Re: temperature increases, volume decreases. why?
From: Bjoern Feuerbacher (feuerbac_at_thphys.uni-heidelberg.de)
Date: 06/30/04
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Date: Wed, 30 Jun 2004 16:21:55 +0200
Anja wrote:
> "Bjoern Feuerbacher" <feuerbac@thphys.uni-heidelberg.de> wrote in message
> news:cbtseu$i6o$1@news.urz.uni-heidelberg.de...
>
>>Anja wrote:
>>
>>>Assume I have a gas in a cylinder that has a piston as a top. the system
>
> is
>
>>>isolated. if I put a mass on top of the piston, then the pressure and
>>>temperature inside the cylinder will increase, while the volume will
>>>decrease.
>>>What is the physics behind the fact that the temperature increases and
>
> the
>
>>>volume decreases?
>>
>>Why the volume decreases should be fairly obvious, isn't it?
>>
>>The temperature increases because the pistron, when it is pushed down by
>>the mass on top of it, increases the energy of the system (essentially,
>>the potential energy of that mass gets transformed into heat).
>>Try to
>>see it this way: the piston tries to fall down, i.e. its velocity should
>>increase due the gravitational force of the mass on it. But the atoms
>>below it constantly bounce against it, and due to these collision, the
>>piston gets pressed upwards (so that it doesn't fall down accelerated,
>>but only moves down a bit slowly), and the atoms get additional velocity
>>(they took the kinetic energy from the piston). This kinetic energy
>>taken from the piston gets then distributed in the whole gas (due to
>>collisions between atoms), and voila: the temperature of the gas has
>>increased.
You keep asking questions here. Is the education in your
school/university so bad? Or do you try to learn that stuff on your own?
And, BTW, a "thank you" to all the people who answered you would be
polite...
> is it possible to get an estimate of how much the temperature pressure and
> volume would change if we assume for example that inside the cylinder there
> are hydrogen atoms, and that I put 1mg of mass on top of the piston. (we can
> also assume that also that T=25oC. P=1atm. V=(10cm)^3,Area of piston =10cm).
Is that a homework exercise, and you expect us to do your work for you?
I'll tell you the steps, but not the solution:
* Calculate the force due to the mass on the piston.
* Calculate the pressure due to this force (BTW, "area = 10cm" makes
no sense - did you mean cm^2?).
* This is an adiabatic process, since you said that the system is
isolated, so one can use the adiabatic equations to calculate the new
volume and the new temperature, using the new pressure and the old
pressure, volume and temperature.
* The fact that gas consists of *hydrogen* atoms is irrelevant, as far
as I can see. It is enough to know that there are atoms, not molecules,
in order to get the adiabatic coefficient right.
Bye,
Bjoern
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