From: Ronald Wong (ronwong@inreach.com)
Date: Fri May 17 2002 - 00:12:26 PDT
Message-Id: <l03102802b909d0e0e8ec@[209.209.20.84]> Date: Fri, 17 May 2002 00:12:26 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Generation of heat
>Subject: Generation of heat
>From: "Jhumki Basu" <sjbasu@hotmail.com>
>Date: Thu, 25 Oct 2001 16:12:41
>
>Hello!
>
>Can someone explain why contact or collision between objects produces heat?
>What's happening at a molecular level?
>
>Thanks,
>Jhumki Basu
Jhumki:
YOU may have come across the answer to your question by now, but last year,
when you posted this question on Pinhole, John Lahrs replied by suggesting
an activity that would demonstrate this effect and Paul Doherty responded
by giving an interesting explanation behind John's demonstration.
Nobody seemed to have come along and answered your question, "why contact
or collision between objects produces heat?".
A nice analogy - remember, it's only an analogy - is to think of the
objects as if they were made up of a very large collection of
randomly-distributed, little spheres ("atoms/molecules") connected to one
another by springs that are open coils (so that the springs can be
compressed as well as stretched between the spheres).
If you started to hammer away on one of the "objects" with a very large
board, the little spheres at the point of contact would get knocked about
and, through their ties to their neighboring spheres - by way of the
springs - start setting their neighbors into a state of random vibration.
Their neighbors, in turn, would set their neighbors into a state of random
vibration and so on until the entire block's population of spheres would be
vibrating away in a random fashion. The average kinetic energy of the
spheres will have increased from zero to some finite value. In other words,
their temperature would have gone up (in this case from an initial
temperature of "absolute zero").
Have your kids take a hammer and drive a large nail into a block of hard
wood.
If they check the temperature of the nail before they start hammering
(simply pinching it between their fingers is usually sufficient) with the
temperature immediately after they've been hammering on the nail for a
while, they'll discover that it has gotten noticeably warmer. The KE of the
hammer has been converted in to thermal energy (actually, the nail has lost
a little PE and that too has gone into heating the nail and the wood as
well).
The only difference between this demonstration and the analogy is that the
atoms of the nail were already vibrating randomly before your students
started hammering away on it. In other words, the temperature of the nail
before the hammering was not at absolute zero but at room temperature.
Collisions are nothing more than one object hammering away on another.
If the "springs" are really stiff, there isn't much randomizing. Most of
the original energy of the system will be found in the KE of the two
objects after the collision and very little in the increased, random
vibrational energy of the objects' atoms/molecules (this is what happens
when two steel spheres collide with one another). In other words, the
temperature of the objects doesn't increase very much.
If the "springs" aren't as stiff, more randomizing occurs and there is
greater increase in temperature (and deformation) as a result of the
collision. More of the system's energy will be found in the random, thermal
vibrations of the atom/molecules of the objects and less in the KE of the
two objects after the collision (try throwing two "springy" sponges at each
other and see how well they bounce off each other).
Just remember not to get too carried away with this analogy.
Paul's remarks regarding the transmission of sound through the spheres in
Lahr's demonstration is directly related to the type of "spheres" and the
stiffness of the "springs". The speed of sound in solids is directly
related to these two factors. There is a "randomness" component too but
it's usually considered to be negligible. The fact that the sound wave can
be focused back to the point of contact due to the geometry of the spheres,
thereby concentrating the energy produced by the collision at the point of
contact, is an interesting one.
ron
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