Cyclic Process in Thermodynamics

The cyclic process

A process in which a system goes from an initial state to a final state and returns back to the initial state is called a cyclic process. The net energy change in a cyclic process is zero. That means, in a cyclic process, the system starts and returns to the same thermodynamic state.

Let consider that a system changes from state A to state B, and then back to state A again. For the change from A to B, ∆U₁ = U_B – U_A.  And for the change from B to A, ∆U₂ = U_A – U_B.

The total energy change,

∆U_total = ∆U₁ + ∆U₂ = (U_B – U_A) + (U_A – U_B) = 0

For a cyclic process, ∆H must also be zero as H is a state function.