Work (thermodynamics)

In thermodynamics, work is a process involving an energy transfer across the boundary of a thermodynamic system, where the energy transfer is due to a force acting through a distance. Crucially, work is defined only if the system boundary moves. If there is a force exerted on the system but no displacement of the boundary, no work is done on or by the system in the thermodynamic sense.

Work is a path-dependent quantity, meaning the amount of work done depends not only on the initial and final states of the system but also on the specific process (or path) by which the system transitions between those states. This is in contrast to state functions like internal energy, which depend only on the system's current state.

Work can be done by the system on its surroundings (e.g., expanding a gas against a piston) or on the system by its surroundings (e.g., compressing a gas with a piston). Work done by the system is typically assigned a positive sign, while work done on the system is typically assigned a negative sign. This sign convention is a matter of choice, and some sources use the opposite convention. However, consistency is crucial within a given problem or analysis.

Common types of thermodynamic work include:

• Expansion/Compression Work (PV work): Work done by a system when its volume changes against an external pressure. This is particularly relevant for gases and liquids.

• Shaft Work: Work done by a rotating shaft, often used in pumps, turbines, and compressors.

• Electrical Work: Work done by an electric field on charges, or by moving charges in an electric potential.

• Surface Work: Work required to increase the surface area of a liquid.

Work is typically measured in units of energy, such as Joules (J) in the SI system. Understanding work is fundamental to analyzing thermodynamic processes, applying the first law of thermodynamics (conservation of energy), and designing efficient energy conversion systems.