Energy is an indirectly observed quantity which comes in many forms. There are many forms of energy such as, kinetic energy, potential energy, radiant energy and many more. Energy is an indispensable prerequisite for performing mechanical work and the concept has great important in natural science.
The law of conservation of energy was first postulated in the 19th century. It was applied to isolated systems. Noether’s theorem states the conservation of energy is a consequence of the fact that the laws of physics do not change over time.
Energy can be used in different contexts in which energy is used. In chemistry, energy is used to describe heat being released from chemical reactions. In biology, energy is responsible for growth and development of organisms. In earth sciences, energy describes the movement of tectonic plates. Humans input energy to sustain life through calories via food.
Energy density is a term used to denote the amount of useful energy stored in a given system or region of space per unit volume. For example, nuclear reactions have a much higher energy density than coal combustion reactions.