What is Energy?

Energy is described as the capability to accomplish a task, like the ability to push a block by applying force on it. It is a quantitative property that is used to accomplish work on an object.

Forms and Properties of Energy

Energy can take various forms in nature, including kinetic energy, potential energy, nuclear energy, electrical energy, thermal energy, chemical energy, and sound energy. Even matter is a kind of energy that can be transformed into another form of energy to perform work later. Various forms of energy are equal and interconvertible.

Energy obeys the universal law of conservation of energy. This means it cannot be produced, nor can it be destroyed; it only alters its form.

What is the Classification of Energy?

Energy can be classified on the basis of its source, like geothermal energy, solar energy, or wind energy, or on the basis of how it is collected, like potential energy or kinetic energy. Some forms of energy are also classified based on the way in which they complete a task, such as mechanical energy, chemical energy, and thermal energy.

There are two primary forms of energy:

  • Potential energy
  • Kinetic energy

Potential Energy

Potential energy is energy collected by an object due to its location, arrangement, or state. The work done by a force is stored as potential energy. For example, a spring reserves potential energy when it is compressed or extended by applying force. We can lift an object by applying force, and the work done by us is stored as potential energy in that particular object.

Potential energy is further divided into the following categories:

  • Chemical energy
  • Nuclear energy
  • Mechanical energy
  • Gravitational energy

Chemical Energy

Chemical energy is the deposited energy of the bonds between atoms and molecules of an object. When these bonds break, chemical energy is released, like in an electric cell. In it, chemical energy is delivered to generate current. The energy in petroleum, biomass, and natural gas is comprised of chemical energy that generates heat when released.

Nuclear Energy

Energy that holds the various particles within the nucleus of an atom together is termed nuclear energy. Nuclear energy is stored within the nuclei of atoms. When such a nucleus breaks, nuclear energy is transformed into various forms of energy in a huge amount, like the atom bomb. The fusion reactor and fission reactor are based on nuclear energy. In these reactors, the liberation of nuclear energy is maintained at a moderate rate.

Working of a nuclear power reactor.
A nuclear power reactor

Mechanical Energy

Mechanical energy is associated with the motion and position of an object. In other words, mechanical energy is a form of energy which an object possesses due to its motion and position. As a mathematical form, mechanical energy is the sum of kinetic energy and potential energy.

As per the conservation of energy principle, mechanical energy remains constant in an isolated system. The total mechanical energy is conserved in elastic collisions, while in the case of inelastic collisions, some part of the mechanical energy is transformed in the form of thermal energy. Mechanical energy can be turned either into kinetic energy or potential energy.

Gravitational Energy

As per Newton’s law of universal gravitation, every body having mass pulls every other body with a force that is directly proportional to the product of masses of the bodies and is inversely proportional to the square of the length between the centers of the bodies. So, every object on the Earth is attracted by gravitational force.

Gravitational energy is the energy that arises when work is done by gravitational force. On an object of mass m, force by the Earth is mg. If the object is lifted to a height h, the work done by the gravitational force is represented as mgh. The gravitational energy of a river or a dam at a height is used in hydroelectric power plants to generate electricity.

Kinetic Energy

Energy owned by a system due to its movement is termed kinetic energy. Electric and sound energy are examples of kinetic energy generated due to the movement of electrons and sound waves.

Kinetic energy is further divided into following categories:

  • Radiant energy
  • Thermal energy
  • Motion energy
  • Electrical energy
  • Sound energy

Radiant Energy

Radiant energy is the energy of electromagnetic emission that moves in the form of transverse waves. The heat that we observe while sitting near a warm object is a form of radiant energy that is invisible. X-rays used in medicine are another example of radiant energy.

These types of radiant energy are similar because they all move in the form of waves with the same speed in space. But they also tend to differ because the magnitude of energy they possess can be different. The higher the frequency of the wave, the greater the energy.

An illustration of the electromagnetic spectrum.
Electromagnetic spectrum

Thermal Energy

Thermal energy is also called heat energy. The thermal energy of an object is a measure of its hotness or coolness. Thermal energy is created due to the oscillation of the atoms in a substance. Examples of thermal energy include geothermal energy and hot springs.

Motion Energy

Motion energy is the energy generated by the motion of objects from one position to another. Wind energy and tidal energy are examples of motion energy. The types of motion energy include:

  • Wind energy
  • Hydropower
  • Electrical energy
  • Sound energy

Wind Energy

Wind energy is energy created by the movement of the wind. Electrical power is produced by utilizing wind turbines. Wind traveling at high speeds strikes the wind turbine and shifts its momentum to the turbine. The turbine blades begin rotating, and the rotational energy of the turbine is transformed into electric energy using magnets.

An illustration of the working of the wind turbine.
Wind turbine


Energy held in bodies of water, such as the waves and tides of the ocean, is referred to as hydropower energy. The kinetic energy of moving waves is used as a source of energy.

Electrical Energy

Electrical energy is generated by the constrained flow of electrons in matter. Electrons are negatively charged particles. They travel from the negative to the positive pole of a cell in a closed path. The movement of electrons is known as the current, which is produces electrical energy. An electrical bulb is an example of electric energy. Electric energy is mainly obtained from the kinetic energy of movable electrons.

Sound Energy

Sound energy is energy produced by the movement of sound waves within a medium. In the air, it is in the form of longitudinal waves. Longitudinal waves are waves that propagate within the medium in the form of compressions and rarefactions alternately while passing through it.

Interconversion of Different Forms of Energy

Various forms of energy can be commonly interconverted. For instance, when we switch on a fan, electrical energy is transformed into motion energy. In the dynamo, motion energy is transformed into electric energy. Energy cannot be destroyed; it just shifts its form.

Common Mistakes

It is easy to confuse fusion and fission. In fission, the heavy nucleus is split into two fragments and a few neutrons, while in fusion, lighter nuclei will combine to create a heavy nucleus.

Students may also get confused between potential energy and kinetic energy. Potential energy is the energy possessed by an object due to virtue of its state of rest or position, while an object contains kinetic energy due to its movement.

Context and Applications

The topic of the forms of energy is very significant in several professional exams and courses for undergraduate, diploma level, graduate, and postgraduate programs. For example:

  • Bachelors of Technology
  • Bachelors in Science
  • Masters of Technology
  • Masters in Science
  • Visible light
  • Conservation of energy principle
  • Index of wave articles
  • Power station
  • Transfer energy

Practice Problems

1. A moving car is an example of:

  1. Radiant energy
  2. Thermal energy
  3. Chemical energy
  4. Mechanical energy

Answer: d

Explanation: As a car moves with high speed, it possesses a high amount of kinetic energy. So, it is an example of mechanical energy. All moving objects, like cars, airplanes, trucks, and many more are examples of mechanical energy.

2. A drum's vibrating diaphragm is an example of:

  1. Mechanical energy
  2. Sound energy
  3. Radiant energy
  4. Chemical energy

Answer: b

Explanation: All musical instruments, like drums, guitars, pianos, and trumpets, are good examples of sound energy. Vibrating musical instruments can produce sound.

3. Which of the following is the ultimate source of energy?

  1. Fossil fuels
  2. Uranium
  3. Wind
  4. Sun

Answer: d

Explanation: The sun is considered the ultimate source of energy. It is a renewable type of energy that is being utilized by all living beings in the universe. All the energies in the universe come directly or indirectly from the powerful sun.

4. In thermal plants, which of the following fuels is used?

  1. Fossil fuels
  2. Water
  3. Uranium
  4. Biomass

Answer: a

Explanation: Fossil fuels are considered hydrocarbons, which combine carbon and hydrogens. These fossil fuels are utilized to produce electricity. They are burnt so that they will produce heat energy.

5. In which case is kinetic energy is transformed into electrical energy?

  1. Tidal energy
  2. Radiant energy
  3. Sound energy
  4. None of these

Answer: a

Explanation: Tidal energy is the form of energy in which kinetic energy will be transformed into electric energy. It is a renewable type of energy that is used to generate and supply electricity in houses.

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