Another physics question

[Erebus the Black]

This is about thermal physics.

Back in Uni I've been told there were three main modes of heat transfer: Convection, Conduction and diffusion. (and a few secondary modes such as radiance/radiation)

 

I understood that Convection is the transference of heat by way of hot particles moving from one point to the other.

Conduction is the transference of heat by way of particle collisions.

But to me Diffusion (traditional diffusion is the movement of particles in such a way that equilibrium is met at its end (e.g. if one side is hot and the other is cold then equilibrium will be met when both sides reach the same temperature) ) seems like a kind of an average of both other modes, so why is it considered an independent mode? (other than because of the mathematics (diffusion is a a natural solution while convection is a forced solution (if I'm not wrong ) ) )

[Randomizer]

I always hated thermodynamics, but I know someone that's had the course more recently will correct any errors.

 

From wikipedia:

It has diffusion and conduction as the same, The third form is radiative.

 

Convection is particles traveling in motion from one place to another, think of it as a current carrying heat from hot to cold and then the loop has cold to hot. This is primarily a one directional flow of the particles. Back in the old days when building windows had two sections that could be opened, you could raise the lower section and lower the top section and the temperature difference between the top and bottom of the window would cause an air current coming in one place and going out the other. It's the same idea as convection ovens where the air inside transfers the heat from the electric elements on the walls to the food in the center.

 

Conduction is a more random transfer of heat as the particles collide. The energy of a hotter particle will be greater and transfer more to the cooler particles. Since it's random motion it transfers mostly from hot to cold, but some collisions will go backwards so it's not a current. A bar of metal with one end heated is a good example.

[Alorael at Large]

For a simpler version, I think Randomizer's right and the main modes for heat transfer are conduction, convection, and radiation. I'm going to just restate what he said in slightly simpler terms.

 

Radiation is transfer of thermal energy as electromagnetic radiation. Basically, everything loses heat constantly by giving off infrared light.

 

Conduction is, on a large scale, the fact that if you put two things at different temperatures in contact with each other, heat will transfer from the higher temperature object to the lower temperature one. "Objects" here can mean one half of a single thing compared to the other, all the way down to individual particles.

 

Convection has to do with faster-moving fluids, usually gas or water, having increased ability to transfer heat compared to slower-moving fluids of the same type.

 

—Alorael, who sees from Wikipedia that conduction is also known as diffusion, which makes sense. But convection is also substantially about the movement, including diffusion, of particles in fluid. So yes, your confusion is reasonable.

[Kelandon]

Conduction: Heat itself flows.

 

Convection: Hot material flows.

 

Radiation: Hot material glows.

[*i]

Think of conduction as a spreading out of stuff. Consider putting a drop of ink in a cup of water. The ink molecules tend to spread out until you have a homogeneous mixture, and not the other way, as per the second law of thermodynamics.

 

The next leap of thought to get conduction is rather than thinking of individual molecules themselves, think about the random kinetic energy of those molecules. If you have a concentration of molecules that are moving very fast next to those that are slower, eventually they will all be moving with (on average) the same speed.

 

The difference from convection is that conduction is a process based solely from the average of random motion of molecules and the second law. Convection is based on non-random bulk motion of a fluid. If I push a hot fluid into a cold fluid (or vice versa), the energy transfers that way. Note that I can move cold to hot, and this does not violate the second law because I had to provide mechanical work to make that happen.

 

Most of the time, you can safely ignore radiation, unless the object gets really hot, then it quickly swamps out all other modes of heat transfer as it gets hotter.

[Erebus the Black]

The thing is I worked with a Phd. student on a project for growing crystals (I was a B.Sc. student) and he was very insistent about diffusion being a separate mode.

The difference between conduction and convection is (and was) clear to me, my only problem is with diffusion,

from this:

α = k / cpρ is the thermal diffusivity, a material-specific quantity depending on the thermal conductivity k, the mass density ρ, and the specific heat capacity cp.

and is interconnected with convection through the heat capacity of an object (in my case, the crystal or the flux).

(And don't forget that all of these, so called, constants are dynamic through their dependancy on temp.).

 

[Alorael at Large]

Your symbols come out as gibberish, but I think the point isn't that diffusion is another mode of heat transfer. Rather, heat diffusivity is a meaningful property of materials that depends on conductivity.

 

—Alorael, who has never heard of the property, but from the variables involved (that he can understand) it makes sense. You can lose more heat into something with high heat capacity, and you can lose it faster if it has high conductance.

[A less presumptuous name.]

[alpha] = k / (cp * [rho]

Wikipedia even has an article about it. It must be a real thing! It sounds like a measure of how easily a material conducts heat.

[Callie]

Originally Posted By: Verily we roll along

Your symbols come out as gibberish

We non-modly denizens do not have access to shiny HTML characters.

[Dantius]

Originally Posted By: Excalibur

Originally Posted By: Verily we roll along

Your symbols come out as gibberish

We non-modly denizens do not have access to shiny HTML characters.

Right. I propose that all STEM-educated people o the board be made mods, so we can better argue about equations with one another!

[Slawbug]

Actually, it would be possible to give certain users, who are not mods, the power to post using HTML (which is how we make the symbols appear as I did for Sokka). But that is probably a bad idea for many reasons.

[Dintiradan]

I think Tyran had HTML enabled on Shadow Vale at one point, and I made him reconsider after putting an animated gif inside blink tags inside marquee tags.

[Gabriel]

Diffusion is random movement.

 

Just look at this example: [wikimedia commons] File:Gaz_molecules

 

Don't think about the end result (equilibrium). The modes of heat transfer describe how the heat moves during the middle part, after the beginning of the measurement and before the end result of equilibrium.

[Aran]

Originally Posted By: Dintiradan

I think Tyran had HTML enabled on Shadow Vale at one point, and I made him reconsider after putting an animated gif inside blink tags inside marquee tags.

So, so tempted. However, I notice that blink tags don't seem to affect images in the current version of Firefox.

[Student of Trinity]

At first I thought, Hey, how nice to have a physics question on my first visit here in a while. (Nothing against this place in particular; I gave up a bunch of Internet stuff for Lent.) Even nicer that it's about my favorite topic of heat.

 

But now I realize that this is awkward. Heat and heat transport is my particular research focus. You might think that this means I know all about it, but with my kind of fundamental theoretical physics, it doesn't work that way. I know almost nothing about heat. I'm not even sure what it is. That's why I'm working on it.

 

I have a blog about it, actually, though I haven't posted anything for about two years. I should write a few more entries. I have some more stuff to say now.

 

[Mosquito---Slayer]

Originally Posted By: Galanos

But to me Diffusion (traditional diffusion is the movement of particles in such a way that equilibrium is met at its end (e.g. if one side is hot and the other is cold then equilibrium will be met when both sides reach the same temperature) ) seems like a kind of an average of both other modes, so why is it considered an independent mode? (other than because of the mathematics (diffusion is a a natural solution while convection is a forced solution (if I'm not wrong ) ) )

Here's what I have been able to understand:-
Diffusion is considered a distinct phenomena because the physical reasoning is different sufficiently from both conduction and convection.
Diffusion basically happens due to random movement of particles, now these particles can either move from a place where there are too many of them to the place where there are too few of them , or they can collide with some of the stationary (i.e lesser energy particles on the other side) and give pretty much the same result.
Now the first situation is obviously different from convection being natural and not forced however the procedure is the same and the laws governing it will be either similar or same to that of convection( Both involving movement of higher energy particles from one place to another).
In the second case the laws will resemble conduction , however over here it is the transfer of movement (i.e velocity) from one particle to another which results in flow of heat whereas in conduction it is the vibration of immediate particles resulting in vibration of next particles and so on.
To sum up the two procedures involved in diffusion are mathematically equivalent to conduction and convection , But physically they are totally different and hence it is only reasonable to call diffusion a seperate phenomena altogether.

[*i]

Quote:

To sum up the two procedures involved in diffusion are mathematically equivalent to conduction and convection , But physically they are totally different and hence it is only reasonable to call diffusion a seperate phenomena altogether.

Not sure everyone would agree with this statement. What is often called, and you are calling, diffusion is more explicitly mass diffusion. There are analogous types of diffusion for momentum and energy. Much confusion arises because the macroscopic quantities were named and explained before we understood the underlying (atomic-level) reasons why they occur.

Taking a step back, there are three conserved quantities that arise from classical kinetic theory: mass, momentum, and energy. Each of these quantities tends to spread out over time because of the same fundamental process, random atomic motion. At a macroscopic level, these processes are called (mass) diffusion for mass, viscosity (or friction) for momentum, and conduction for energy.

To make things more confusing, convection has its own ambiguities with terminology. What is clear is a term called advection, which is the process of moving conserved quantities (mass, momentum, or energy) via bulk fluid flow. Convection can either be just advection or the sum of both advection and diffusion, depending on the definition.

How does advection differ from mass, momentum, and energy diffusion? The latter results from random motions of particles and tends to lead to an increase in a quantity called entropy. Bulk (non-random) movement of material and transfer of conserved properties is conservative and reversible -- in other words, no entropy is generated in the process. Of course, this is only in theory. In practice, you cannot move a material without some diffusion occurring (think friction, for example), but this is a distinct, having different underlying atomic processes.

[*i]

Quote:

But now I realize that this is awkward. Heat and heat transport is my particular research focus. You might think that this means I know all about it, but with my kind of fundamental theoretical physics, it doesn't work that way. I know almost nothing about heat. I'm not even sure what it is. That's why I'm working on it.

I have a blog about it, actually, though I haven't posted anything for about two years. I should write a few more entries. I have some more stuff to say now.

I know what you mean about things you thought you understood well as an undergraduate or grad student becoming more and more mysterious as you learn more about it. I really look forward to hearing what you have to say on this topic.

Trying to understand what has been termed the mesoscale regime of physics really is a hot topic these days. We've understood how things behave at the macroscopic scale (things we can physically see) for quite some time. More recently, we've come understand the atomic and molecular levels, with interactions of a few particles fairly well. Connecting the two is very difficult, and while we may have a good idea of why macroscopic properties such as enthalpy, conductivity, elasticity, etc. come about, we don't really understand how.

For example, take plasticity (the irreversible stretching of a material). We know it is meaningful to talk about it in the context of rods of steel. We also know that it is not meaningful to talk about the plasticity of one iron atom. But, how many bonded iron atoms do we need before a property such as this becomes meaningful to speak of? 1,000? 10,000? If not that, then 11,000? Is there some intermediate type of plasticity? If so, what characterizes it?

All of these are related fascinating questions that hopefully we'll have answered someday within our lifetimes.

[Harehunter]

Quote:

To make things more confusing, convection has its own ambiguities with terminology. What is clear is a term called advection, which is the process of moving conserved quantities (mass, momentum, or energy) via bulk fluid flow. Convection can either be just advection or the sum of both advection and diffusion, depending on the definition.

I don't think I remember that term, advection, before, but I can see where it would be most applicable to electric current. It always bugged me that while electons (negatively charged particles) flow from negative to positive, but current always flows from positive to negative. While advection is probably more closely related to convection, and not so much to electric current which is a prime example of conduction, it does imply that the motivating factor for convection is not so much of a pushing motive, but rather more like a pulling motive.

I see a similar situation with regards to centrifugal force. What we perceive as a force pushing us around the orbited boy, it is actually the pulling force of gravity that keeps us in orbit.

I am guessing that there is a similar term which applies to conduction. The Electro-Motive Force (EMF) that pushes the electrons down the wire is not so much as pushing the electrons, but rather it is a pulling force.

Either that, or this bunny needs to return to his hutch and watch dancing with the starskys.