Conduction, Radiation, and Convection? Is that all there is?

[mm]

In high school physics, the three methods of heat dispersal were presented as conduction, radiation, and convection?

Is that all there is? Is diffusion a fourth or is it subsumed by convection?

I could be wrong but: Conduction seems to be limited to within a solid, or from the surface of a solid to that part of a liquid or gas in contact with the solid.

ICBWB: Convection seems to be limited to liquids and gases.

And ICBWB: radiation seems to be limited to from a solid or maybe a liquid through a gas to another solid or maybe a liquid.

As to convection, it was always described and seems to be limited to broad currents, such as hot air rising and cold air sinking, but is that all that happens? In, say, a room with moderate cooling in the summer or moderate heating in the winter, while in general the hot air rises, doesn't the random motion of some of the hot air cause it to go downward and to mix with the cooler air below it? Is this radiation? Is it still convection? Or is it diffusion and for reasons of definition, not one of the other three?

[mmeron]

Diffusion is conduction.

No, conduction is present in liquids and gases as well, though it is dominated by convection for significant temperature gradients (and in the presence of gravity).

That much is certainly true.

In principle radiation can be present within liquid or solid as well. In practice, under normal circumstances, its range is so limited there that it is indistinguishable from conduction.

Collective motion of masses of air is convection, random motion of individual molecules is conduction.

Mati Meron | "When you argue with a fool, [e-mail] | chances are he is doing just the same"

[zero]

Transporter beam in a pinch or properly used, a Deflector Dish.

HTH,

-zero

[Keith Williams]

I'd say conduction, rather then convection. Heat is transfered by the molecules bouncing off each other = conduction.

No, liquids and gases also conduct. Convection may be dominant though.

Sure, something's gotta move for convection. Solids don't move too well. ;-)

No, radiation occurs with all substances but at moderate temperatures can be dwarfed by conduction and convection. We see Jupiter (a "gas giant") by radiation, for instance.

Some by radiation, some by conduction, mostly convection though.

Diffusion normally relates to different gases/liquids. If they're the same I'm not sure how you track the molecules. They'll swap heat by bumping into each other (conduction).

[Igor]

That's about it. At least that's what's still being taught at all levels, these days.

Depends what you actually mean by diffusion. If you're talking about the ability of a gas to spread out to fill it's container, then yes, you're talking convection.

Conduction can occur in any medium. Remember it's just the direct transfer of heat energy through the media.

In normal, everyday situations, yes. But geologists talk about convection in rock in the earth's mantle, albeit very hot rock, but still technically a solid at the pressures considered.

Well, radiation can be generated in any medium and absorbed by any medium, regardless of state.

Basically. Convection is the transfer of heat by a moving medium, such as air.

Statistically yes, but on average, warm air will rise and cooler air will fall. It is thermodynamically possible for the very opposite to occur, but the probability of that happening is astronomically small.

Radiation is light, UV, IR, etc.

In your above example, it is practically all convection, since the air is moving and carrying the heat with it. There could also be some conduction occuring, as energy is directly transferred through the air between hot and cold regions,.and perhaps some radiation if sunlight is shining through a window or there is a light turned on.

[Pat]

Time travel, too. You could go back to before the heat.

For adding heat, don't forget laser beams, microwaves, and spontaneous combustion :-))

zero wrote:

[Andy Resnick]

That's a tricky question to answer. Partly, I don't understand what you mean by 'dispersal'. In any case, heat can be transported by material particles or by nonmaterial processes. That is, energy (heat) can be convected by the motion of hot particles flowing into colder regions, transported by infrared radiation, or transported by nonmaterial diffusion (conduction). Hot particles diffusing into colder regions is not typically found here on Earth due to bouyancy- hot stuff is less dense and so there is a bouyancy-driven flow of material which convects the heat.

Only when bouyancy effects dominate. In space, for example, conduction occurs in liquids and gases as well. Or if the liquid is of high viscosity (if the Nusselt number is small). Thermal greases will conduct heat rather than convect heat. There's also the Grashof number and Peclet number. If these numbers are large or small, a particular heat transfer mechanism occurs. For example, if the Grashof number is large, there will be large bouyancy effects. If the Grashof number is small, viscous effects dominate and the thermal energy will instead diffuse.

Yes, usually. Again, take away gravity and convection doesn't really occur. There still are some effects like Marangoni flow (surface tension-driven convection) that occur. It's an open question for granular media, IIRC.

There's a difference between thermal radiation and radiative heat transfer. You seem to be thinking about heat transfer here, not thermal transport.

[nicksanspam]

It's a part of conduction. Some people also add "phase change" as a method of heat transfer, eg evaporation from lower soil layers and condensation above.

Not much. Then again, room air may fall near an outside wall, slide across the floor, rise up near an inside wall, then slide across the ceiling to make a loop.

No, but that can also be important. Surfaces with mean absolute temp T (R) and emissivity 1 have a linearized radiation conductance 4x0.1714x10^8xT^3 Btu/h-F-ft^2, eg U1 (R1) for T = 400 + 70 (F.) = 530 R.

Nick

[Richard J Kinch]

"Thermal" radiation? What's that?

[Andy Resnick]

As commonly used, "thermal radiation" means radiation in the waveband from about 3 microns to about 20 microns, and represents the peak wavelength of blackbody radiation for most objects of interest- of interest to those using the term "thermal radiation".

As a general concept, the electromangtic field carries energy at all wavelengths.

[tadchem]

Conduction: transfer of heat from one molecule to its neighbor by means of thermal collisions.

Convection: transfer of heat from one location to another as kinetic energy of a group of molecules of a fluid. (usually driven by the effects of gravity on differential densities as in the Archimedes effect)

Diffustion: *also* transfer of heat from one location to another as kinetic energy of a group of molecules of a fluid. Distinguished from convection in that the random thermal motion of the molecules provides the transport.

Radiation: transfer of heat from location to another by means of electromagnetic radiation. No medium is required as radiation can traverse a total vacuum. [The sun sure is hot today, isn't it?] If a medium is present it must be at least partially transparent to the particular wavelengths of radiation involved.

Heat can also be transported by converting it to another form (typically electrical or chemical energy) top be placed within a material, which itself is transported (or in the case of electricity is electrically conducted) to another location where the heat can be released.

HTH

Tom Davidson Richmond, VA

[Richard J Kinch]

OK, it sounded like you were advocating some 19th century notion of radiant heat.

[CWatters]

How about some of the tricks used to achieve near absolute zero temperatures. I recall they are quite exotic and involve lasers and optical cavities and the like - but perhaps they all amount to conduction or radaition?

[Stormin Mormon]

I could be wrong but: Conduction seems to be limited to within a solid, or from the surface of a solid to that part of a liquid or gas in contact with the solid. CY: I'd think that conduction is possible within gasses,b ut much less so. Cause they would be more likely to convect.

ICBWB: Convection seems to be limited to liquids and gases. CY: Well, if you wanted to be a smartass, you could mention a semiliquid like sand. But that's not really fair.

And ICBWB: radiation seems to be limited to from a solid or maybe a liquid through a gas to another solid or maybe a liquid. CY: Sounds about right. Of course, radiation from the sun is supposed to be a superhot ball of gas (plasma??)

As to convection, it was always described and seems to be limited to broad currents, such as hot air rising and cold air sinking, but is that all that happens? In, say, a room with moderate cooling in the summer or moderate heating in the winter, while in general the hot air rises, doesn't the random motion of some of the hot air cause it to go downward and to mix with the cooler air below it? CY: yes, that would happen. Some air currents push hot air down.

Is this radiation? CY: No.

Is it still convection? CY: Pushing hot air down is still convection.

Or is it diffusion and for reasons of definition, not one of the other three? CY: Only three ways I know to transfer heat.

[Stormin Mormon]

Yes, convection stands there with her high heels, black leather and whip. She says to the conduction which is tied to the bed.....

Know what you mean, though. Since fluids vary in density with temp, that convection would be more a factor.

[CWatters]

Take a look at sealed double glazing.... The gap between the panes is choosen so that it's as large as possible (to mimimise conduction losses) but not so big that it allows convection losses.

It's worth noting that convection is only an issue because conduction occurs between the gas and glass. If there was no such conduction it wouldn't matter if the gas had convective flow.

[Neill Massello]

[mm]

Can't they tie little transmitters to them, like they do with wild animals?

I guess not.

I guessed radiation and convection and you and most of you said it's conduction, the only one I iddn't guess. Oh, well.

Thanks to you and everyone (except the trekkies ;) )

I didn't answer right away because I read alt.home.repair and it has so much traffic, I missed the whole thread for 2 days (until I remembered to go looking for it.)

[dpb]

mm wrote: ...

That's _natural_ convection, there's also _forced_ convection. Difference is whether there is an external causative factor for the fluid motion (fan, say) or only the thermal gradient.

[David Combs]

...

...

Nope -- no "ether" or anything else required for radiation to travel through (eg, onto you).

It's "electromagnetic" radiation, exactly the same as for light from a candle or light-bulb or flourescent light, heat and light from the sun, or radio or tv transmitting-antenna.

Travels best, and even fastest, through a vacuum.

Cheers!

David