according to the "first law of thermodynamics", it is useful to separate changes to the internal energy of a thermodynamic system into two sorts of energy transfers. work refers to forms of energy transfer which can be accounted for in terms of changes in the "macroscopic" physical variables of the system, for example energy which goes into expanding the volume of a system against an external pressure, by driving a piston-head out of a cylinder against an external force. this is in contrast to heat energy, which is carried into or out of the system in the form of transfers in the "microscopic" thermal motions of particles.