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Old Sep 10th 2017, 05:27 AM   #1
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Question Physical significance of PV of a gas mass

Hi
This might seem to be a simple question off the bat, but I would like your comments on it.
When pressure of a gas is P and specific volume is v what is the physical meaning of Pv?

I tried to interpret it as the "Compressive Energy" of the gas mass. Assume the air mass is at the volume it is under the pressure P. Therefore the pressure has compressed the air mass to the current volume, and in that process store energy in the gass mass like in a spring.

I'd love your comments on what Pv of a gas means
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Old Nov 2nd 2017, 09:34 AM   #2
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It is sometimes referred to as the "mechanical energy" term. You should probably use this term rather than "compressive energy" because if you're delving into internal energy or equations of state, temperature and many other parameters, such as molecular properties, start to get involved. For example, what would you say the product of pressure and volume is for, say, different states of matter (e.g. liquids) or diatomic gases? The superior theories that will correctly describe the relationship between energy transfers under compression may involve fluid physics, statistical mechanics or other things where the product of pressure and volume is only a small part of the problem.

You find these pages interesting:

https://en.wikipedia.org/wiki/Equation_of_state
https://en.wikipedia.org/wiki/Internal_energy
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Old Nov 2nd 2017, 12:59 PM   #3
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Welcome Thanitha, it's good to see someone thinking about their subject like this.



pressure and volume, specific volume or just straightforward volume are a pair of variables whose product has the units of energy (or specific energy if you use specific volume).

That is PV = energy (or work done, if there is a volume change.)

Before all the concepts benit (quite rightly) mentioned came along, Physics was much simpler and two sorts of quantities were recognised.

Heat and work. Energy itself came later, after these two were shown to be interconvertible and equivalent.

During the industrial revolution, steam engines had what was called an 'indicator'.
This device could display or even trace out the PV diagram for the machine.
So a PV diagram came to be called an 'indicator diagram'.

The point of all this is that the directly observable pressure and volume could be used to measure the work done or available on one of these diagrams.

This was hugely convenient for steam engineers.

So the work element of 'energy' was covered and people wondered if there was a similar pair of variables for heat.

This is how the concept of entropy was born -and it makes it so simple, not some mystical thing in the universe but the quantity which pairs with temperature to form an indicator diagram for heat.

Understanding this will stand you in good stead for your future studies, not only because it helps to understand the difficult subject of entropy, but also because many more such pairs have been identified over the centuries since, including magnetic and electric ones.



Please ask if any of this is not clear.
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Last edited by studiot; Nov 2nd 2017 at 01:10 PM.
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