Fig The infinitesimal work done by the system (gas) during the small expansion dx is dW = p A dx.

Fig The work done equals the area under the curve on a p-V diagram. (a) Volume increases, work and area are positive. (b) Volume decreases, work and area are negative. (c) A constant-pressure process: Volume increases, W > 0

Fig (a) Three different paths between state 1 and state 2. (b)-(d) The work done by the system during a transition between two states depends on the path chosen.

Fig The final states are the same. The intermediate states (p and V) during the transition from state 1 to 2 are entirely different. Heat, like work, depends on the initial and final states AND the path between the states. (a) Heat is added slowly so as to keep the temperature constant. (b) Rapid, free expansion does no work and there is no heat transfer.

Fig In a thermodynamic process, the internal energy of a system may increase, decrease, or remain the same: (a)  U > 0 (b)  U < 0 (c)  U = 0 The First Law of Thermodynamics Q =  U + W or  U = Q - W

Fig Daily thermodynamic process of your body (a thermodynamic system)