Energy

Energy : The capacity for doing work. Head, electricity and radiation are forms of energy. Fuels release chemical energy when they burn. Kinetic energy is the energy possessed by moving objects. potential energy is energy that is stored in a system-an object that can fall has gravitational potential energy.

In a closed system, the overall energy content remains constant no matter what chemical or physical changes take place therein: this being termed the conservation of energy. Only in nuclear reactions does the conservation law not apply. Then energy is created by the destruction of matter, for the two are equivalent, related by Einstein's equation E = mc² .
The standard unit of energy is the joule.

Energy level or energy state; one of a number of states in which electrons or other atomic particles can exist. In each state a particle or atom possesses a certain energy. When it moves into another higher-energy state. In this state it is said to be excited.