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Q & A: How Different Metals Conduct Heat

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Most recent answer: 12/21/2012
Q:
Why do some metals conduct heat better than others?
- Vasken (age 9)
Clovis,CA,USA
A:
Vasken -

First, let me explain why metals generally conduct heat better than other solids do. In metals, some of the electrons (often one per atom) are not stuck to individual atoms but flow freely among the atoms. Of course, that's why metals are such good conductors of electricity. Now if one end of a bar is hot, and the other is cold, the electrons on the hot end have a little more thermal energy- random jiggling- than the ones on the cold end. So as the electrons wander around, they carry energy from the hot end to the cold end, which is another way of saying they conduct heat.

Of course, how fast they conduct heat depends a lot on things like how many free electrons are around, on how fast they move, and especially on how far they usually go before they bump into something and change direction. Those are the same factors that determine how well the metal conducts electricity. So there's a rule that works very well, saying that the thermal conductivity of a metal (at some temperature) is proportional to the electrical conductivity. That's convenient because it's much easier to measure electrical conductivity than thermal conductivity.

So now I'll get a little closer to answering your question. The biggest factor giving different conductivities for ordinary metals is the difference in how far the electrons go before they hit something. It turns out, for amazing reasons connected with the wave nature of electrons, that they can flow right through a perfect crystal without bouncing, the same way light travels through a clear crystal. Lots of metals (stainless steel, brass, etc) are alloys of several elements, and the electrons bounce off all the irregularities in the arrangement of the different atoms. So those aren't good conductors. Even in a pure metal, the electrons still bounce some, because the thermal jiggling of the atoms keeps them from ever forming a perfectly exact crystal arrangement.

Mike W.

(published on 10/22/2007)

Follow-Up #1: heat conduction of alloys

Q:
Why do elements conduct heat better than alloys ?
- Joe (age 13)
USA
A:
If you compare a metallic alloy with the pure metals itís made of, youíre right that the alloy tends to be worse. Thatís because the heat is conducted around by waves- mostly electron waves but also some sound waves. The variation from one type of atom to the next in an alloy makes for a sort of bumpy environment, where the waves bounce around instead of traveling a long way in one direction. So they donít conduct heat as well from one place to the next. The same principle works pretty well for insulators, where the heat is carried around just by sound waves, not by electron waves.

Of course, some of the biggest differences are not between different metallic alloys but between metals as a whole and insulators as a whole.

mike w

(published on 10/22/2007)

Follow-Up #2: Stir tea and see

Q:
what practical could you do to prove copper is a better conductor of heat compared to steel iron and zinc?
please give a list of apparatus and a fair test
- Anonymous
england
A:
Apparatus:  One silver spoon (or copper if you can find one) *
                   One stainless steel spoon
                   One cup of hot tea
                   One spoonful of sugar (optional)

Procedure:  Brew tea and pour into cup.  Stir first with stainless steel spoon, then with the
                   silver or copper spoon.   The sensations in your fingers should convince you                            one  way or the other.

LeeH

*N.B.  A silver plated spoon won't give as good results, it should be silver through and through

(published on 10/22/2007)

Follow-Up #3: Metals good at conducting heat

Q:
which metals are best at conducting heat?
- helena (age 14)
uk.
A:
pure silver, copper, and aluminum are good
Mike W.

(published on 10/22/2007)

Follow-Up #4: vibrating solids

Q:
when particles in a solid gain more (heat) they vibrate more and as a consequence collide more often, what effect does this have?
- Lydia (age 13)
UK
A:
It's true that the vibrations are bigger in the hotter solid. However, there's no clear distinction between 'colliding' and 'not colliding'. The reason is that the atoms are all exerting forces on their neighbors all the time- that's what makes the solid crystal rigid. When they vibrate, those forces fluctuate up and down.

Of course increasing the vibrations (raising the temperature) has many effects. If the temperature is raised enough, the atoms move around too much to stay in a regular arrangement, and the crystal will melt. Before that, all sorts of different effects (softening, loss of magnetism, increase in electrical resistance, etc. ) can happen as the temperature increases.

Mike W.

(published on 04/29/2009)

Follow-Up #5: Conductivity of metals?

Q:
of theses five, in what order would they be in for best conductor? copper lead steel brass aluminium? a list please, not just teh best one
- steve jones (age 14)
England
A:

See:     http://www.coolmagnetman.com/magcondb.htm

(published on 06/29/2010)

Follow-Up #6: cooling a car engine

Q:
Most automobile engines transfer the heat resulting from combustion into a "coolant solution" (ie- Glycol/Water). If minute particles of a conductive metal were suspended in the engine oil, why couldn't this oil replace the coolant solution?
- John Caulkins (age 65)
Cleveland, Ohio, USA
A:
The key role of the coolant is to get the heat out of the engine. It carries the heat to the radiator, where it can get dumped into the rapidly flowing air. Increasing the thermal conductivity of the oil wouldn't help, since the oil doesn't have much contact with the outside world.

Even if someone were to devise an engine in which there was a lot of oil flow out to some sort of radiator, adding bits of metal to the oil would be a bad idea. Single metal atoms or tiny clusters of a few atoms wouldn't help the thermal conductivity significantly, since their conduction electrons are confined to the metal itself. They don't help carry heat between the particles. Bigger clusters would defeat the whole purpose of the oil by scratching up the working surfaces of the engine.

Mike W.

(published on 12/21/2012)

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