Reversible process (thermodynamics)
The classical Carnot heat engine
In thermodynamics, a reversible process -- or reversible cycle if the process is cyclic -- is a process that can be "reversed" by means of infinitesimal changes in some property of the system without entropy production (i.e. dissipation of energy). Due to these infinitesimal changes, the system is in thermodynamic equilibrium throughout the entire process. Since it would take an infinite amount of time for the reversible process to finish, perfectly reversible processes are impossible. However, if the system undergoing the changes responds much faster than the applied change, the deviation from reversibility may be negligible. In a reversible cycle, the system and its surroundings will be exactly the same after each cycle.
Reversible processes are a means of allowing entropy to be a state function, where a state function is something that does not rely on the pathway that it took to determine its quantitative value. In an ideal thermodynamically reversible process, the energy from work performed by or on the system would be maximized, and that from heat would be minimized; heat cannot fully be converted to work and will always be lost to some degree (to the surroundings). This phenomenon of maximized work and minimized heat can be created treating a reversible process as a set of steps, with smaller and more frequent steps leading to a more idealized and reversible process.
An alternative definition of a reversible process is a process that, after it has taken place, can be reversed and causes no change in either the system or its surroundings. In thermodynamic terms, a process "taking place" would refer to its transition from its initial state to its final state.
In an irreversible process, finite changes are made; therefore the system is not at equilibrium throughout the process. At the same point in an irreversible cycle, the system will be in the same state, but the surroundings are permanently changed after each cycle.
Irreversibility is thermodynamically described as prohibiting the movement of the system or surroundings from going in the opposite direction. A classic example of this is allowing a certain volume of gas to be released into a vacuum (such as outer space). By releasing pressure on a sample and in turn allowing it to occupy an infinitely large space where no work is being done on the surroundings as the gas expands, there would be no means to reverse the process, since no work was performed to make it happen in the first place.
Boundaries and states
A reversible process changes the state of a system in such a way that the net change in the combined entropy of the system and its surroundings is zero. Reversible processes define the boundaries of how efficient heat engines can be in thermodynamics and engineering: a reversible process is one where no heat is lost from the system as "waste", and the machine is thus as efficient as it can possibly be (see Carnot cycle).
In some cases, it is important to distinguish between reversible and quasistatic processes. Reversible processes are always quasistatic, but the converse is not always true. For example, an infinitesimal compression of a gas in a cylinder where there exists friction between the piston and the cylinder is a quasistatic, but not reversible process. Although the system has been driven from its equilibrium state by only an infinitesimal amount, heat has been irreversibly lost due to friction, and cannot be recovered by simply moving the piston infinitesimally in the opposite direction.
Historically, the term Tesla principle was used to describe (amongst other things) certain reversible processes invented by Nikola Tesla. However, this phrase is no longer in conventional use. The principle was that some systems could be reversed and operated in a complementary manner. It was developed during Tesla's research in alternating currents where the current's magnitude and direction varied cyclically. During a demonstration of the Tesla turbine, the disks revolved and machinery fastened to the shaft was operated by the engine. If the turbine's operation was reversed, the disks acted as a pump.
- Carnot cycle
- Entropy production
- Toffoli gate
- Time evolution
- Quantum circuit
- Reversible computing
- Maxwell's demon
- Stirling engine
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- Lower, S. (2003) Entropy Rules! What is Entropy? Entropy
- Giancoli, D.C. (2000), Physics for Scientists and Engineers (with Modern Physics), 3rd edition (Prentice-Hall.)
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- "Tesla's New Monarch of Machines". New York Herald Tribune, Oct. 15, 1911. (Available online. Tesla Engine Builders Association. )