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Thermodynamics: Carnot Cycle & Ideal Cycles

Reversible and Irreversible Processes

When studying thermodynamics the processes that are talked about are almost always reversible processes. The definition of a reversible process is a process that can be reversed without leaving traces of the process on the surroundings. In other words the process can always be returned to its initial state. Reversible processes do not occur in nature, but instead are ideal processes. The opposite of a reversible process is an irreversible process. Any process that cannot return to it original state is an irreversible process. Irreversible processes are real processes.

So why do engineers focus on reversible processes instead of irreversible processes? They do this because a reversible process is much easier to analyze. An irreversible process can have unknowns that can' t be accounted for. Also, if you solve the reversible process, which is an ideal process. You can use those results and compare them to the results from an actual process that has been measured. Basically the reversible process is a theoretical limit for its corresponding irreversible process. If your irreversible process provides better results than your reversible process, then you either calculated your reversible process wrong, or something is wrong with your measurements.

So what causes a process to have irreversibility's? Well first most reversible processes assume that a process is quasistatic. In real life, however, it's not practical for a process to happen slow enough to be quasistatic. This in turn results in unrestrained expansion, which can actually cause a pressure difference within the cylinder that the gas is expanding, or be compressed in. Another common irreversibility is friction. Friction causes heat, which is lost energy from the system that cannot be reversed. Heat transfer can also cause irreversibility. An example is a hot liquid that cools in air, the heat will not readily go back into that liquid, so the process cannot be reversed. Finally, another example of an irreversibility is an inelastic deformation. If something has be permanently deformed it will not release the same amount of energy that was used to deform it.

Carnot Cycle

The Carnot Cycle is one of the most widely used ideal and reversible cycles. It is stated that nothing in real life can be more efficient then the Carnot Cycle. Basically the Carnot Cycle is a theoretical heat engine that has two Isothermal processes and two adiabatic processes. It is typically represented by a piston that is in a quasistatic state where the gas inside is expanded by a heat source, and then a heat sink is used to slowly remove the heat from the piston. Refer to image below.

Carnot Cycle

From the Kelvin Planck and Clausius statement a formal definition of a Carnot Cycle is as follows.

"A heat engine cannot operate by exchanging heat with a single reservoir source and a refrigerator cannot operate without net energy input from an external source."

1. The efficiency of any irreversible heat engine is always less than the efficiency of the reversible cycle that represents that heat engine.

2. The efficiency of all reversible heat engines operating between the same two reservoirs are the same.

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