Re: [Phys-l] T dS versus dQ

["LaMontagne, Bob" <RLAMONT@providence.edu>]

The force for a sudden movement is greater because of the mass of the accelerating piston - and it will be less to decelerate the piston to stop it. If the gas is ideal, the pressure as a function of volume - hence the force of the piston on the gas - will be the same. If the piston is pinned - as Carl proposed - and is allowed to drop until it hits a stop, the force of the piston against the gas is still just the force of the gas against the piston - any excess force goes into the acceleration of the piston.

Unless the system is allowed to radiate the sound waves to the outside, it seems the final temperature is again the same as before since the sound waves soon become part of the random molecular motion. It seems that the final entropy is the same as for the slow reversible case.

Bob at PC

> For that matter, the motion doesn't even need to be
> supersonic;  I reckon Carl mentioned that just by
> way of emphasis.  Any reasonably _sudden_ motion of
> the piston will launch sound waves which will sooner
> or later become thermalized, raising the temperature.
>
> To say the same thing yet another way, the force
> required for a sudden movement will be greater than
> the force required for a gradual movement.  This
> means the F dot dx (aka P dV) will be larger than
> you might have expected.