Energy
Introduction
In our daily life, we see many activities such as walking, running, lifting objects, moving vehicles, flowing water, and glowing bulbs. All these activities require energy. Without energy, no physical activity or change can take place.
Energy is one of the most important concepts in Physics. It helps us understand how work is done and how motion, heat, light, sound, and other physical changes occur.
Definition
Energy is defined as the capacity or ability of a body to do work.
Energy = Ability to do work
Whenever work is done on an object, energy is transferred to that object. Similarly, when an object does work, it loses energy.
SI Unit of Energy
The SI unit of energy is the joule (J).
Since energy is closely related to work, both work and energy have the same SI unit.
\(1\,J = 1\,N \times 1\,m\)
One joule of energy is the energy required to do one joule of work.
Forms of Energy
Energy exists in many forms. Some common forms of energy are:
- Kinetic energy
- Potential energy
- Heat energy
- Light energy
- Sound energy
- Electrical energy
- Chemical energy
- Nuclear energy
In mechanics, the two most important forms of energy are kinetic energy and potential energy.
Kinetic Energy
Definition
The energy possessed by a body due to its motion is called kinetic energy.
Any moving object has kinetic energy. A faster moving object has more kinetic energy than a slower moving object of the same mass.
Formula of Kinetic Energy
If an object of mass \(m\) is moving with velocity \(v\), then its kinetic energy is given by
\(K=\frac{1}{2}mv^2\)
where,
- \(K\) = Kinetic energy
- \(m\) = Mass of the body
- \(v\) = Velocity of the body
Figure
Explanation
The above figure shows a body of mass \(m\) moving with velocity \(v\). Since the body is in motion, it possesses kinetic energy.
The kinetic energy depends on both mass and velocity. If the mass of the body increases, kinetic energy increases. If the velocity increases, kinetic energy increases more rapidly because velocity is squared in the formula.
\(K \propto m\)
\(K \propto v^2\)
Examples of Kinetic Energy
- A moving car has kinetic energy.
- A running athlete has kinetic energy.
- Flowing water in a river has kinetic energy.
- A flying bird has kinetic energy.
- A moving cricket ball has kinetic energy.
- Wind has kinetic energy because air particles are moving.
Potential Energy
Definition
The energy possessed by a body due to its position, shape, or configuration is called potential energy.
Potential energy is stored energy. It can be converted into other forms of energy when conditions change.
Examples of Potential Energy
- Water stored in a dam has potential energy due to its height.
- A stretched rubber band has potential energy due to its changed shape.
- A compressed spring has potential energy.
- A stone kept at a height has potential energy.
- A raised hammer has potential energy before striking a nail.
Gravitational Potential Energy
Definition
The energy possessed by a body due to its position or height above the ground is called gravitational potential energy.
When a body is lifted upward against gravity, work is done on the body. This work gets stored in the body as gravitational potential energy.
Formula of Gravitational Potential Energy
If a body of mass \(m\) is raised to a height \(h\) above the ground, then its gravitational potential energy is given by
\(U=mgh\)
where,
- \(U\) = Gravitational potential energy
- \(m\) = Mass of the body
- \(g\) = Acceleration due to gravity
- \(h\) = Height of the body above the ground
Figure
Explanation
The above figure shows a body of mass \(m\) kept at a height \(h\) above the ground. Since the body is at a height, it has gravitational potential energy.
To lift the body to this height, work is done against the force of gravity. The force required to lift the body is equal to its weight.
Force = Weight = \(mg\)
Work done in lifting the body is
\(W=Fs\)
Here, force is \(mg\) and displacement is \(h\).
\(W=mg \times h\)
\(W=mgh\)
This work is stored in the body as gravitational potential energy.
\(U=mgh\)
Examples of Gravitational Potential Energy
- A book kept on a table has gravitational potential energy.
- Water stored at a height in a dam has gravitational potential energy.
- A stone held at the top of a building has gravitational potential energy.
- A raised hammer has gravitational potential energy.
- A person standing on a hill has gravitational potential energy with respect to the ground.
Comparison Between Kinetic Energy and Potential Energy
| Kinetic Energy | Potential Energy |
|---|---|
| It is the energy possessed by a body due to its motion. | It is the energy possessed by a body due to its position, shape, or configuration. |
| It depends on mass and velocity. | Gravitational potential energy depends on mass, gravity, and height. |
| Its formula is \(K=\frac{1}{2}mv^2\). | Its formula is \(U=mgh\). |
| A moving car has kinetic energy. | Water stored in a dam has potential energy. |
Importance of Energy
- Energy is necessary for doing work. Without energy, no work can be done.
- Energy helps us understand motion, force, work, heat, electricity, and many other physical phenomena.
- The concept of energy is used in machines, vehicles, power plants, construction, and daily life activities.
- Kinetic energy helps us understand the energy of moving objects such as vehicles, wind, water, and projectiles.
- Potential energy helps us understand stored energy in objects due to height, position, or shape.
Examples
- A moving bus has kinetic energy because it is in motion.
- A ball kept on a roof has gravitational potential energy because it is at a height.
- Flowing river water has kinetic energy and can be used to generate electricity.
- Water stored in a dam has gravitational potential energy. When it falls down, this energy changes into kinetic energy.
- A stretched bow has potential energy. When the arrow is released, this energy changes into kinetic energy.
Important Points
- Energy is the capacity or ability of a body to do work.
- The SI unit of energy is the joule (J).
- Energy and work have the same SI unit.
- Kinetic energy is the energy possessed by a body due to its motion.
- The formula of kinetic energy is \(K=\frac{1}{2}mv^2\).
- Kinetic energy depends on the mass and velocity of the body.
- Potential energy is the energy possessed by a body due to its position, shape, or configuration.
- Gravitational potential energy is the energy possessed by a body due to its height above the ground.
- The formula of gravitational potential energy is \(U=mgh\).
- Energy can be converted from one form to another.
- A moving object has kinetic energy, while an object kept at a height has gravitational potential energy.
Conclusion
Energy is a fundamental concept in Physics. It is defined as the capacity to do work. The two important forms of mechanical energy are kinetic energy and potential energy. Kinetic energy is possessed by a moving body and is given by \(K=\frac{1}{2}mv^2\). Gravitational potential energy is possessed by a body due to its height and is given by \(U=mgh\). Understanding energy helps us explain motion, work, force, and many natural and practical phenomena.
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