Re: Isobaric Expansion
- From: baez@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx (John Baez)
- Date: Sat, 30 Dec 2006 02:24:37 +0000 (UTC)
In article <1167107391.846979.32480@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
renai <bensegev@xxxxxxxxxxx> wrote:
I've got what's most likely a very simple thermo. 101 question that I
hope someone can answer. When a cylinder is heated quasi-statically
with a free moving piston the expansion is said to be isobaric and
[... the pressure is ...]
equal to the sum of the external atmospheric pressure and that from the
weight of the piston itself. I understand why the inital and final
pressures would be so because the piston initially is not moving
meaning the internal and external pressures are balanced as well as
when it's finally stopped expanding.
Right.
But during expansion doesn't the
internal pressure have to be higher than the previously mentioned
pressure for it to move.
The pressure inside just needs to be a *tiny* bit higher
than the pressure outside plus the pressure caused by the weight
of the piston for the piston to accelerate near the beginning of
its motion upwards. It just needs to be a *tiny* bit less for
the piston to decelerate near the end of its motion.
Here I'm neglecting friction.
I read somewhere that the key to this isobaric assumption lies in the
quasi-static (very very slow) expansion of the gas [...]
Right - that's the key. For the piston to accelerate a very tiny bit,
the pressure differential only needs to be very tiny. So, in the
"quasistatic limit" the pressure differential is zero!
Again, I'm neglecting friction. In reality, the presence of
"static friction" means you need some pressure just to get the
piston moving at all - otherwise it will stay stuck in position.
Your thermodyamics book is probably neglecting the piston's
friction, which is okay if you're trying to understand concepts
instead of design actual pistons.
.
- References:
- Isobaric Expansion
- From: renai
- Isobaric Expansion
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