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## Physics library

### Unit 10: Lesson 2

Specific heat and heat transfer# Intuition behind formula for thermal conductivity

Intuition behind formula for thermal conductivity.

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- But when we increase the thickness of the wall, it doesn't affect the amount of thermal energy transferred, doesn't it? All it does is that it takes a longer time to get transferred from one side to the other? So how can we say that it helps with insulation? Im sorry if if my question is too confusing.(12 votes)
- Your right it only delays the transfer of heat energy. If you put an insulation box in the contact with heat ,the time will come when inside and outside temp of box will be same. But suppose there is hot sunny day and you are in the metal house it will quickly become hot, you have thick layers of Polystyrene or Styrofoam on the walls, it will slow down the heat transfer and may be before thermal equilibrium there will be sunset and night, or small cooling system will work.(22 votes)

- Why doesn't the heat transfer speed depend on the absolute temperatures themselves?

I mean, Imagine 600 Kelvin degree and 500 Kelvin degree temperatures, the molecules are moving really fast, and the time delay between collisions of a molecule with the wall is short.

But if you have like 105 kelvin degree and 5 kelvin degree temperatures, the molecules would move much slower, and therefore it would them more time to get to the wall, so the collisions wouldn't only be weaker than in the case with 600 and 500 degree temperatures, but they would also be happening**less often**.. wouldn't they?

So why don't the temperatures themselves affect the heat transfering speed?(4 votes)- Orangus, what you said is correct, though not in contradiction to what Sal was saying in the video. Sal was explaining the "Intuition behind the formula for thermal conductivity," he was not solving for specific heat ratios. Think about your question then watch the video again and you will see that Sal is reenforcing your point using generalized intuition as opposed to specifics. There are reasons he teaches this way and they are all good ones. It is much more powerful to know the system than it is to know the design, because if you have a good intuition for the general principals you will, generaly, be able to figure out the specifics. Also, Sal himself actually answers your question at around the3:30mark. (Ta - Tb)

I hope this helped, Happy Learning:)(0 votes)

- I still don't get it. We derived this formula on the basis of one assumption.i.e there exists a steady state. But I've rummaged through google and I still couldn't find the answer for the question as to why heat flows in the first place. Like, from what I've been reading in books, is the fact that the temperature is constant everywhere along the conductor during steady state.So , why does heat flow in the first place?(2 votes)
- Heat flows through contact whenever there`s a temperature gradient. In other words, if two points have different temperatures and are connected by a thermal conductor, heat will flow between them.

Heat flows espontaneously only from the hot source to the cold source.(3 votes)

- When we consider total heat transfer over some time t,

Q=k*A*dT*dt/thickness

but won't the temperature differential (dT) change (reduce) with a change in time(dt)?

Shouldn't it deal with something like integrating the LHS wrt dt as dT changes(reduces)?(2 votes) - sir

can you make a video on the concept of thermal diffusivity {including the formula derivation}? Please...(2 votes) - Does the temperature of the barrier affect the equation?(2 votes)
- What Sal should have said is that one side of the barrier (think of it as the interior of a wall) had surface temperature of Ta and the other side (exterior) had a surface temperature of Tb.

The temperature of the barrier itself varies linearly from one side to the other.

Also, the thermal conductivity CAN very with temperature, but just don't worry about that.(1 vote)

- Why if I increase the area of the surface the rate heat transference increase? It doesnt makes sense if i have to cover more area the transference will decrease.(1 vote)
- Each particle of the surface is involved in the conduction of heat, a bigger surface area means there are more particles working on transferring heat from the hot side to the cooler side. This is why a room with bigger windows will loose more heat than a room with smaller windows even if they are made of the same type of glass and are the same thickness.

It is increasing the thickness of the material that decreases heat transfer as the heat needs to travel through more particles (further) to escape.(2 votes)

- Is d=0 if objects are side by side?(1 vote)
- Yes... and no.

Yes, because, well, the distance*is*0.

No, because the equation only applies to a 3-object system, and having d=0 means that there is no object in-between that conducts the heat, and the system is then no longer has three objects, and the formula doesn't apply anymore.(1 vote)

- How the temperature difference is proportional to rate of conductivity?(1 vote)
- What do we do if one side of the wall has a greater area than the other side of the wall? For example, in sal's sphere, as long as the sphere had some thickness, the outside of the sphere would have a greater surface area than the inside of the sphere. Which side of the sphere should we put into the equation?(1 vote)

## Video transcript

- So I have an interesting system over here. I have two compartments, on the left compartment I have a gas that is at a temperature of T sub a and on the right side of this I have gas that is a temperature of T sub b and they are separated by a wall of depth, d, or I guess you say of thickness, d, and the contact area of the wall, or the contact area of the gas onto the wall that area is A, and I'm just drawing a section of it, we're assuming that these two compartments are completely separated. Now what I am curious about, and we're going to assume that the temperature on the left is higher than the temperature on the right, and so because of that you're going to have a transfer of thermal energy from the left to the right, and that thermal energy that gets transferred we call that heat, and we'll denote that with the letter Q, I'm curious about how does the rate at which heat is transferred, or so how much heat is transferred per unit time, that's the rate at which heat is transferred, how would that change depending on how we change these different variables. So for example, if our, if our area, if our contact area were to go up what would that do for Q over t? Well then Q over t would also increase per unit time 'cause I have, I have more area for these hot molec--these hot air particles or hot air molecules to bump into and they'll heat that wall and then there will be more heated wall to heat up the colder air particles. So in that case our rate of heat transfer would also go up. Well what if we, what if we, and obviously if I made my area smaller, maybe I should just write that explicitly, if I made my contact area smaller then my rate of heat transfer, my rate of heat transfer would go down, and that feels like common sense. Now what about, what about the thickness? If I were to make it, if I were to make the thickness larger, if I were to make this a thicker wall, what would that do to my rate of heat transfer? Well then I would have, I would have more things that I would have to heat up to get it to a certain temperature before I can, which then can heat up the particles on the right, and obviously this is a continuous process, it'll always be happening, but there will just be more stuff to heat up and it's going to take longer and more of that, and more of that more of that kinetic energy, that average kinetic energy is going to get dissipated in this wall so if this wall becomes thicker, if the wall becomes thicker then the rate of heat transfer is going to go down, or if you, if the wall became thinner, if this depth decreased then the rate of heat transfer, then the rate of heat transfer would go up. So you could say the rate of heat transfer is going to be inversely proportional to the thickness of this wall. Now what else could we think about? Well we could think about the temperature differential, the temperature differential, that's T sub a minus, minus T sub b, minus T sub b. Well if this temperature differential, if this temperature differential were to go up, well what's going to happen? Well it's common sense that well if this is super hot, if this is super hot over here, this is way hotter than what we have on the right, well we're going to have more heat transferred so the rate of heat transfer, you're going to have more heat transferred per unit time, your rate of heat transfer is going to go up. And likewise, if this differential were to go down and you could take the extreme case if there was no differential, if T sub a was the same as T sub b then you would have no heat transfer frankly in any unit of time. So it makes sense that the rate of heat transfer is going to be proportional to the temperature differential. So how can we encapsulate all of this intuition into maybe a formula for describing thermal conductivity, for thinking about how quickly some, how quickly this heat will be transferred, the rate of heat transfer? Well we could say the rate of heat transfer, and this is really, hopefully you know comes out of a little bit of common sense or intuition of what would happen here, the rate of heat transfer, I could say it's going to be proportional to well, what are the things it's going to be proportional to? It's going to be proportional to the area, the more surface area we have on this wall, more contact area, the more heat we're going to have transferred per unit time, so it's going to be proportional to that contact area. It's also going to be proportional to the temperature differential so let's multiply this times the temperature differential, so T sub a minus T sub b, minus T sub b, and it's inversely proportional to the thickness of the wall so all of that over the thickness of the wall. And now another thing that you might be saying, okay I have this proportionality constant but wouldn't this be different for different materials? For example, if this was a metal wall wouldn't this conduct the heat quicker than if this was a wood wall? And you would be correct, a metal wall would and so this K, this K right over here, this is dependent, this is dependent on the material, so what is the wall made of. So material... material of the wall, and you can actually, you can actually measure this thing and different materials will have different thermal conductivities, which this, which this variable right over here would actually represent. So going through a little bit of intuition we were able to come up with what looks like a fancy formula, and you will sometimes see this formula, formula for thermal conductivity through a solid barrier but it really comes out of hopefully common sense, the rate of, the amount of heat transferred per time is going to be proportional to, and the proportionality constant is going to be dependent on the material. Styrofoam for example would be very low here, that's why coolers are made out of Styrofoam, and it's going to be dependent on the area, it's going to be proportional to the area, the temperature differential, and then inversely proportional to the thickness. So if you wanted to really insulate something you would want to minimize the surface area and you would want to, and you would want to maximize the thickness and you would want to have something with a very low thermal conductivity, so a thick Styrofoam wall that's maybe shaped in a sphere might be a pretty good container for keeping something hot or for keeping something cool.