Heat Capacity of Air at Constant Volume


by s.p.q.r
Tags: capacity, constant, heat, volume
s.p.q.r
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#1
Oct1-07, 06:28 AM
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Hi

I have an ongoing dispute with my mate on this one, please help to clarify this before I open up a can of whoop *** on that sorry mo-fo.


300 litres of air are compressed into a 3 litre tank. What is the heat capacity of this air?

Thanks in advance.
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Hootenanny
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Oct1-07, 06:39 AM
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What do you think it is?
s.p.q.r
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#3
Oct2-07, 03:51 AM
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The Cp J mol is 29.19. But because I ask for constant volume, it is definately lower then this. This is what I think. I can find no references to constant volume anywhere and unfortunately I have no teacher to ask as I study archaeology, not physics.

Do you have the answer?


Thanks in advance.

Andrew Mason
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Oct2-07, 09:53 AM
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Heat Capacity of Air at Constant Volume


Quote Quote by s.p.q.r View Post
The Cp J mol is 29.19. But because I ask for constant volume, it is definately lower then this. This is what I think. I can find no references to constant volume anywhere and unfortunately I have no teacher to ask as I study archaeology, not physics.

Do you have the answer?


Thanks in advance.
Air is almost entirely a diatomic gas, [itex]\gamma = C_p/C_v = 1.4[/itex] (7/5)

AM
s.p.q.r
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#5
Oct2-07, 10:54 AM
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Hi,


Thanks for the reply. Is 1.4 per gram or mol?

Also,

How can you measure a gram of gas and how much is 1 mol?

Cheers.
mgb_phys
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#6
Oct2-07, 11:03 AM
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1.4 is a ratio ( actually nearer 1.3 for dry air at room temp)
see http://en.wikipedia.org/wiki/Heat_capacity_ratio
1 mol of air is roughly 30g or 22.4litres at STP ( 0deg C 1 atm)
Loren Booda
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#7
Oct2-07, 10:30 PM
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Is heat capacity independent of volume for an ideal gas?

Stupid question - gas performs work while being compressed.
mgb_phys
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#8
Oct3-07, 08:30 AM
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For an ideal gas heat capcity just depends on the amount (number of moles) present and the number of vibration states of the molecular.
For a real gas it also depends on the pressure because the molecules close to each other change the vibration state/bond energy.
Loren Booda
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#9
Oct3-07, 11:19 AM
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In a modification of the "ideal gas" law, I seem to recall an equation with correction terms for the volume and pressure, respectively. Has anyone run across this?
s.p.q.r
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#10
Oct8-07, 08:36 AM
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Hi,
This ratio of 1.4, does this just mean that you divide the constant pressure capacity (1.020J/g) by 1.4?
Andrew Mason
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Oct8-07, 12:53 PM
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Quote Quote by s.p.q.r View Post
Hi,
This ratio of 1.4, does this just mean that you divide the constant pressure capacity (1.020J/g) by 1.4?
[itex]\gamma = 1.4[/itex] is the ratio of the specific heat (heat flow per gram or per mole per degree K change in temperature) at constant pressure to the specific heat at constant volume. [itex]\gamma = C_p/C_v[/itex]. What you want to find is Cv. You also have to find the number of moles of air in this container to find its heat capacity (heat flow per degree K change in Temp.).

AM


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