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Define Reversible process

Posted: Wed Jan 29, 2020 10:08 pm
by Phuong Tran 1G
So, from what Ive read on the textbook, a reversible process means any small change can cause the object working against opposing force to move in or out?

Also, can someone explain what this means?

Expansion against external pressure that differs by a measurable amount from the pressure of the system is an irreversible process in sense that an infinitesimal change in external pressure does not reverse direction of travel in piston.

Re: Define Reversible process

Posted: Wed Jan 29, 2020 10:20 pm
by Dina Marchenko 2J
The whole point of a reversible process is that the tiniest change results in work being done. I think overall reversible is a confusing term. I just think of irreversible as doesn't change easily.

If a system is reversible it means that all surrounding and system variables are basically the same (at equilibrium) so the tiniest change in any variable results in work. If the external pressure is far larger than the internal pressure of the system, the tiny change in the variable will result in a tiny amount of work being done which won't have any real effect on the vastly larger external pressure. It's like a fly trying to hurt an elephant. So, in order for a process to be reversible, the external and internal pressure have to be very close to each other or the same, so that a fly can hit a fly and impact it.

Re: Define Reversible process

Posted: Wed Jan 29, 2020 10:26 pm
by Sebastian Lee 1L
A reversible expansion is basically one that takes place in really small, slow steps. Conversely, an irreversible expansion is one that happens very quickly. I'll give examples of both. So for both processes, you can think of a beaker in which a piston is compressing a gas at say 2 atm. In an irreversible process, the surrounding pressure can be at 1 atm for example, so the compressed gas will very quickly expand and do work equal to the constant pressure (1 atm) times the change in volume. In a reversible expansion, you would have the initial outside pressure be equal to the inside gas pressure at 2 atm. Then you would VERY slowly decrease the external pressure. For example, you would take away .00001 atm from the surroundings. As a result, the compressed gas would do work and push the piston to reach equilibrium at each very small step. You would repeat this process many, many times until you got to the same volume and pressure as the irreversible process. The reversible process will do more work because the external pressure is higher (though decreasing) throughout the expansion. It will start at 2 atm at the initial volume and slowly decrease to 1 atm at the final volume. The irreversible process starts at 1 atm and stays at 1 atm from the initial to final volume.