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 . Work refers to forms of energy transfer which can be accounted for in terms of changes in the external macroscopic physical constraints on 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 .