In short , the law of conservation of energy states that energy can not be created or destroyed , it can only be changed from one form to another , such as when electrical energy is changed into heat energy . Work and heat are processes which add or subtract energy , while the internal energy is a particular form of energy associated with the system . Thus the term `` heat energy '' for means `` that amount of energy added as the result of heating '' rather than referring to a particular form of energy .