Unravelling Heat Transfer: The Mystery of Conduction & Convection

[permagrin]

I am confused as to how heat transfer really works. I've read all about conduction/convection/radiation. Radiation makes sense to me, electrons enter a higher band, come back down and give off energy in the form of EM waves. These waves can then be absorbed by other atoms causing electrons to move to higher bands and the process repeats itself. At least that's how I understand it. When reading about conduction most materials state that heat is transferred because atoms with more heat energy are "vibrating" more and they end up "contacting" or "colliding" with one another ultimately leading to heat energy being transferred from one with higher energy to one with lower. But particles don't ever really touch one another! At the atomic or subatomic level, nothing really actually "touches" something else does it? Most of an atom is empty space, electrons or atomic nuclei never really" collide" with one another except in some extreme circumstances like a supernova or something correct? If this is true then how does heat transfer because molecules or atoms or whatever "collide"? What is meant by "collide" at this level? Is it all really just radiation when you get down to this level, or is there some other "force" that comes into play when these atoms/particles get close to one another that causes heat energy to transfer from one to the other? Same with convection, it seems to me convection is just conduction happening on a large scale, explanations for conduction at least attempt to explain the heat transfer by something that sort of makes sense (atoms colliding), but descriptions of convection just say that fluids move around and take the heat with them to the cooler areas. This doesn't help me understand what is actually happening intuitively that causes heat energy to transfer. Or does this just boil down to 'in the end, we have no clue how anything actually works, but there is some quantum probability that everything your brain can think of can happen' :)

 

[Lsos]

I'm no quantum physicist, but on the macroscopic level nothing ever really collides either. When two cars crash into each other their atoms never actually touch...yet they clearly transfer forces between each other electromagnetically. Why could not this work on an atomic level as well? Perhaps the atoms never collide, but they can still transfer vibrations from one to another simply due to their proximity and the electromagnetic force.

 

[permagrin]

Thanks for replying. I tend agree with you, my first thought was that all heat energy transfer, either via conduction or convection, is due to EM force or radiation when you break it down to that level. But if that was the case I would think that would be explained somewhere, I just haven't found that explanation anywhere - the method of transfer is always broken out into the three (or more depending on the source) distinct methods which drives me nuts.

 

[marcusl]

Transport of heat by conduction is carried by two particles: electrons and phonons. Transport is by electrons in good electrical conductors (which are always metals), while it is solely phonons in insulators. Other materials have a mix.

Electrons travel freely in a conductive metal (this is the so-called electron gas). Energetic electrons at the hot end diffuse through the metal and give up their energy through collisions with atoms in the crystal lattice at the cooler end until the entire material is at a single temperature. Yes, electron-atom collisions are common; they also give rise to electrical resistance.

Phonons are quantized vibrations of the crystal lattice. Atoms in the lattice vibrate and transfer energy amongst themselves through chemical bonds, van der Waals forces, and other mechanisms (spin coupling, etc.). In the classical view, atoms are simply modeled as masses interconnected by springs, so that vibrational energy is readily spread to neighbors.

Convection is macroscopic and doesn't need quantum mechanics to understand. It occurs only in gases and liquids, which physically move around. A radiator in a cold room, for example, warms the air molecules that it contacts (yes, collisions again, between a gas molecule and the hot metal surface). Molecules in warm air are, on average, spaced farther apart than cold. Warm air's lower density makes it rise in the Earth's gravitational field (that means there's no convection in the space station...). This sets the air in the room into motion. Warm air rises, pushes across the ceiling, sinks along the opposite wall as it cools, and flows back across the floor to fill the vacuum left by the rising warm air. That is called a convection cell. The heat transfer results from collisions of air molecules with each other and with the contents of the room.

 

[russ_watters]

Thanks for replying. I tend agree with you, my first thought was that all heat energy transfer, either via conduction or convection, is due to EM force...

That is correct. All interactions involving "contact" are via electromagnetic force.