Work (thermodynamics) of the energy went out as heat, and how much as work. This can be summarized by saying that heat and work are not state functions of the system. This is in contrast to classical mechanics, where net work exerted by a particle is a state function. Pressure–volume work (or "PV" work) occurs when the volume of a system changes. "PV" work is often measured in units of litre-atmospheres where = . However, the litre-atmosphere is not a recognised unit in the SI system of units, which measures P in Pascal (Pa), V in m, and PV in Joule (J),
Elastic energy of compressible fluids, and especially gases, demonstrates the essence of elastic energy with negligible complication. The simple thermodynamic formula: formula_2 where dU is an infinitesimal change in recoverable internal energy "U", "P" is the uniform pressure (a force per unit area) applied to the material sample of interest, and "dV" is the infinitesimal change in volume that corresponds to the change in internal energy. The minus sign appears because "dV" is negative under compression by a positive applied pressure which also increases the internal energy. Upon reversal, the work that is done "by" a system is the negative of the