If the process of expansion shown in Fig 21.1 is carried out isothermally (constant temperature T) the gas absorbs a certain amount of heat q. If the process is done reversibly, the work done by the system is maximum (w max) and the heat absorbed by the system is also maximum(q max). If irreversible work is done, then the work done is less than w max and the heat absorbed is also found to be less than q max . These observations lead us to the well known law of conservation of energy in the familiar form of the first law of thermodynamics. |
According to the above law, when a system changes from state 1 (V1 ,T1 and P1) to state 2 ( V2 T2 and P2) the change in internal energy,  U, is independent of the path. In a cyclic process, wherein the system returns to its original state after passing through several steps, energy is conserved, i.e. it is neither created or destroyed and |
