### Re: Hair dryer in a can?

Date: Mon Jul 17 09:26:32 2000
Posted By: Michael Weibel, Battelle Chemist
Area of science: Engineering
ID: 962546721.Eg
Message:

Heat can be thought of as "thermal energy".  For example, if you have a
cool gas (room temperature air) and a hot item (heated pan), the air will
pick up some of the energy from the pan and the pan will lose some energy
to the air.  This occurs until thermal equilibrium sets in, when the
temperatures are the same.  Because a pan is small, relative to the amount
of air in the room (and the air's ability to absorb energy), the room
doesn't get appreciably hotter.  This isn't the case when you use an oven,
however, where your heated item is large and generates lots of heat.

Now, let's imagine this room temperature gas under pressure.  You've
already noted that things seem to cool down when gas under pressure is
released.  This is true.  It can be shown that:

(T2/T1)= (P2/P1)^ (gamma)

Where T is temperature in kelvin, P is pressure (any units), gamma is the
ratio Cp/Cv (= 5/3 for monoatomic gases and 7/5 for diatomic gases)
(reference:  Methods of Experimental Physics, Vol II Atoms and Molecules,
F.B. Dunning and R. Hulet Eds.)

Then, as the pressure drops, the temperature of a gas drops as well
(because of a requirement for conservation of entropy).  To generate heat,
you'd have to reverse the process, adding energy.

To generate heat, you could use a chemical process (disposable hand
warmers like you find at the outdoor recreation stores or a combustion
reaction:  both liberate energy stroed as chemical bonds), or a physical
process such as friction (bend a piece of plastic back and forth for a
while and it will heat up).  An electrical process is a physical method,
as electrons dissipate power through a friction-like mechanism in a
resistor.

Hope this gets you started...
well, and apply it to a problem or question.  This is how many inventors
find success.  For example, you noticed that air is cooled upon
expansion.  Maybe think of a way to use that process, rather than try to
make something happen which doesn't naturally.

Good luck,
Mike

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