# I Two or three types of heat transfer?

#### seb7

Heat transfer: Conduction - Convection - Radiation, but I was wondering if conductive transfer is actually radiation transfer, but at contact distances; well not really contact since no material actually touches each other (at a quantum level).

So, is conductive heat actually being transferred by radiation?

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#### BvU

Science Advisor
Homework Helper
Yes. So is convection -- with a little help from gravity.

#### DrClaude

Mentor
So, is conductive heat actually being transferred by radiation?
No! Conductive heat transfer is through direct contact because of, e.g., coupling (for example, in a solid where local vibrations propagate to neighboring lattice sites) or free electrons in metals. No radiation is ever emitted or absorbed.

#### BvU

Science Advisor
Homework Helper
The governing force in collisions between molecules is electromagnetic, hence my slightly ironic first reply.

#### DrClaude

Mentor
The governing force in collisions between molecules is electromagnetic, hence my slightly ironic first reply.
Fair enough. The irony was lost on me.

Indeed, what is convection but conduction + gravity?

#### sophiecentaur

Science Advisor
Gold Member
This is another classification question which can be useful in as far as it makes you think. But fitting actuality to arbitrary rules that are learned by rote is a bit of a dead end activity.

#### BvU

Science Advisor
Homework Helper
@seb7 : keep wondering !

#### Dale

Mentor
So, is conductive heat actually being transferred by radiation?
I would say no. The reason that we split heat into conduction, radiation, and convection is that different equations are used. You could say it is all radiation but then you would have to distinguish between contact radiation and distant radiation in order to use the right equations

Since we have to make that distinction anyway, we may as well use the words “conduction” and “radiation” to do so.

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#### sophiecentaur

Science Advisor
Gold Member
The reason that we split heat into conduction, radiation, and convection is that different equations are used.
That's a good observation and effectively sorts out the problem.

#### nasu

The governing force in collisions between molecules is electromagnetic, hence my slightly ironic first reply.
Electromagnetic interaction is not equivalent to "radiation". The atoms in a solid transfer thermal energy without emitting any electromagnetic radiation.

#### BvU

Science Advisor
Homework Helper
Care to elaborate ? Subtle differences between virtual photons and (almost) on-shell photons ?

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