Energy is considered a fundamental aspect of physics. It acts as the driving force behind every physical phenomenon in the universe. Everything can be related to energy – from the formation of galaxies to the destruction of stars and carrying out our daily chores. It can be defined as the capability to perform work or manifest a change in matter. As energy is defined in terms of work, the SI Unit of energy is Joules, denoted by J. 

Characteristics of Energy

The facts and characteristics of energy depend upon the form in which work is being done. These are:-

• Energy is the intrinsic property of matter. As energy is not a substance or a material, it is not made of matter. Matter consists of energy in the form of mass, while light comprises energy in the form of motion, but none of them is made of energy.
• According to the Law of Conservation of Energy, energy can neither be created nor be destroyed. This means that the energy of the universe remains constant. It can neither be gained nor be lost.
• Energy can only be transformed into different forms, keeping the system’s total energy constant. For example, consider a cart at a mountain top. The cart has potential energy due to its height from the surface. As the cart moves down the mountain slope, the potential energy gets converted into kinetic energy due to its motion, further accelerating the cart. This energy state is maintained until the cart meets the surface of the mountain and skids to a stop under the force of friction. The friction force, sometimes known as a dissipative force, does work upon the cart in order to decrease its total mechanical energy. Thus, as the force of friction acts over an increasing distance, the quantity of work increases and the mechanical energy of the cart is gradually dissipated. Though the energy has been converted from one form to another form, the system’s total energy remains constant.
• Mechanical energy can be  classified into kinetic energy and potential energy. Kinetic energy is determined as the energy due to work done by an object in motion. Potential energy is the energy due to the position or configuration of the object relative to other objects.
• Energy can be stored in the form of potential energy, which is by the position of an object relative to other objects.
• Energy travels from one system to another and can be transported in waves or pulses and they do not require medium for travel. It converts from one form to another and travels from one place to another. 
• Energy flows from a region of higher potential to lower potential. This is the reason why hot things tend to cool down.
• Energy can be measured in N-m. Although, according to the International System of Units, it is measured in Joules, denoted by J. The quantity and energy density are different for different forms and energy sources. It is measured in Jkg-1.
• Energy is measured relative to a common datum and does not have an absolute value. For example, the temperature is measured relative to a temperature of 0 K as heat energy is considered to be zero. Similarly, potential energy is measured relative to ground level, at which it is considered to be zero.
• Albert Einstein gives the characteristic energy formula. It relates energy and mass by considering them a single physical entity that can be changed into one another. It is given by, 

               E = mc2

where E is the energy of the system, m is the system’s mass, and c is the velocity of light.

The Different Forms of Energy

1. Thermal energy is the energy due to the vibration of particles that produce heat.
2. Mechanical energy is the energy due to the movement of objects.
3. Electric energy is the energy due to the movement of electrons in a wire.
4. Magnetic energy is the energy that causes a push or a pull.
5. Chemical energy is the energy that is stored in fuel or food.
6. Elastic energy is the energy stored when particles are stretched.
7. Nuclear energy is the energy possessed by the nucleus particles, making them small and dense.
8. Gravitational energy is the energy stored in objects above the earth’s surface.

Conclusion

Energy is the intrinsic property of matter and drives everything in the universe. The facts and characteristics of energy depend on the form it exists in. According to the Law of Conservation of Energy, energy can neither be created nor be destroyed. It can only be transformed into different forms, keeping the system’s total energy constant. Energy exists in various forms. Energy travels from one system to another in waves or pulses and a medium. Energy tends to flow from higher potential to lower potential. There is no absolute value for energy. It is always measured relative to a common datum.