A book that I have contains the following lines:
For a process to be reversible, the dissipative forces such as viscosity and friction should be absent.
My question is why?
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$\begingroup$ Could it be, that converting mechanical energy to heat is not simply reversible? Such as pendulum works nice in theory trading positional energy for moving and back, but if you try it in water then it would stop really soon, leaving water just a little warmer? $\endgroup$– gilhadCommented Dec 31, 2015 at 10:41
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$\begingroup$ Basically, yes. You definitely have the right idea. In addition to dissipation of mechanical energy, irreversibility is caused by heat transfer related to finite temperature gradients within the system. $\endgroup$– Chet MillerCommented Dec 31, 2015 at 13:58
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2 Answers
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Because, dissipative forces convert some of the work to heat. If we want to have a reversible process we must be able to return system and its environment back to their initial states without any change in universe. For this purpose, we must extract heat from environment and convert whole of that to work and it is impossible due to the second law of thermodynamics.
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If you allow a cylinder with a piston where there is friction present between the walls of the cylinder and the piston to expand, some of the work done is used in overcoming the frictional force (released as heat). Now if you supply the same amount of heat which was released as work and try to compress the piston back to its original state, some of the work is released as heat again and hence you can never reach the original state without changing the state of the surroundings.
In other words, if you keep compressing till you reached the initial state, you'd have supplied more heat while reversing than it took to get it to the expanded state.
This happens because friction is a dissipative force. Whatever the direction of movement of the piston maybe, some part of the work is always released as heat. If the process was to be reversible, the frictional force would have to absorb heat from the surroundings and must convert it to work but we know that never happens.
