Types of energy
Did You Know?
The word energy is derived from the ancient Greek word ἐνέργεια (pronounced energeia), meaning activity/operation. This term was probably coined first by Aristotle around 4th BCE., according to the available and discovered past records.
To fully grasp the working of the universe, one must be acquainted with the various kinds of energy. Every single event that occurs in this universe is an energy transformation of a particular type. The law of conservation of energy establishes two things―the sum total of energy in the universe is constant, and energy manifests itself in various forms, which can undergo transformation within these forms.
Almost every physical quantity can be precisely defined, except energy, which can only be indirectly observed and measured as it manifests itself in different forms. Therefore, work and energy are very closely related, and have the same unit. Energy is a scalar physical quantity, i.e., it can be completely described by specifying the magnitude. Also, it should be noted that when the perspective of studies related to energy changes from macroscopic to microscopic and vice-versa, the form also might change. For example, mechanical energy like friction in the macroscopic view might be only thermal energy at the microscopic scale. The unit is joules according to the International System of Units (SI). When it manifests itself in the form of heat, it is measured using the unit of 'calorie' or 'kilocalorie'.
Different Forms of Energy
Energy can be further characterized through its observed properties. All the types can be broadly divided into two types―Potential and Kinetic Energy. The sum total of both these energies of a particle always remains constant, when there are no frictional forces operating on it.
Mechanical Energy
It is basically defined as the summation of potential and kinetic energies of a body, which is affected by external forces. If the body is not affected by any external force, then the mechanical energy 'Me' remains constant, i.e., the body is isolated from any external forces. This is a hypothetical scenario, and in reality, forces like friction act on all bodies, though their values are very less. Thus, this energy can be simply represented as:

Me = Ep + K

where, 'Ep' is the total potential energy, and 'K' is the kinetic energy.

Numerous modern devices convert other forms into mechanical energy and vice-versa, like thermal power plants (heat to Me), electric generators (Me to electricity), turbine (Kinetic energy to Me), etc.

The conservation of mechanical energy is also dependent on whether two bodies experience collision that is either elastic or non-elastic. In the former type, energy is conserved as the original shape and form is regained, whereas in the latter type, deformation of the bodies is permanent, and a different form of energy like heat may emerge from it. In this case, energy may not be conserved but might increase or decrease, depending on the nature of collision and the extent of deformation.
Potential Energy
The inherent and dormant entity stored in any physical system, due to its position and structure in an environment, along with applied forces is called potential energy. The mass value of a body plays an important role in deciding it. For example, imagine an archer with a bow and an arrow is ready to launch it. When the arrow is made ready for launching and the taut bowstring is pulled back, at that position, the string has elastic potential energy stored in it. In this position, the string has the 'potential' to perform the work to launch the arrow.

There are various forms of potential energy, depending on the kind of forces involved, such as gravitational potential energy, chemical potential energy, electrical potential energy, magnetic potential energy, and nuclear potential energy. When a force is applied on a body, work is done in a specific direction. This work is represented by taking into account the potential energy of that body, which is denoted by a negative sign, as the energy may increase or decrease depending on whether the work is done against or in the force direction, respectively. This is represented as:

W = -δEp

where, 'W' is work done, and 'δEp' is the potential energy present in the body.
Kinetic Energy
It is mainly possessed by a particle or a body due to its motion. It is subdivided primarily into rotational kinetic energy and vibrational kinetic energy. In the above example, when the archer releases the bowstring, the arrow gets launched when the stored elastic potential energy gets converted into kinetic energy. The bowstring in motion possesses kinetic energy. Thus, any particle in motion has this kind of energy. Kinetic energy of a body that is not undergoing rotation is given by the following formula:

K = (M × V2) ÷ 2 ----- equation 3

where, 'K' is the total kinetic energy, 'M' is mass of the body, and 'V' is the velocity at which it is traveling. For a rotating body, the kinetic energy is represented as:

K = (I × W2) ÷ 2

where, 'I' and 'W' are the moment of inertia and angular velocity of the body.

Kinetic energy varies according to the frame of reference of an observer, along with inertia. For example, if a car passes an observer who is stationary, then the speeds of both objects are relative to each other, and hence the car possesses kinetic energy with a positive value. But, if both the observer and car are traveling at the same speed, then this energy is equivalent to zero.
Thermal Energy
It can be studied or estimated by measuring the temperature of the body or substance under consideration. It exists due to the vibrational, rotational, and translational motion of the body, along with the potential energy of its atoms and molecules. It is a part of the internal thermodynamics of an object, and mainly exists due to the loss of kinetic energy occurring during atomic collisions. This energy is a combination of both kinetic and potential energies of the object, and is characterized by the heat absorption aspect of the atoms, molecules, and other sub-atomic particles. In case of a gas that consists of atoms of the same element, then the thermal energy is equivalent to the entire kinetic energy of that gas. Thermal energy can be easily represented in the form of an equation that describes a mono-atomic gas in the following manner:

K = (M × V2) ÷ 2 ----- from equation 3

Thus, if a gas has 'N' molecules, then its thermal energy can be represented as.

U = (N × M × V2) ÷ 2

= (N × kT) ÷ 2

where, 'k' is the Boltzmann constant, and 'T' is the measured temperature or the heat of the body.

From the above formula, it is clear that this energy operates by the processes of absorption or emission of heat, during its transfer from one portion of the system to another.
Electric Energy
It is derived from the electrical potential energy that exists between charges, which is delivered in the form of an electric current. When you connect the terminals of a battery with a bulb, electrical energy flows between the two terminals, in the form of an electric current. This process takes place due to the transfer of electrons through the wire, between the terminals. This type of energy can also exist in combination with other fundamental energies, which are stated below:
Electromagnetic Energy
As the name suggests, it is present in the form of electromagnetic waves that vary in frequency and amplitude. Both the electric and magnetic components are perpendicular to each other, and also to the direction of energy propagation.
Electrochemical Energy
The generation of electricity with the help of chemical reactions involves electrochemical energy. An amazing example is that of the fuel cell, wherein electricity can be generated due to the reactions triggered inside a device that contains a mixture of different components.
Electrostatic Energy
It is the least harnessed one, and is present when two bodies undergo a frictional interaction or collision, which can create minor electrical charges. For example, rub a comb on a woolen material and hold it over small paper pieces; they are lifted up because of the static electricity created by rubbing both the objects.
Magnetic energy
When an object or body is characterized by polar movement, i.e., the existence of two poles, which have exactly opposite characteristics, then the entity that controls all the related processes is called magnetic energy. The force that is exerted is in the form of a magnetic field, and the North and South poles of this field are situated exactly opposite to each other. A popular example is that of our planet, the Earth, which behaves like a giant magnet. The magnetic energy travels in the form of magnetic lines, which extend from the North to the South pole, creating the magnetic field.

Often, the terms 'electromagnetic energy' or 'electromagnetism' are used, as electricity and magnetism can exist in combined form in the form of waves. In case of this type, the strength of the field depends on several factors such as magnetic dipole moment, strength of the current produced, amount of magnetic material present, etc. A common example that incorporates the use of this energy is that of the electromagnet. This device is utilized in our everyday lives, and it consists mainly of a wire coiled around a metallic material. When an electric current is passed through the wire, a magnetic field is formed, which can be further used for different purposes depending on its strength and the associated magnetic forces.
Chemical Energy
It is the fundamental physical entity that controls the reactions occurring or involving both organic and inorganic compounds and substances, and also controls life-related processes. Chemical energy can be manifested in other forms such as heat, light, electricity, etc., from different sources. When the energy decreases after a reaction, it is then transferred to the surrounding environment or media, and hence the process is called exothermic. Similarly, if a body absorbs energy, its energy value increases, thus making it an endothermic process.

The motive force that powers the human body is provided by the chemical energy that is derived through the process of respiration, which involves the formation and breaking of inter-atomic molecular bonds. Through molecular rearrangements, along with compound formation and breakdown, the biological world derives energy. For example, the formation of glucose from the process of photosynthesis is useful for energy generation in a plant cell.

This type of energy is often represented in the form of the Rydberg constant, which is given as:

R∞ = (Me × E4) ÷ 8e02H3C

= 1.097 × 107 × m-1

'Me' is the mass at zero motion, 'E' is the charge, 'eo' is the space permittivity, 'H' is the Planck constant, and 'C' is the light speed.
Sound Energy
Sound is heard as the result of compressions and rarefactions produced in air as a medium. Thus, the sound energy is derived from the oscillatory motion of air molecules. The vibrations produced when the waves travel through this medium are absorbed and interpreted accordingly. These vibrations are parallel to each other and are in the same direction as that of the wave propagation. Humans and other living beings have the extraordinary character of hearing sound waves with the help of special ear components. When the ear catches sound energy, the waves are amplified and are passed onwards with the help of auditory nerves. The brain then interprets the signals, thus providing us with the feeling of hearing. Sound does not travel in vacuum, i.e., outer space, as compression and rarefaction is not possible in such a medium.

When sound energy is released from an object, the waves spread in all directions, and are a combination of both potential and kinetic energy densities of the body. For example, if a car passes an observer, the first kind of energy that is experienced by the person consists of the sound waves, and their strength depends on several parameters like wave frequency and amplitude, distance between the observer and the vehicle, the total area of the surroundings, etc.
Light/Radiant Energy
It is propagated by electromagnetic waves through space; for example, the light received from the Sun is an example of radiant energy. The spectrum of electromagnetic radiation is vast―from radio waves to the high-frequency gamma rays. The energy derived from this source is directly proportional to the frequency of waves. Humans can only detect the visible light spectrum of electromagnetic radiation, and all other wavelengths are invisible. Majority of light energy that is received by our planet is in the form of the Sun's rays.

Light energy or power is measured mainly by a unit called radiant flux. There have been several theories that attempt to explain the propagation of light waves through any medium including space. The most famous one is the Quantum theory, which states that light travels in the form of small packets of particles called quanta, and each quantum shows dual personality, i.e., it can behave as a wave as well as a particle. Light energy is often accompanied with other kinds like heat, sound, chemical, and magnetic. It can be said that this energy is a secondary form and exists only when another type undergoes transformation due to several processes. These might include chemical reactions, nuclear fission and fusion processes, absorption, reflection, refraction, etc.
Gravitational Energy
The force of attraction that exists between two bodies having substantial mass values is called gravitational force, and this phenomenon is controlled by the entity called gravitational energy. According to Newton's law of gravitation, any two bodies having masses will exert a force on each other that will tend to attract both of them. This force is directly proportional to the product of their masses and inversely proportional to the square of distance between them. This force is represented as:

G = (g × M1 × M2) ÷ R2

where, 'G' is the gravitational attraction, 'g' is the gravitational constant, R is the distance between the two objects, and 'M1' and 'M2' are the masses of both the bodies, respectively.

Gravitational energy is the weakest one of all in our Universe, but the force caused by it could be very strong in some celestial objects like black holes, wherein it is theorized that the gravitational forces would be so strong that not even light can escape from its attraction. On our planet, this energy helps to keep us stable and balanced. The heavier the body in terms of mass, the higher would be its gravitational attraction. Hence, as the Sun contributes the maximum mass of our solar system, its high gravity makes it possible the revolution of every planet around it.
Nuclear Energy
It is a type of potential energy, and it is mainly derived from processes involving nuclear fission and nuclear fusion. In the former one, a radioactive elemental atom is divided or separated, further giving rise to daughter elements, and releasing a tremendous amount of energy. This principle is used in case of nuclear reactor and other associated technological applications. In the latter type, two atoms of an element combine with each other and fuse. This process also leads to the release of high amount of energy, and the prime example where this process is said to occur is that of the Sun; it is theorized that in this star, nuclear fusion is taking place at its core portions.

Nuclear power has several applications in the modern world, and since several decades, this energy is utilized to produce electricity and heat supplies. Entire ships and submarines can be operated on the basis of a nuclear source. Some nations also use this energy form to make nuclear weapons. The electricity production is done with the help of a nuclear reactor and radioactive material. The atomic nuclei are bombarded with electrons, which cause them to split and form daughter elements. The energy released is used to power generators, which further produce electric power.
Elastic Energy
When you stretch a rubber band and then release it, the inter-atomic forces makes it snap back to its original condition. The stored elastic potential energy is converted into kinetic energy to create the reversible motion, which brings the elastic band to its original position. Thus, elastic energy more or less makes it possible exert tensional and compressional forces on an object. The work done depends on the magnitude of these forces.

For example, when a spring is extended, the stored potential energy makes it possible for the stretching of the material, and when the extensional forces are removed, it reverts back to its original position. Another example is the one, which was described earlier in this article―the bow and arrow description. In this instance, the bow string is stretched till a particular point, and after the arrow is released, it reverts back to its original position due to the elastic energy that is present during its stretching.

After a certain point, elasticity might get converted to plasticity, wherein the object gets permanently deformed. This happens because each material has its own limit of elasticity, and beyond this limit, the elastic forces stop operating. This can be easily observed with the Young's modulus experiment.
Surface Energy
It is defined as the energy, which is present by virtue of existence of tensional forces on an object's surface. Such forces are typically present on still water, viscous liquids, stretched rubber material, etc. When two materials come in contact with each other (mostly liquids) and do not form any sort of mixture, surface tensions are created, which are governed by this type of energy. For example, the capillary motion in plant tissues, the formation bubbles and soap films on water, immiscibility between oil and water, etc are all instances of surface energy. This type exists under a particular limit of external forces, and when these forces increase beyond a certain limit, then the energy is released. Surface tensional forces are represented as:

dW = γ × dSa

where 'dW' is the work done and represents total surface energy of the body, 'γ' is surface tension, and 'Sa' is the surface area of the body.

In solid objects, surface energy is usually present in combination with elastic energy. When a solid is stretched this energy is mostly measured in the form of heat. The volume of the deformed body remains more or less same, as compared to the original object. Contact angles are also measured in order to determine this type of energy.
As seen in the above-described sections, the physical entity called energy can work in myriad forms and kinds, and can also exist in combination with the various types. This entity is governed by a single doctrine, which is also known as Newton's 3rd law of motion. It states that energy can neither be created nor destroyed, and only can be changed from one form to another. This law is applicable to the entire Universe, at least till the extent discovered by mankind.