When a system expands in an isobaric process, the thermodynamic work, W , done by the system on the surroundings is the product, P Δ Δ V , of system pressure, P , and system volume change, Δ Δ V , whereas − − P Δ Δ V is said to be the thermodynamic work done on the system by the surroundings. The change in internal energy of the system is: