Energy is a fundamental quantity that every physical system possesses; it allows us to predict how much work the system could be made to do, or how much heat it can exchange. Energy can be used not only to produce observable change, it also is used to prevent change in which case unaided observation of this kind of energy can be difficult. It can be readily transformed from one form into another; for instance, using a battery to power an electrical heater converts electrical energy into thermal energy.
