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Suppose you have a cylinder filled with an ideal gas and a piston on top which can slide up and down. If you want to get work done using isobaric, isothermic or any other process (other than adiabatic) you apply heat. Makes sense.

But in adiabatic process, you can't apply heat. So do you shout to the gas "Hey gas, lose some internal energy and do work!" or what do you do exactly to?

I have two ideas: one is that you pull the piston upwards, but that makes no sense to me because then I'm the one doing work, not the gas

The other is that I reduce pressure on the piston, and then the volume will increase. But once again I'm applying some sort of work to change the pressure so... is the gas doing anything really?

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You can reduce the pressure without doing work on the system. Imagine that the piston is weighted down by many many thin disks. Reduce the pressure on on the gas by moving the disks off of the piston "sideways". The displacement of the disks is perpendicular to the force due to pressure, so no work is done in moving them, but the system does work when the volume expands.

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  • $\begingroup$ So is this the most efficient process? Because you basically do nothing and you get work out of it $\endgroup$ – M. Wother Mar 14 '17 at 15:40
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    $\begingroup$ You get work from the internal energy of the gas. I get work out of a automobile battery while basically doing nothing ... $\endgroup$ – garyp Mar 14 '17 at 16:23
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    $\begingroup$ This is the way. Keep the surrounding pressure lower than your system's pressure, you will get the work from the internal energy of the system. $\endgroup$ – user115350 Mar 14 '17 at 17:15

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