Re: Thermal Energy

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Thu, 7 Mar 2002, Hugh Haskell wrote:

> ... It seems to me that the reasonable criteria are: any
> energy associated with the translation of the center of mass
> and any net angular momentum (that is gross translation of the
> system) should be excluded from the thermal energy. only that
> translational motion which is truly random (that is, does not
> contribute to any motion of the center of mass), and
> rotational motion which is truly random (that is, does not
> contribute to the net angular momentum of the system) should
> be considered when figuring thermal energy.

Consider the following two systems:

1. Two identical particles attached at the ends of a spring and
oscillating linearly.

2. To identical disks sharing the same axis, joined by a coil
spring, and oscillating angularly.

There is no contribution to motion of the CM and no contribution
to the net angular momentum of the system in either case.  So do
these oscillations represent thermal energy?

If you try to exclude these two situations by adding simple
vibrations to the list, I'll suggest you consider a bell which has
a large number of relatively low frequency relatively simple
vibration modes and an increasing density of increasingly complex
vibration modes at progressively higher frequencies.  When first
struck, the bell oscillates with a *lot* of energy in the low
frequency modes and less and less energy in the higher and higher
frequency modes.  Will you consider all of this to be thermal
energy?  If not, where do you draw the line?

I mention all of this only to show that "thermal energy" is a
tricky if, I do believe, still useful concept.

John Mallinckrodt      mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm