.

Electrical Energy

Two charged bodies either attract or repel each other. And, if they are free to move, the charged bodies either move towards each other or move away from each other. This happens because of energy possessed by a charged body called electrical energy. Electrical energy is also required to move the electric charge (current) through a conductor, to run a T.V., machine, electric bell, etc.

Atomic Energy

The energy stored in the nucleus of atoms is called the atomic or nuclear energy. This energy is used in atomic reactors to produce electrical energy. This energy is also used in making atom bombs, used for destruction in wars. Thus, atomic energy has bad as well as good effects.

This is a preview of the first 3 pages. To get the complete book, click below.