is a branch of physics which deals with the energy and work of a system. Thermodynamics deals with the large scale response of a system which we can observe and measure in experiments. As aerodynamicists, we are most interested in the thermodynamics of propulsion systems and high speed flows. The propulsion system of an aircraft generates thrust by accelerating a working fluid, usually a heated gas. In response to Newton's third law of motion, the propulsion system and the aircraft are accelerated in the opposite direction. To understand how a propulsion system works, we must study the basic thermodynamics of the working fluid.