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Re: [Phys-l] teaching energy

p.s. One of the Amateur Scientist articles explains in detail why vulcanization is necessary to cause polymerized isoprene to be elastic. Polyisoprene alone has a rather low coefficient of restitution. Another article discusses the thermodynamics. I think I've already posted that one.

June 1960 Concerning Experiments with Rubber ...

April '71 Some Delightful Engines Driven by Heating of Rubber Bands

Bernard Cleyet wrote:


You gave it away. I wanted XX to discover it themselves.


Jeffrey Schnick wrote:

My students do the rubber band experiment when they study the first law
of thermodynamics. It works with small rubber bands of low quality too.
I think the effect is more obvious if you use an uninflated latex
balloon. I buy a bag of cheap party balloons the day the students do
the experiment and open it in their presence. (Who wants to touch an
old balloon to their lip?) Here is some more detail: Without
stretching the balloon, hold one end of the balloon in one hand and the
other end in the other hand so that the balloon extends from left to
right in front of you. The interface between your upper lip and the
facial skin just below your nose is quite sensitive to temperature
differences. Touch the balloon momentarily to that interface. Now
stretch the balloon rather quickly about as far you can comfortably hold
it stretched, and as soon as you have stretched it, while holding it
stretched, touch the balloon momentarily to the lip/face interface
again. You will notice that the balloon is warmer than it was before
you stretched it. Now, keeping the balloon stretched, wait 10-20
seconds and touch it momentarily to the lip/face interface again. It
will feel cooler than it did the previous time. Now quickly bring your
hands together to the point where the balloon is unstretched (but still
extending from left to right in front of you). As soon as you have
allowed the balloon to become unstretched, touch it momentarily to the
lip/face interface again. You will notice that it feels colder than it
felt at any other touch.

(Dr. Allen Wasserman, now a Professor Emeritus at Oregon State
University, had us do this experiment when I took his Thermal Physics
class at Oregon State in about 1983.)
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