Message text written by "Jeffrey J. Barteet"
>And for purposes of comparison, is a material's ability to conduct heat
ALWAYS directly porportional to conducting electricity? Is there any
material out there that's great at conducting heat, but not electricity or
vice versa?
<
Let's see if I remember my junior year physics correctly.
Yes. The same phenomenon is at work here. Heat is a measure of electron
activity. As a material heats up the electrons move faster and faster and
jump to higher energy orbits. In a conductor the electron orbits of
adjoining atoms overlap and the electrons can pass from one atom to
another. When heat energy is transfering the electron movements are at
random and the net movement of electrons is zero. Electical conduction
works the same way except with the introduction of electromotive force
there is a net electron flow from the negative terminal to the positive.
Heat transfer in insulators is accomplished by the movements of electrons
of one atom generating electromagnetic fields which excite the electrons of
near by atoms. This phenomenon also takes place in conductors so heat
conductivity and electrical conductivity is not exactly proportional but it
is generally so. This is a slower process and explains why the best heat
conductors are also good electrical conductors.
If my memory serves me right. It's been almost 30 years.
>And finally, is the property of a material to conduct heat also make it a
>good property to dissipate heat?
That depends on what you mean by heat dissipation. In the truest sense of
the word heat dissipation and heat conduction are the same thing. But I
think you are refering to a material's ability to serve as a backing
material in a welding operation. In this case a material's melting point
and the ability to absorb heat from an adjacent material (in this case
sheat metal) come into play. In this case the material's thickness is a
factor as well as its shape. There are too many vairables for a succinct
scientific answer. Perhaps a welder is more qualified to answer.
>Every heat sink I've ever seen was made of aluminum. And I don't think
>they did it for weight savings. Maybe cost savings?
Yes. I've seen copper heatsinks but not very often. Aluminium is a
cheaper material but more importantly it is easer to form into heatsinks.
>And for the most part, the VAST majority of wires I've worked with were
>copper, or an alloy that was mostly copper. As I recall, back int the 70s,
>they had a disasterous experiment using aluminum core wiring for homes.
>The result was a lot of melted wires where they met copper termininals.
In theory a thicker aluminium wire will condict electricity as well as a
copper wire. Note that the problem was at the terminations. This relates
to not the materials ability to conduct electricity but rather on of it's
other mechanical properties, mallebility. Aluminium is more malleable than
copper. Copper will maintain a pressure against a screw whereas the
aluminum would eventually accept its new profile. When this happened the
connection was less tight and the gas-tight connection is compromised and
ohmic connections result. Electric utilities use aluminium wires in their
distribution systems all the time but the connections are a bit more than
wrapping it around a screw and tighening.
>If it's a super-important installation, we find gold contacts which I
>believe is the BEST conductor of electiricty, but to the best of my
>knowledge, nobody has ever made a radiator of gold.
The real reason gold is used is for it's corrosion resistance. Gold is too
expensive for anything more than that. In a PBS program about gold they
made a frying pan of gold and let Julia Child cook with it - to get her
opinion. Whan she was done they took it back and melted it down again.
(She said it cooked very well)
Now, who want's an explaination of the enthalpy tables?
Dave
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