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Q & A: heat through wires

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Most recent answer: 10/22/2007
Q:
I read some of your questions and answers about heat conductivity, and noticed that metal is a good heat conductor. I have been experimenting with trying to conduct heat through different guages of metal wire, but the heat never travels very far through the wire from the heat source (flame). It seems to only travel for about 1/2-3/4 inch and that seems to take forever. Why isn’t the heat traveling on farther through the wire?
- Kayla (age 13)
Franklin middle School, Springfield Illinios
A:
The temperature increase is determined both by the heat flowing in one end and by the cooling of the wire by the air.
Probably the air convection is cooling the wires pretty well. You could test this idea by taking two similar pieces of wire and insulating a half inch of one with cloth, to see if you can then feel the heat farther down.

As for how long it takes the heat to move down the wire, that depends on the thermal diffusion coefficient of the wire and the distance from the source. The typical time goes as the square of the distance from the source. I would have guessed that at, say, 3/4 inch from the source the time needed for the wire to get as hot as it is going to get would be a few seconds. Is it really taking much longer? Perhaps the air cooling is really effective, and the wire is only heating up that far away by radiant heat from the flame. You could check that too by using a little aluminum foil shield to keep light from the flame from hitting the wire..

I do hope your being very careful with the flame, and using sturdy non-flammable little clips rather than fingers to hold things in it!

Mike W.

(published on 10/22/2007)

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