# The Naked Scientists Forum

### Author Topic: The speed of heat.  (Read 5383 times)

#### paul.fr

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##### The speed of heat.
« on: 16/08/2007 15:53:14 »
Does heat have a speed? If so how fast does it go?

For example: If i am sunbathing, i know the light getting to me from the sun is some 8 minutes or so behind. Is the heat i am feeling from the sun also delayed? I am i actually getting tanned by something that's not here yet.

Hope that makes sense.

#### daveshorts

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• Physics, Experiments
##### The speed of heat.
« Reply #1 on: 16/08/2007 16:07:57 »
What you are referring to as heat from the sun is electromagnetic radiation heating you up. Visible light will do this to you but a lot of the heat comes in the form of infra-red radiation. This being electromagnetic radiation will move at the speed of light.

Heat moving through a solid is actually transferred by electrons moving about at random and phonons (Quantum particles of vibration or sound) phonons move at the speed of sound in the material, and electrons which move very fast.

#### syhprum

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##### The speed of heat.
« Reply #2 on: 16/08/2007 16:21:32 »
I know that electrons can move very fast in a vacuum but I wonder how fast they move in a metal conductor?.
Obviously when you apply a voltage to a wire individual electrons do not travel along it and come out the other end, a voltage appears at the other end because an electromagnetic wave travels along it at a speed determined by its inductance and capacitance at near the speed of light.
How far and how fast does a typical electron move between atoms

#### daveshorts

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##### The speed of heat.
« Reply #3 on: 16/08/2007 16:30:31 »
The overall dift velocity in a conductor is very small, of the order of mm/s, but the electrons are moving around thermally hugely faster than this

#### another_someone

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##### The speed of heat.
« Reply #4 on: 16/08/2007 17:08:37 »
We also have additional to this heat transfer by convection, which would be a different speed again.

#### lyner

• Guest
##### The speed of heat.
« Reply #5 on: 21/08/2007 18:36:00 »
Quote
The overall dift velocity in a conductor is very small, of the order of mm/s, but the electrons are moving around thermally hugely faster than this
Yes but only a very short way- and randomly. The 'speed' of thermal conduction is not likely to be any higher than for electrical conduction - I guess a few mm per second (effectively)  for heat to diffuse through a metal would agree with experience.
The unbelievable slow  drift velocity of electrons in an electrical circuit is justified by the fact that there are so many of them - it's a bit like the Newton's Cradle discussion, elsewhere. The energy passes through very quickly but the electrons (or ballbearings, in the other case), themselves hardly go anywhere - particularly with AC, where there is no net flow of charge at all .

#### The Naked Scientists Forum

##### The speed of heat.
« Reply #5 on: 21/08/2007 18:36:00 »