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Take the case of an isothermal reversible expansion of a piston to lift a weight. As the piston expands, the temperature remains constant, but the volume increases. This volume increase implies a pressure decrease. But a pressure decrease implies that the piston should not be able to expand anymore! I think my main problem here is how we can keep the force of the gas equal to the force of the weight.

The only way I see this working is if we progressively decrease the weight little by little, but it seems to be that reversible isothermal expansions have been presented to me without this characteristic.

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  • $\begingroup$ pressure equalizes very fast (determined by the speed of sound) so you should reduce the weights mentioned by @John_Rennie at a slower rate than the rate at which the temperature in the cylinder is equalized with the outside. The speed with which the temperature homogenizes also depends on the thermal conductivity of the gas. $\endgroup$
    – hyportnex
    Mar 18, 2019 at 12:21
  • $\begingroup$ What you said is correct. You very gradually decrease the weight on the piston, by removing small gravel pellets (or grains of sand) from the top of the piston. You can also do it manually, by gradually easing up on the force you apply to the piston. In all the cases, the cylinder is in contact with a constant temperature bath so that the expansion is isothermal. $\endgroup$ Mar 18, 2019 at 12:23

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The reversible expansion is an ideal process that doesn't exist in the real world, though real processes are often very close approximations to the ideal reversible process.

In this case we require that the weight of the piston exactly balances the pressure of the gas, then we reduce the weight of the piston very slowly (ideally infinitely slowly). As we reduce the weight of the piston the gas expands and the pressure decreases until the decreased pressure balances the reduced weight.

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