From: pauld@exploratorium.edu
Date: Thu Feb 24 2000 - 10:17:17 PST
Message-Id: <200002241817.KAA17335@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole Sound waves part II Date: Thu, 24 Feb 2000 18:17:17 GMT
When one vibrating thing, the bell, is in contact with another non-vibrating
thing, the vacuum, the amount of energy transferred from one to the other
depends on the different density (and speed of sound) of the two materials.
Roughly the more different the two materials are the less energy goes from one
to the other. (Quantitatively, the product of density and speed of sound
matters.)
Now a bell is very different from air so only a little of the bells energy goes
into the air.
But the bell is even more different from a vacuum so even less energy goes into
the vacuum.
Most of the energy of the vibrating bell in a vacuum goes into heating the bell.
Only a little goes into moving the molecules of the vacuum.
If the mean free path of the molecules were greater than the bell jar radius
then molecules hit by the bell would carry that energy to the walls of the bell
jar without colliding on the way. However all bell jar vacuums are so bad that
the mean free path is quite short and the molecules will collide many times on
their way to the wall.
Paul D
> Thank you John and Paul for your replys to my question regarding sound waves and a
vacuum.
My student, however, had posed the
> question with respect to our bell jar demo in the classroom. He understood that at
great
distances, the sound would invariably be
> converted into heat before it reached the edge of our atmosphere. He wondered why the
last layer of atoms on the bell did not fly
> across the vacuum and strike the glass of the jar, and if not, what happens to the
energy
of the bell's motion?
> -- Adam Singer
> Marina Middle School
>
>
>
>
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