 Welcome back Mechanical AI. Out of all the energy that goes into a system, only available energy or exergy is the one that does the actual work. This makes us wonder, what is availability? Before we jump in, check out the previous part of this series to learn about what expansivity and compressibility is. Now, the availability or the exergy of the system is the maximum work that can be extracted from it until it reaches the state of thermodynamic equilibrium with its surroundings. This equilibrium state where all the spontaneous processes end is called the dead state of the system. The minimum energy that has to be rejected by the second law is called the unavailability of the system. Exergy depends both on the system and its surroundings. It is discussed under two groups. First is the exergy of heat input in a cycle. From energy balance, we can understand that the net input in a cycle is a sum of available energy and unavailable energy. With eta being the efficiency of the cycle, we get available energy to be equal to the product of efficiency eta and the heat input Q. Second is the concept of availability in a steady flow process, which is given by the difference between the energy steady state function psi and dead steady state function psi naught of the system. The exergy function is given as a sum of Keenan function denoted by B, kinetic energy denoted by mv squared upon 2 and potential energy denoted by mgz. Hence we saw what available energy is, then applied it to a cycle and then to a steady state process. So like, subscribe and comment with your feedback to help us make better videos. Thanks for watching. Also thanks a lot for those constructive comments. You help the channel grow. So here are the top mechanical EIs of our last videos. In the next episode of Mechanical EI, find out what irreversibility is.