Helmholtz Free Energy
A thermodynamic potential measuring maximum reversible work obtainable from a closed system at constant temperature and volume.
Governing formula
F = U - T S, dF = -S dT - P dV
SI unit
J
In depth
In constant T and V processes, spontaneous changes minimize Helmholtz free energy ($dF ≤ 0$). Connected to canonical partition function via $F = -k_B T ln Z$.
Examples in the real world
Calculating thermodynamic stability and equilibrium phase boundaries in solid-state crystalline systems.