Refer to figure. Let ΔU1 and ΔU2 be the change in internal energy in processes A and B respectively, ΔQ be the net heat given to the system in process A + B and ΔW be the net work done by the system in the process A + B.


As seen from the figure initial state of A is the final state of B and vice-versa.


So, the change in internal energy of process A and B will be equal in magnitude but with a negative sign.


So ΔU1=-ΔU2


ΔU1+ΔU2=0.

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