V. Second Law of Thermodynamics Chapter 20 The second law: The entropy of an isolated system never decreases. The entropy change for a slow quasi-static process is dS =dQ/T and for a slow isothermal process, AS =0/7 . The equipartition theorem: The average energy stored in each degree of freedom of a system in thermal equilibrium is ,7, where ke = 1.380649*102 J K-1. The potential energy of a mass m on a spring with spring constant & and oscillation amplitude & is U = kx . A simple pendulum of length / in Earth's gravity g has an angular frequency for small oscillation amplitudes @ =