# Some confusion between radiation and conduction

#### klng

"I pour hot water into a metal container. The inside surface of the container gets hot."

It is quite clear the inside surface gets hot due to conduction (criteria of process is satisfied).

What i am not so sure about is whether radiation is involved. Theory and some gut instinct tells me there should be. But if this were the case, then wont radiation be present (though it may not be the doninant process however) in all thermal applications which involve conduction as well?

Thanks in advance for the guidance.

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#### magpies

Is the container made out of uranium lol?

#### Mapes

Homework Helper
Gold Member
Is the container made out of uranium lol?
king is referring to electromagnetic radiation, which includes visible light, infrared, etc., and which is a valid mechanism of heat transfer. Yes, radiative heat transfer is always present, though it can be assumed to be negligible in many situations. It's generally negligible for the cases of heat transfer through metal or heat transfer through air between objects at less than several hundred °C, for example.

#### klng

Hi Magpes,

I can understand why you said radiation through metals is negligible, as there are many mobile electroons which will make conduction the predominant process.

But i cant fully understand why radiation through air is negligible if the temp is less than several hundred degC. Is there a benchmark temp when radiation becomes significant?

Also take a look at the following example:
A bunsen flame is applied at one end of a metal rod. Note the flame is directly touching the metal rod (ie no air space between flame and the rod). Is the main transfer process between flame and metal rod conduction or radiation?

Had there been an air space in between,we can argue it is radiation, since air is a bad conductor of heat. But w/o the air space, it gets kinda puzzling. But if i were to make a bet, i will still bet my money on radiation.

(",)

#### Mapes

It's something you can calculate: the heat flux from radiation is $\sigma \epsilon (T^4-T^4_\infty$), where $\sigma=5.67\times 10^{-8}$ and $\epsilon$ is the emissivity. Compare to convection, $h(T-T_\infty)$, where the convection coefficient can be around 10 W m-2 K-1 for natural convection or much higher for forced convection. I shouldn't have said negligible, though; let's say instead that radiation isn't the dominant mechanism for temperature differences less than a hundred degrees.