Work is done when an object is moved against an opposing force. It is equivalent to a change in height of a body in a gravimetric field. The energy of a system is its capacity to do work. When work is done on an otherwise isolated system, its capacity to do work is increased, and hence the energy of the system is increased. When the system does work its energy is reduced because it can do less work than before. When the energy of a system changes as a result of temperature differences between the system and its surroundings, the energy has been transferred as heat. Not all boundaries permit transfer of even when there temperature difference between the system and its surroundings. A boundary that does not allow heat transfer is d adiabatic. Processes that release energy as heat are called exothermic, whereas processes that absorb energy as heat are called endothermic is The mathematical expression of the first is