What thermodynamic property ensures that Delta U is zero in a cyclic process?
**Carnot engine** reversible engine operating between T_h and T_c has maximum efficiency η =1 - T_c/T_h, T in kelvin, e.g., T_h=400 K T_c=300 K η=0.25, real engines less due to irreversibilities, second law defines direction of spontaneous processes and entropy increase. Internal energy ( U ) is a state function, meaning its value depends only on the state, not the path. In a cyclic process, returning to the initial state means Δ U = 0 , regardless of intermediate changes. Using first law ΔU = Q - W, W = ∫ P dV, isobaric W = P ΔV, isothermal W = n R T ln(V₂/V₁),
Ref: NCERT > Physics Book > Thermodynamics > Second Law Heat Engines and Kelvin-Planck
