You must have come across the term energy quite often while studying any branch of science. And even without much scientific information, you must have an understanding of what energy is. A person without scientific information may think of energy as some sort of property that allows objects to function. For example, energy makes our cars' engines go, and energy can illuminate light bulbs. However, the scientific view of energy is a little more complex and far more widespread than one might be inclined to think.

What is Energy?

The scientific definition of energy may be stated as follows:

Energy is the quantitative property that is transferred to an object and is recognizable either in the form of heat, light, sound, etc. or in the performance of work.

This is just scientific jargon, which an average reader might have trouble understanding. So, instead of mugging this definition up, let us try and figure out what it means.

First of all, energy is defined as a quantitative property, which means that it can be measured and expressed as a number. There are a lot of quantities that have no numeric representation. For instance, a color cannot be expressed as a number. However, energy is not one of them and you can always express its amount with a number and an appropriate unit.

The second point you should note is that energy is recognizable either in some form like heat or light or in performing work. That is, energy provides the ability for an object to do some sort of work. Indeed, this forms a basic, albeit a little confusing definition of energy: “Energy is the capacity to do work”. At the same time, energy can be recognized as heat, light, sound, etc. Thus, energy has multiple forms.

The above two paragraphs are what you need to know about energy to get started. Now, we will turn our attention to the types of energy.

Types of Energy

Energy can be classified into various types, depending on the form it takes. However, the simplest classifications of energy are listed as follows:

Potential Energy

Potential energy is often connected to the position of an object. For example, an object on Earth has gravitational potential energy that increases as we go upwards from the surface. Similarly, an electron in an atom has potential energy that rises as it approaches the nucleus. Another common example is the potential energy of an object that is attached to a spring.

Kinetic Energy

Kinetic energy is associated with movement and velocity. When an object is moving with a certain velocity, it possesses energy we term kinetic energy. The formula for kinetic energy is quite simple. It is given as $\mathrm{K=\frac{1}{2}mv^{2}}$

Mechanical Energy

Mechanical energy is the sum of the potential and kinetic energies of an object. This is not a separate form of energy. However, the forces that cause potential energy and motion are both studied under the concept of mechanics. Hence, we have the term mechanical energy.

Thermal Energy

The energy that is associated with heat is known as thermal energy. It can be generated in a large number of ways. For instance, an object on fire generates enormous amounts of thermal energy. A lot of chemical reactions are exothermic and release thermal energy as well.

Sonic Energy

Just like the heat is a form of energy, the sound is energy as well. The energy associated with sound is known as sonic energy. It is carried across space by sound waves, which cannot propagate in a vacuum.

Law of Conservation of Energy

The law of conservation of energy is one of the most fundamental laws of physics. It forms the foundation of almost all the theories and concepts we have in Physics in one way or another. Indeed, if the conservation of energy were to be violated, everything we knew about Physics would go into upheaval. Here is the statement of this ubiquitous law:

Energy can only be changed from one form to another. But it can never be created or destroyed.

Energy conservation implies a constancy of energy. That is, energy is either changed into a different form or remains constant. You cannot create energy out of anything, and you cannot make it vanish into nothingness.

  • For example, suppose that two steel balls are approaching each other. There are no external effects. When they collide, they come to a halt and stick together. A novice would say that there is clear energy loss here since moving objects had kinetic energy and it vanished. However, that is impossible due to conservation law. You know that sound and heat are forms of energy. When two balls collide, they create a “bang” sound, and they might even heat up. Thus, the kinetic energy of the object merely got converted into sonic and thermal energies.


Energy conservation is used almost everywhere in Physics:


There are various situations in which, using energy conservation leads to simpler equations and faster solutions than other formulae. For instance, when an object is falling due to gravity, its kinetic and potential energies can be equated to get information about its speed and height.

Similarly, in problems related to central force motion, we can work in terms of total energy and obtain information about the object’s orbit with far less effort.

Developing Theories

Oftentimes, we need to work on theories and hypotheses. Using energy conservation can give us an idea of how to proceed. For instance, out of two theories, A and B, B might lead to results that violate energy conservation and that would make A the obvious and only choice to proceed further.


Studying the energy changes associated with a chemical reaction can allow us to understand how the reaction will proceed and the conditions required for a reaction to occur. For example, an endothermic reaction only occurs when there is thermal energy available outside.


Energy is defined in simple terms as the capacity to do work. This simple definition does not do justice to the enormous uses that the concept of energy has in every single field of science. Energy is found in a large variety of forms, ranging from mechanical to thermal to chemical.

The law of conservation of energy implies the constancy of energy. In simple terms, it states that the total energy of the universe is a constant. That is, energy cannot appear out of thin air and it cannot vanish into nothingness. It can only be transformed into its different forms. This law allows us to solve various problems in physics, chemistry, and biology with far more ease than conventional formulae would afford us.


Q1. What is mass-energy equivalence?

Ans. Mass-energy equivalence refers to the way mass and energy are interconnected properties. We can convert energy into a mass or vice versa. The mathematical relation for such a process is given by the equation $\mathrm{E=mc^{2}}$.

Q2. What are the units of energy?

Ans. The SI unit of energy is given as Joules. Other units include calories, eV, erg, and kW-h, among others.

Q3. Can we transform all energy from one form to another?

Ans. No. Practically, 100% efficiency is not possible. We always incur some sort of loss, which appears as heat, sound, or light.

Q4. When a box slides downhill, where does its kinetic energy come from if we cannot create energy?

Ans. Objects in a gravitational field have gravitational potential energy. When they slide downhill, the kinetic energy does not appear anywhere. Instead, the potential energy changes forms and appears as kinetic energy. Indeed, wherever there are forces involved, there is energy involved. In any system, force and potential are connected via the equation $\mathrm{F=-\frac{\partial U}{\partial r}}$

Q5. If a process does not violate energy conservation, then can it necessarily occur?

Ans. No. Some processes are forbidden despite following energy conservation. This is encountered most often in thermodynamics.