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Q & A: Direction of heat flow

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Most recent answer: 04/19/2011
Q:
We are having a discussion in our office here if heat transfer in a solid is affected by direction. If a steel rod is held in the middle in a verticle position - and heat is added at the bottom and at the top - will the heat transfer faster from the bottom or from the top. or -- if you have a large block of solid steel and heated it somehow from the very center -- would all directions heat identically?? I've always worked on heat rises -- I think I may be getting mixed up here though when pure conduction is in play?? Hope you can help - Thanks
- Jack (age 45)
Beamsville, Ontario, Canada
A:
The equations of heat flow in solids do not depend on which way is up. The direction of heat flow in a small element within a solid only depends on the temperature of its neighboring elements. Heat within the solid will flow from hot to cold.  It will cheerfully flow downward if it is cooler down there. Your intuition about heat rising is probably due to the fact that heated air does rise since it is less dense than cold air, for example in a hot-air balloon or thermal convection currents.

LeeH

(published on 05/16/2013)

Follow-Up #1: heat in solids

Q:
What happens to the particles in a solid when heated?
- Tanya (age 12)
Australia
A:
Nice question. I'll tell you what happens to the particles in a solid which doesn't get heated too much, so it stays a solid. (If you heat it more it will melt or evaporate.)

Even in a solid the particles are jiggling around. You can think of them as sort of like little balls connected to their neighbors by springs. What happens when you heat the solid up is that the jiggling gets stronger. The particles move faster and the little springy connections get stretched or squashed more.

Mike W.

(published on 04/19/2011)

Follow-up on this answer.