# What is the difference between convection and conduction?

I was having a conversation with my dad the other day and he described something as happening due to convection, and it dawned on me that I have no real clue what the difference between and convection and conduction is.

I started to do some reading on this and haven't really found any good clarification.

Here is what I do understand:

Convection:

• Involves transfer of heat within a single substance by mass flow
• Does not take place in solids due to a solid's inability to behave like a fluid
• The heat flow is governed by diffusion of higher energy molecules and thus a distribution of that energy evenly, or by advection which basically seems to be bulk diffusion to me

Conduction:

• Transfer of heat within a body by diffusion
• This diffusion differs from convection because convection supposedly happens through a material carrier
• Can take place in solids

Confusion:

No doubt, some of the points I just put up there are wrong or else I basically see no point in making a distinction between convection and conduction when the processes can be described more easily by diffusion which redistributes energy by principle of maximizing entropy.

So why define these two different terms when they are essentially the same process perhaps just happening in different mediums?

Contradictions:

I was looking at the Wikipedia pages, which I usually find to be reliable, for convection and conduction and was only confused more.

For instance, from the conduction page:

"In gases and liquids, conduction is due to the collisions and diffusion of molecules during their random motion."

Now, from the convection page:

"Convective heat and mass transfer take place both by diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid."

Those two statements seem to be basically the same. The mechanism for the heat transfer is the diffusion of randomly moving moving particles.

On the other hand, the conduction page explains that conduction takes place in solids through,

"In solids, conduction is mediated by the combination of vibrations and collisions of molecules, of propagation and collisions of phonons, and of diffusion and collisions of free electrons."

That seems like a perfectly good explanation of what takes place when heat is being distributed towards thermal equilibrium in a solid.

Questions:

What about the above statement that conduction in solids is mediated through collisions of electrons and vibrating atoms makes that process conduction rather than convection?

What of the above points are incorrect?

On a molecular level, (i.e. using typical chemistry ideas) what makes a distinction between convection and conduction?

Finally, what is meant by the statement that convection happens through a material carrier?

Does that imply that conduction does not happen through a material carrier? If so, how?

I hope you can see my confusion between these two terms. They seem very convoluted to me.

I would point out that good and correct answer need not address each of my sub-questions specifically because a good answer may clear up my confusion and hence eliminate one of those sub-questions.

• Can I have an explanation from whoever downvoted this? The criteria for a downvote is, "This question does not show any research effort; it is unclear or unuseful." I did my best to present this question as clearly as possible and I believe it's useful for anyone interested in thermodynamic phenomena. Sep 2 '15 at 21:56
• I am not the downvoter, but I think one reason could be that this question is only very vaguely related to chemistry. It might serve a better purpose on Physics. Sep 3 '15 at 4:48
• @Martin-マーチン It's more chemical engineering than chemistry -- perhaps it should be on Engineering.SE? Sep 3 '15 at 13:50

## 1 Answer

Convection is a mechanism that enhances conductive heat transport, rather than being a separate physical phenomenon. It operates by bringing hotter parcels of fluid and colder parcels of fluid into closer physical proximity with one another so that conduction can take place more easily and rapidly. One way this is accomplished is by shearing the fluid near a heated wall in such a way that the colder faster-moving layers further from the wall flow over the hotter slower-moving layers closer to the wall to physically cause the temperature gradient normal to the wall to increase. This enhances the rate of conductive heat transfer. Another situation is where the fluid is being mixed by a stirrer to create thin striations of hotter fluid and colder fluid throughout the fluid. Conduction between these hotter and colder striations increases, again because of the much larger temperature gradients that have been created. This is typical of how convection works.

What I'm saying is that the deformations experienced by the fluid while flowing are such that they cause the temperature gradients in the fluid to get higher, and this increases the rate of heat transfer.

Imagine a deck of cards split in half, with all the red cards (hot) in one half and all the black cards (cold) in the other half. If you bring these two slabs of hot and cold material together and wait, heat transfer will occur very slowly between the hot and cold regions. However, if you shuffle the deck so that there is a hot (red) card sandwiched in between every pair of cold (black) cards, and a cold (black) card sandwiched in between every pair of hot (red) cards, the heat transfer between the hot and cold layers will be much more rapid. This shuffling effect is what convection does.

• Haha you'll notice I edited the question so as to not be presumptuous about engineers. Wasn't trying to be offensive. Frankly I'm just jealous I'll never make as much money as any of you guys :) So, you're saying that convection is essentially the mass transport (where the mass being transported has a temperature differential with the rest of the system) and conduction is the actual heat transfer taking place? Sep 3 '15 at 3:43
• Welcome to Chemistry.SE! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. While it may seem strange at first we do not use salutations in our Questions and Answers. Hence I removed it. Sep 3 '15 at 4:50
• this has alluded me as well, in high school physics books you typically have heat transfer divided into 3 categories. convection surely seems little bit out of frame for me. as you said it's not the samke kind of "thing" as radiation and conductivity. Jul 13 '19 at 10:06