First Law of Thermodynamics

• Let us imagine a control-mass system with a fixed mass of gas. If  amount of heat is added to the system across the system-boundary and if  is the work done on the system by the surroundings, then there will be an eventual change in internal energy of the system which is denoted by de and we can write

(38.24)

This is first law of thermodynamics. Here, de is an exact differential and its value depends only on initial and final states of the system. However,  and  are dependent on the path of the process. A process signifies the way by which heat can be added and the work is done on/by the system. (Note that heat added to system is taken as positive and work done on the system is taken as positive)

• In this chapter we are interested in isentropic process which is a combination of adiabatic (no heat is added to or taken away from the system) and reversible process (occurs through successive stages, each stage consists of an infinitesimal small gradient and is an equilibrium state). In an isentropic process, entropy of a system remains constant ( as seen in the following lecture).