 no shearing forces), the stress tensor is simply the negative of the pressure times the unit tensor so that

The trace of the strain tensor (formula_31) is the fractional change in volume so that the above reduces to formula_32 as it should.In a similar way, temperature differences drive changes in entropy, and their product is the energy transferred by heating.Temperature is the driving force, entropy is the associated displacement, and the two form a pair of conjugate variables.