volume (V) on a diagram. By following the path of this P–V diagram, we can study
how the system is affected as it moves from one state to another.
Work is done on or by the system when the piston is moved and the volume of the
gas changes. For example, imagine that the gas pushes the piston upward, causing
an increase in volume. The work done by the gas during its expansion is W = FΔs,
but since F = PA, we have W = PAΔs, and because AΔs = ΔV, we have
W = PΔV
This equation is also true if the piston is pushed down and the volume of the gas
decreases. In this case, ΔV is negative, so W is negative. In general, then, W is
positive when the system does work against its surroundings, and W is negative
when the surroundings do work on the system.