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 molecular kinetic and potential energies, a state variable independent of the path. It does not include the system’s macroscopic kinetic energy (e.g., a moving bullet), making option C incorrect. Using first law ΔU = Q - W, W = ∫ P dV, isobaric W = P ΔV,