Why does the First Law of Thermodynamics allow some processes that the Second Law prohibits?
**First law of thermodynamics** ΔU = Q - W, ΔU internal energy change (J), Q heat added to system (J), W work done by system (J), sign convention physics Q positive when added, W positive when done by system, energy conservation, for isochoric W=0 ΔU=Q, for adiabatic Q=0 ΔU=-W, for isothermal ΔU=0 Q=W, for cyclic ΔU=0 Q_net=W_net. The First Law ensures energy conservation ( Δ Q = Δ U + Δ W ), permitting any energy-balanced process. The Second Law introduces directionality and efficiency limits (e.g., no 100% heat-to-work conversion), restricting feasible processes. Using first law ΔU = Q - W, W = ∫ P
Ref: NCERT > Physics Book > Thermodynamics > First Law of Thermodynamics Applications