A-to-Z Guide to Thermodynamics,
Heat & Mass Transfer, and Fluids Engineering

One of the most important applications of thermodynamic processes is to do work. Work is any interaction between the system and its surrounding which can be used to lift a weight. The definition of work is

(1)

i.e., force multiplied by displacement in direction of force. The unit of work is 1 N m (Newton meter) = 1 J (Joule). Any force, such as the weight of a body, the force of a compressed spring or the attraction between two magnets can produce work. However, the force most commonly used is the pressure force, for example, in an internal combustion engine.

(2)

where p is the pressure, A the area, ds the distance moved and dV the change in volume.

If the fluid expands, V2 > V1, the piston moves upwards and work is done by the system. If the fluid is compressed, V2 < V1, the piston moves downward and work is done to the system. From this the definition of work follows: Work done by the system is positive. Work done to the system is negative.

In an open system, such as a steam turbine, the flow work during inlet and exit into the system contributes to the total work. The shaft work Ws which may be obtained by a rotating or reciprocating shaft is

(3)