Go Back   Physics Help Forum > College/University Physics Help > Advanced Thermodynamics

Advanced Thermodynamics Advanced Thermodynamics Physics Help Forum

Like Tree3Likes
  • 2 Post By benit13
  • 1 Post By studiot
LinkBack Thread Tools Display Modes
Old Sep 10th 2017, 04:27 AM   #1
Junior Member
Join Date: Sep 2017
Posts: 1
Question Physical significance of PV of a gas mass

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
Thanitha is offline   Reply With Quote
Old Nov 2nd 2017, 08:34 AM   #2
Senior Member
Join Date: Oct 2017
Location: Glasgow
Posts: 369
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:

topsquark and studiot like this.
benit13 is offline   Reply With Quote
Old Nov 2nd 2017, 11:59 AM   #3
Senior Member
Join Date: Apr 2015
Location: Somerset, England
Posts: 1,035
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.
topsquark likes this.

Last edited by studiot; Nov 2nd 2017 at 12:10 PM.
studiot is offline   Reply With Quote

  Physics Help Forum > College/University Physics Help > Advanced Thermodynamics

gas, mass, physical, physical meaning, pressure volume, pv meaning, significance, thermodynamics

Thread Tools
Display Modes

Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Physical significance of temperature kelsiu Thermodynamics and Fluid Mechanics 1 Dec 31st 2014 05:29 PM
what is primary color and what is its significance?? kenny1999 Light and Optics 1 Sep 2nd 2009 10:42 AM
Article from Physical Review Letters alessia_dip General Physics 0 Aug 14th 2009 05:44 AM
Physical quantity that is conserved? sensei Advanced Mechanics 1 Oct 10th 2008 12:01 PM
Physical Constants topsquark Physics Resources 1 Aug 18th 2008 09:43 AM

Facebook Twitter Google+ RSS Feed