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From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: comp.sys.ibm.pc.misc,sci.electronics
Subject: Re: Peltier effect device
Keywords: cooling
Message-ID: <13385@milton.u.washington.edu>
Date: 22 Dec 90 11:06:09 GMT
References: <1990Dec14.213730.10078@spool.cs.wisc.edu> <5070@optilink.UUCP>
Organization: University of Washington, Seattle
Lines: 38

In article <5070@optilink.UUCP> cramer@optilink.UUCP (Clayton Cramer) writes:
>
>A Peltier effect device is essentially a thermocouple run backwards.
>
>I can visualize the process by which heat causes electron flow from
>one metal to another (since different metals have different electro-
>negativities) -- for some reason, I can't picture how electron
>flow causes cold.

	You're SO close!  Stand back and look at what you just said.
Heat causes electron flow from one metal (carrier-rich) to another 
(carrier-poor), by diffusion from high carrier concentration to low
carrier concentration.  This diffusion, since it spontaneously
creates a charge separation, requires energy.  The heat
that CAUSES this electron flow is used up thereby.
	Entropy still increases, because the charge carriers
are spread out more evenly (diluted, as it were.)  Osmosis
is a VERY similar phenomenon.
	It is the difference in charge carrier densities at
the two disparate temperatures that drives the complete circuit
as a thermocouple, by the way.  The back-EMF caused by the charge
transfer would otherwise equilibrate at a relatively low voltage,
and only if mechanical work (lifting one material off the other)
is done on the charge would any significant energy be noticed.
That is the principle of rubbing materials together to generate
a static charge (and the original 'electricity' phenomenon.)
	
	All this is actually very close to common elementary
electric experimentation that we all did in our youth.
	It is common nowadays to show the charge separation in
the absence of rubbing, by the way (to clarify the fact that
friction has nothing to do with it).  The usual technique is to
pull a piece of sticky tape off of another (identical) strip
of tape.  The sticky-side has different charge density from
the slick side, so the two strips, once separated, have opposite
charges.
	
	John Whitmore