Energy is a state function that is best defined as the capacity to do work or to produce heat. There are many forms of energy (e.g., radiant energy, kinetic energy, potential energy, etc) each of which can be converted into other forms of energy. The fundamental law of thermodynamics states that the total energy of the universe is fixed and that energy can not be created or destroyed—only converted from one form to another.

Energy can be changed, or transformed, from one form into another. Energy transformation is also called energy conversion. The Système International d'Unités (SI) unit for energy is the joule (J), named after James Joule, who demonstrated that work can be converted into heat. The Joule is the fundamental unit of energy for both work and heat and is the work done by a force of one Newton acting...