Index Home About Blog
From: Steven B. Harris <>
Subject: Re: Helium Trivia
Date: Sat, 04 Apr 1998 19:06:44 GMT

>   Another way of looking at it, is sound is transmitted by atoms
>bumping into each other in chains, like a billiard ball shot.  How fast
>that happens will depend on how fast they move, but shouldn't depend on
>how many there are, or how packed they are (perfectly elastic
>collisions).  And indeed, temperature is the only variable which
>controls how fast molecules of a given weight move.  The speed of sound
>is a given fraction of the average speed of molecules in a gas
>(generally about 70%).
>   Now all this isn't QUITE right because of the fascinating
>contribution of gamma.  There is one other way for the energy of a
>sound wave to be absorbed in an adiabatic compression, besides heating
>the gas and giving the molecules a kinetic kick in the butt.  Some of
>the energy in diatomic gases goes into rotating the molecules.  So
>sound goes slower in diatomic gases of the same mass, because some of
>energy is absorbed in molecular rotation, and each compression wave
>doesn't heat the gas as much, and kick the molecules as hard into
>translation.  Thus, sound travels faster at the same temperature in
>helium (mol. weight 4) than it does in deuterium (mol. wt 4).  This is
>one of the things that makes physics so neat.
>                                        Steve Harris

      I will add a bit about a physics demo which impressed me
mightily when I was a an undergrad student.  The physics lab had
a bottle with a rubber cork in it, and stuck into the cork was
a very finely made glass tube of non-varying diameter.  In the tube
was a ball-bearing JUST slightly smaller than the tube diameter, so
that the bearing could move freely, but not allow any gas past it.
When the bearing was dropped, it underwent oscillation in the tube,
as on a spring, due to compressing the gas in the bottle, than being
pushed up again.  The gas here was acting like the spring (as in gas
shock absorbers).

    Now the fun part was that you could time the oscillation and infer
the force constant involved from the mass of the ball.  THat, with the
area of the tube, told how the pressure changed with volume in this
adiabatic compression.  From that, you could calculate gamma, the
ratio of heat capacities at constant pressure and temperature.  Which
in turn told us that the gas in the container was a monatomic gas, and
not a diatomic gas, like air.   Simply put, monatomic gases are more
"springy" on compression, because all of the compressive work goes into
increasing the gas temperature, and none into rotating gas molecules.
So from looking at this dumb ball bearing going up and down and
doing a little algebra, we could tell that somebody had put
an inert gas (helium and argon the only practical choices) into
the bottle.  An example of how all things are connected together,
and a lesson I never forgot.  (It was indeed argon).

                                 Steve Harris

Index Home About Blog