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Ideal Gas Law help please - Page 3

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Posted by J. Clarke on January 2, 2011, 2:36 pm

I should have mentioned it before, but if you're a new to thermo or it's
been a long time since you took the course, you might want to work
through the Schaums <(Amazon.com product link shortened)
Thermodynamics-Engineers-2ed/dp/0071611673/ref=pd_cp_b_1#_> as well.

Posted by daestrom on December 24, 2010, 7:15 pm
On 12/24/2010 12:38 PM, Morris Dovey wrote:

Actually, for chemistry and ideal gas purposes, STP is defined as 1 atm
and 273.15K (0 C), not 293.15K (20 C).

This is valid for 'ideal' gasses and only if the volume is held constant.

But in a lot of thermodynamics you have to consider a change in the
(n/V) term.  Then it gets a bit more complicated (but not too bad).

For a fixed mass of gas, we can get a couple of different formulas
depending on what process you're interested in.

For constant volume:
T1/T2 = P1/P2 (this is what you have above)

For constant temperature:
V1/V2 = P2/P1 (PV= K )

Both of the above however require that you add/remove heat as either T
or V changes.

For the case of expansion or compression where there is little time for
heat to be transferred in/out, (i.e. the process is adiabatic), we have:

PV^y = K

Where y is the ratio of specific heats (1.4 for air, 1.66 for monoatomic
gases and 1.3 for steam).  You can combine this with the ideal gas law
(PV=nRT) and find the temperature change during compression/expansion as
well as volume changes.  Substitute P=nRT/V into the above and you get:

nRTV^(y-1) = K
T1/T2 = (V2/V1)^(y-1)

Or substituting V=nRT/P, we get
T1/T2 = (P2/P1)^(1-y)/y

Good Luck


Posted by Morris Dovey on December 24, 2010, 8:21 pm
 On 12/24/2010 1:15 PM, daestrom wrote:

Ok. I found multiple definitions and used the one with which the 22.4
liter/mole figure was associated. Do I need to adjust that if I use 0C
as the Standard Temperature?

<grin> Understood, but I'm such a rank beginner that these are exactly
the conditions I'm struggling to establish as a baseline...

I kinda wondered about that, and I decided that I'd better wrap my head
around the most simple case first - then add complications as necessary.

So far so good. In what actually I'm working on, neither the volume nor
the temperature will be constant - but examining the beast with a
constant volume assumed seems like a good starting point.

Ah - you've just touched on one of the really big holes in my mental
model: time. I'm a long way (I think) from incorporating a t variable
into the process. Methinks I'm going to spend a fair amount of time
learning heat transfer basics to satisfy that need.

Hmm - forewarned is forearmed. This looks like Good Stuff so I've filed
a copy of your response for future reference. For the moment I'm going
to stick with dry air or pure water for modeling, although I'm beginning
to suspect that air will be of decreasing interest.

Many, many thanks!

Morris Dovey

Posted by J. Clarke on December 25, 2010, 1:09 am
Just a comment but if you're working with steam the "right" way to go
about it is to get a set of steam tables and work from them.  When I was
at Georgia Tech we had a full semester of doing nothing but working with
the steam tables.  That was going on 40 years ago though and I've
forgotten most of it.

The "Bible" is Keenan and Keyes--Amazon has used copies of older
editions for as little as four bucks and shipping.  Wark's
thermodynamics text also has a usable set IIRC and they've got those
used for around a buck.

There are also online calculators (google "steam tables" and you'll find
a bunch of them) and there's software available but it's not cheap.

Oh, and there's a very very short animation at
file=th020403f.swf&course=th&chap_sec.4> tht I think you might find
helpful--it shows (in metric) the deviation from the ideal gas law at
various points on the PV diagram for water.

Posted by Morris Dovey on December 25, 2010, 4:17 pm
 On 12/24/2010 7:09 PM, J. Clarke wrote:

Um, yeah - I know how that works (except in my case classes were even
farther back). This stuff is all new to me, so there isn't anything to

I have links to more tabular data than I really know what to do with.
What (I think) seems to be most useful are formulas that, at least
approximately, match the tabular data so I can get a gut feel for what
happens and produce an (evolving) software model.

I just finished ordering both.

Already DAGS and have downloaded PDF versions of some tables. I haven't
found any software yet to do the simulation I want. I've already figured
out that even my own home grown software is going to be fairly expensive
in terms of non-dollar resources required.

I've bookmarked it. Thank you - I hadn't seen the deviation portrayal

John, whether you've recognized it or not, you've provided some very
helpful input over a fair number of years - and I don't think I've ever
thanked you properly. Many thanks!

It was your commentary (in r.w) on how poorly your neighbors' solar
panels had held up that ultimately led to mine growing a tougher skin so
they'd last longer. It did take me a while to figure out how I wanted to
go about it, but you can see the result of your influence in the bottom
photo at the link in my sig. :)

Morris Dovey

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