Practice question

**Heat engine** absorbs Q_h from hot reservoir T_h, does work W, rejects Q_c to cold T_c, efficiency η = W/Q_h =1 - Q_c/Q_h, second law limits η <1, Kelvin-Planck forbids perfect engine with Q_c=0, Clausius statement heat cannot flow spontaneously from cold to hot without work, equivalent to Kelvin-Planck. Internal energy ( U ) is the sum of the kinetic and potential energies of the molecules within a system, excluding the kinetic energy of the system as a whole. It is a state variable dependent on the system's state, not its motion as a whole. Using first law ΔU = Q - W, W =