This should be high school physics, but well, I never took high school physics. I’m having trouble understanding the difference between convection and conduction.
The way it’s explained to me is that convection is heat transfer through a fluid medium, whereas conduction is heat transfer via direct contact.
But what’s happening on the atomic level – or do we have to go even smaller?
I THOUGHT that in conduction, energized electrons move from one atom to another (or is that wrong)?
In convection in a cup of water, then, wouldn’t it be mass conduction between water molecules? Is convection just a way to describe that in aggregate, or is there something more I’m missing?
In conduction, atoms or molecules that are moving fast (thereby making the region that they are in hot) pass on some of their kinetic energy to other atoms of molecules that are adjacent to them, thus making the adjacent region hotter (and the original hot region a little cooler). When it is happening within a solid, the ‘hot’ atoms or molecules in question are not really moving from their place, but simply vibrating with greater speed or amplitude than the cooler ones nearby that eventually pick-up some of the energy.
Conduction happens withing gasses and liquids too, but in their case convection will transfer heat much faster. In a gas or liquid the atoms or molecules are actually free to move around, and not just vibrate, so they do, carrying away the heat with them. As a hot fluid is generally less dense than a cooler one (because the atoms or molecules are moving faster, and thus bashing into one another harder and pushing others away) the warmer regions of the gas or liquid, close to the heat source, tend to move upward and get replaced by cooler fluid, which in turn gets heated up and moves upward. This moves the heat around much faster than conduction alone could.
In short, conduction is just the heat (vibrational energy) moving, with the matter staying in place, but in convection the hot stuff itself (hot regions of the fluid) moves.
If you have convection from point A to point B, then there’s a flow of actual material from A to B. With conduction, none of the molecules of the material move very far away from their original location, but just bump into their neighbors.
By way of analogy: Picture a long line of people all standing side-by-side. The guy on the end punches his neighbor on the shoulder. That guy then gets upset, and punches his neighbor, and so on, until eventually the guy on the other end gets punched. The punch moved down the line, but no person moved out of place. That’s conduction.
Now imagine that instead, the first person in line ran all the way to the other end and punched that guy directly. Both the punch and the person moved. That’s convection.
Thank you both. That explains the difference very well.
A followup, if I may: In both, how is the energy stored and transferred? If you “heat” an atom, what happens to it? Do the electrons spin faster? Does a part of it become photons? Does the atom, as a whole, vibrate more?
In general, the atom as a whole vibrates more. In a molecule, the bonds between the atoms may flex or rotate as well. Some of that heat energy will be given off as radiation (light). It would take a lot of heat for that radiation to be in the visible spectrum though.
I have no idea about the electrons spinning faster.
Now imagine that instead, the first person in line was picked up by another guy and carried all the way to the other end and now this person punches that guy directly. Both the punch and the person were moved. That’s* forced* convection.
