Natural Science Forum / Physics / General Physics / July 2008

On Jul 13, 3:11 pm, Agent Smith <agent-sm...@two-blocks-on-your-
left.com> wrote:

Actually, most scientists simply call it "The Perfect Gas Law."  It's
expressed:  PV = nRT

Quite right.  Gamma is not employed no nuseful where the Perfect Gas
Law can be applied.

Wrong, look up Boyles Law in an Physics 101 text.

It may also be taught there, but usually it is taught ealier than
when students usually study Thermodynamics. As a undergraduate physics
major I was required to take Therm as 3 separate course from three
different departments, Physics, Chemistry, and Engineering. Thermo as
taught by the Physics and Chem departments were virtually the same,
except used slightly differents symbology for Entropy, Enthalpy, Free
Energy and other termo parameters.  There was no coverage of the gas
laws, because they were assumed to be alread know by majors in these
subjects.  Thermo as taught by the engineering department was of a
very different flavor, and focused more on things like gas laws and
simple physics problems, and less on what I consider to be formal
Thermodynamics,

Actually, I did provide you with a rather complete anwser.  I did a
double check and it appears in both of the two best know Physics 101
textbooks ued by major universities.  The are:

University Physics, by Sears and Zameski
Fundamentals of Physics by Halliday and Resnick

It also appears in:
Elements of Phsical Chemistry, by Samual Glasstone

If you wish, I can give you chapter and page in each of these classic
texts.

I think it's trivial, but then I'm a physicist...not everyone posting
in sci.physics is.
Not trying to sell you short, but you evidently have not made any
effort at all to find this informaton on your own, or even tried using
Google.

If this is level of research you are willing to put in to learning
physical science or egineering, consider a major in retail sales!   :-
(

Harry C.

Both equations apply to ideal gases.  The equation pV^gamma=const applies
only to adiabatic compression or expansion; that is, expansion/compression
in which no heat is going into or out of the gas.

I have no idea if it has a name -- if I had to call it something, I would
probably refer to it as "the equation for adiabatic compression/expansion"
or something like that.

It's apparent he's talking about pV=nRT while you're asking about
pV^gamma=const.  This sort of misunderstanding happens a lot here.

Depends on the gas.  It's 5/3 for monatomic gases, 7/5 for diatomic gases,
and other values for other types or for mixtures.

There's a nice freshman-level experiment to measure it in which you start
with a big flask full of air with a rubber stopper at the top connected to
a vertical glass tube.  Then you take a metal ball with the same diameter
as the tube, drop it in the tube, and measure the period of oscillation as
the gas contracts and expands beneath the ball.

You can also work it out from statistical mechanics; you need not just the
ideal gas law, but also the expression for the internal energy of the gas,
which depends on the temperature and how many internal degrees of freedom
the molecules have.  So by a macroscopic measurement of gamma, you can,
without too much work, learn something about the structure of the gas
molecules.  I think that's pretty cool.

Sorry I can't be of any help with the name.

On Jul 13, 8:39 pm, John Schutkeker <jschutke...@sbcglobal.net.nospam>
wrote:

Perhaps. I can't find it in the literature. It is not commonly
credited to whomever did it.

It is derived directly from the ideal gas law. If you do not know how
to do that, then I suggest you pick up any calculus-based introductory
physics book. If you have one in your possession, I might be able to
suggest a section reference.

PD

On Jul 13, 9:39 pm, John Schutkeker <jschutke...@sbcglobal.net.nospam>
wrote:

PD, I believe yoou're correct on both points.  In fact, if you check
an edition of University Physics prior to around 1975 or even the 1986
Second Edition of Halliday & Resnick, you won't find the gamma
expression yet being used.  I haven't yet checked any of my
Thermodyanics texts, so that it may be in more common use in that
field.

The simple fact is that except for some fairly new observations made
in Statistical Mechanics (which is in itself can produce some pretty
amazing things, the subjects taught at the Physics 101 level have not
changed over the past 100 years, and much of it has not changed at all
since the days Newton and Boyle. Still, every new kid that comes along
want to change the terminology used with, so far as I can tell, no
value added. Also, every physics professor that earns tenure is
writing his own textbook. (No, I'm not kidding.)

Out of couriousity, I on the the explanation of the Perfect Gas Law/
Ideal Gas Law (your choice of terms) in Halliday & Resnick, and while
that in Sears and Zemansky is based on the classical gas laws of Boyle
and Gay-Lussac, that in Halliday Resnick first cites the principles of
Statistical Mechnics nearly entirel, specifically, the work of J.
Williard Gibbs (1839-1903) and Ludwig Boltzmann (1844-1906) but this
is mostly name mentioning.  They then on page 408 run though a very
brief and overly simplified explanation of how PV = nRT based once
again on the laws of Boyle and Gay-Lussac with virtually none of the
math shown.

Now I believe Tadchem cited Marks as referencing adibatic situations,
which is correct, I'm personally not aware of Mark's publications,
although I believe we works in the field of Mechanical Engineer
primarily.  I'm not even sure about that.  Then too, I may be thinking
about the "Marks Handbook" used in machine design.  Please let me know
if this is incorrect.

For those rare individuals whose interests in physics are spurred by
this thread, you should own a recent edition of University Physics
(Sears and Zemansky) or Fundamentals of Physics (Halliday and
Resnick).  Now here is a what I hope will be a helpful hint.  Both of
these books run well over 1,000 pages, an sell new for generally over
$100 each. (Check the prices for new copies on Amazon.com.)  Now here
is the secret for saving a good bit of money.  The basic physics
contained in a Physics 101 text has not changed over the past 100
years!  You can purchase used copies in good condition from Amazon for
1/2 this price, or if you live near a majory university, visit their
bookstore where you can often find students selling last years edition
of the text for a little as 1/4 the current price. Second, for anyone
with very serious interest in physics, try to find a pre-1975 copy of
University Physics, if you understand an introductory level of
calculus, because the explanations and derivations of basic equations
ten to run much deeper than those in more recent editions where basic
content was edited to make room for useless explanations of new
physics.  Beware of a book called College Physics by Sears and
Zamansky, because this is primarily a watered down version of
University Physics also written by Sears and Zemansky, but designed
for use by non-physical science majors lacking math skills.
Fundamental of Physics (Halliday & Resnick) is a good general Physics
101 text, but whose content is critcised for having enchanced its
market by including a lot of grapics white space and useless
photography at the expense of its basic physics content.  Again, this
is simply my opinion since I own both and have taught and tutored
using both.   Again, these are simply my personal opinions.

Harry C.

On Jul 13, 9:39 pm, John Schutkeker <jschutke...@sbcglobal.net.nospam>
wrote:

John, realize that when I precede or follow a statment with a caveat,
readers should pay attention. I'm now retired now, but have spent
roughly 50 years of my life working in physics and technology, and I'm
pretty familiar with the "street language" of the professional crowd.
This is often not the same terminology used in academic teaching
circles.  Also yes, every decade or so I've watched the terminology
change a bit, yet never with any significican "value added".  This is
why I, like my fellow who work in the trenches on applied physics use
terms like "Perfect Gas Laws" and particles energies measured in Bev
and Not Gev, know precisely what each other means. Saves time at
converences and in casual converstations. Trust me, it's not
ignornance and we are a bit more careful in our choice of wording a
paper for formal presentation and publication. For these papers, I'm
just damn thankful that I don't have to write them in Latin, as Newton
did. Tough to believe, but even as and older physicist, my German is
nearly as bad as my Latin!  :-)

Still, my point is:  If you understand the physics concept and learn
how a working formula is derived, don't sweat it formal name or who
originally derived it, but still be generally aware of it's history
and origins. I've spent most of my career working with theoretical
mechanics (usually referred to a Tmech, and electromagnetic wave
theory (called simply EM by working physicists).

I tend to believe that the world would be left in pretty poor shape if
the world were left entirely in the hands of the academics and their
formal definitions.  It simply does not work that way in the real
world!

Hate to tell you this, but except in a few rare situations (my
apologies to NIST), but there are few things in science or engineering
that require more than 3 decimal places behind the decimal point, and
most practical computations can be performed with slide rule accuracy,
if you remember these were.  If when I was just starting out we needed
the values of physic constants of almost anything to more this
accuracy, we simply looked them up in a muli-volume set of books whose
data was develped by the National Bureau of Standards (NBS), at least
IIRC. NIST is, I believe their successor and is funded at roughtly 4X
the budget of the NBS, but produces less than 1/8 of the results once
delivered by the NBS.

Ok, explanantion and rant concluded.  Here the caveat: This is simply
my opinion.

Harry C.

p.s., Note that I cite my sources to prevent a lot of arm waving
baseless objections.

On Jul 13, 9:39 pm, John Schutkeker <jschutke...@sbcglobal.net.nospam>
wrote:

I agree that this would be an intresting thing to learn, if nothing
other than historic importance.

John I believe I previously cited the origins of PV = nRT, and gave
you a textbook reference. I even posted the value of R in the MKS
system.

If your question is who first combined the Laws of Boyle with those of
Gay-Lussac, my guess it that the merger between these two laws to
acuire the result PM = nRT, the Perfect Gas or equally known as the
Ideal Gas Law were so multiple and relatively conincident, what who
did this first is incidental and to my knowledge has never been
documented.

That said John, given the scientific ego, it is likely that someone
did publish a paper linking these two separate Laws to create the
Perfect Gas Law, but out of curiousity after finding a reference to
the combiner in my textbooks at hand, I spent two hours in at the MIT
library searching for the answer and searcher their extensive library
of journals without locating any hits.  Harvard's library goes back
much farther than that at MIT, so when I have a chance, I'll run the
same search there.

John, you have tickled my curiousity about this, and given enough time
I will find out the name and a citation to the paper. Tomorrow, I will
contact a close friend at Princeton, and have him search on the
subject at Princeton's Firestone Library (whose records go way, way
back.)

Now I can predict what will happen next, knowing this guy quite well.
Your question is likely to spread like a computer virus to the major
scientific library thoughout the world.  Likely, a paper citation will
be ultimately revealed, if one exists.

John, you have raised a very good and interesting question,
particulaly since PV = nRT is among one of the most basic
relationships know to both physicists and engineers aside from F =
MA.  Still, the answer may be well like that to the question of who
invented blotters.

Harry C.