Path Function
A quantity whose value depends on the specific route taken between two states.
Formula / equation
δq, δw
Unit
J
In depth
Heat and work are the two path functions of thermodynamics: the same change of state can be brought about with different amounts of each. Only their sum, which equals the change in internal energy, is path-independent. Path functions are written with a small delta rather than a capital one.
Examples in the real world
An expanding gas doing maximum work along a reversible path but far less if it expands freely into a vacuum.