On the other hand, if a net amount of heat exits the system, it must have entered in the form of work. If there is a net amount of heat that comes into the system, it goes out of the system in the form of work – heat enters the gas, which in turn expands and does work on the piston.
Workdone in thermodynamics full#
This means that for a full cycle we can use the first law to conclude: One of the state variables that returns to its original value when the cycle is complete is the internal energy. In a process diagram, it forms a closed loop: Simply put, this is a process that returns to the same thermodynamic state at which it started. One process that we will discuss in detail later is called a cyclic process.
One might think that this is not supposed to be allowed, but knowing the work doesn't define the path in the \(PV\) diagram – any path between the endpoints that result in the same area-under-the-curve would work equally well. Interestingly, in this case, the fact that we know the change in internal energy and that there is zero heat exchanged means that we also know how much work is done. Once again, it should be emphasized that the gas is not following a quasi-static process, so it does not go through the intermediate states of the adiabat, but as we are interested in the change of a state variable, we can choose any path that passes through the endpoints that results in no heat exchange.
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