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## Physics library

### Course: Physics library>Unit 10

Lesson 2: Specific heat and heat transfer

# Thermal conduction

Intuition behind how heat gets transferred through thermal conduction.

## Want to join the conversation?

• I still don't get the intuition behind heat. How is it different from thermal energy or kinetic energy of molecules and what exactly do we mean when we say that kinetic energy is lost because of friction to heat. And some places on the internet tell me that heat is a form of energy while other places say it is not so. What exactly is it?
(7 votes)
• The trick is that, at the heart of it, heat and energy (and work) are the same thing, they are all measured in joules. Kinetic energy is the work added to a particle (truck, bullet or gas molecule) when it is set in motion. Thermal energy is basically the kinetic energy bound up in individual molecules of a gas, liquid or solid, and temperature is how we describe the average thermal energy of those molecules. Heat, in one sense, is the total amount of work done to a group of molecules to get them from no thermal energy at all (at 0 degrees Kelvin) to their current temperature, Usually, though, we use heat to mean the energy or work (same thing, remember?) added to or taken from a system when we do something with it.
Now remember that, other than in atom bombs, energy/work/heat is conserved, which means that we can change it from one form to another but when we add up all the forms it can take in a system, we have to end up with exactly the amount we started with. When we say we lose kinetic energy to heat because of friction, the friction is a process that transforms some of the kinetic energy to heat. Sometimes we mean to like when we apply the brakes on a car or bicycle, and sometimes we just can't avoid it like the tiny amount of heating that goes on in a good set of wheel bearings.
(15 votes)
• Thermal conduction is ascribed to the molecule collision between two different gases. What is the thermal convection? Could you please explain the difference between thermal conduction and thermal convection? Thank you very much.
(7 votes)
• Thermal conduction happens between molecules when the more energetic one transfers some of its energy to the less energetic one. In a solid, that is pretty much the only way to move heat. However, in liquids and gasses, as soon as a tiny pocket of molecules gains some heat by conduction, it becomes less dense and starts to rise, which moves the heat energy to different, higher part of the container, where it will eventually lose the heat it gained to other, cooler pockets of molecules it comes in contact with.
Note that we need gravity to make convection work, they've done experiments, and there is no real convection on the International Space Station, although fans still work to cool their computers.
(5 votes)
• In some sites that i research, i see that heat transfer is also referred to the amount of free electrons. How does that relate to the particles bumping into one another and transferring heat?
(7 votes)
• Yes that's true, but it only occurs in metal. Metal has a giant metalic structure, so it possesses positive metal ions and a 'sea' of delocalized (free-moving) electrons. When conduction takes place in metal, it is assisted by free electron diffusion, where the higher energy and free moving electrons collide with the particles in the solid, causing them to vibrate more vigorously, and speeding up the process of transfer of kinetic energy and achieving thermal equilibrium.
(3 votes)
• how is momentum different from kinetic energy qualitatively?
(7 votes)
• - Momentum is 𝑚𝑣 and kinetic energy is 1/2𝑚𝑣^2

- Momentum has a direction, kinetic energy not

- Momentum is conserved, kinetic energy not (but energy is)

- Momentum depends linear on velocity, kinetic energy depends quadratically on velocity
(2 votes)
• If temperature is the average K.E, then why doesn't it have the same unit (Joule) as K.E?
(3 votes)
• Temperature is not equal to the average kinetic energy.
Average kinetic energy of an ideal gas molecule is proportional to the temperature of the gas. Which means that increasing the temperature of the gas will make the molecules of the gas move at higher speed and therefore the average kinetic energy per mole of the gas will increase as well.
(5 votes)
• the gas molecules in the pot can also be heated by convection other than conduction as explained in the previous video? So what is the primary reason for transfer of heat here? Conduction or convection?
(2 votes)
• When Sal started talking about the pot, he mentioned that it was empty, so we are just thinking about the heat moving through the metal by conduction. In real life, of course, we would also be heating the air in the room by convection from both the inside and outside of the empty pot.
(4 votes)
• How does a thermometer measure the average kinetic energy in a system?
(2 votes)
• Depends on which type of thermometer. A couple of examples are thermometers based on the volume change of a known substance (like quicksilver) and thermometer based on the electromagnetic wave emission pattern of a dark body.
For volume change thermometers, you just need to put it in contact with the object you're interested at until equilibrium is reached. By then, you should be able to observe a volume change in mercury due to temperature change, a known and studied effect. The manufacturer surely marked the relation between volume and temperature on the scale, so you can quickly read it.
The other type I mentioned deals with the fact that dark bodies emit a known spectrum of electromagnetic waves which corresponds with its temperature. By building the measuring conditions in such a way that the energy absorbed is similar to that of a dark body (usually putting the thermometer in a cavity), one can read the absolute temperature with a good precision.
(4 votes)
• A cup of tea at 90°c has less heat than a bathtub full of water at 70°c. Please explain ..me in 7th std
(2 votes)
• Does it take more energy to get a cup of tea to 90 c or a tub full of water to 70 c?
(3 votes)
• In the gas molecule diagram, the molecules are rather evenly spaced apart in both gases. However, shouldn’t the gas with the lower temperature have molecules that are closer together than the molecules in the gas with the higher temperature?
(2 votes)
• Yes, you are right! Assuming both gases are made up of same molecules, the gas on the right with lower temperature should be more dense—it's molecules should be drawn closer together.
(2 votes)
• Does it mean that heat is a form of energy which is added or removed to increase avg. KE (only) of molecules present?
(2 votes)

## Video transcript

- Let say that I have two different gases at two different temperatures that just got in contact with each other. So, this is my magenta gas. Right over here, let me draw a bunch of the molecules of my magenta gas right over here. And it's just, this system over here, I guess, whatever you want to call it, has just come in contact with this blue gas. This blue gas, right over here. And lets say that right where when we're starting our simulation, our experiment, that this magenta gas has a higher temperature. Higher temperature. And our blue gas has a lower temperature. Lower temperature. So lets just remind ourselves what temperature is. Specially if we think about it on a molecular scale. So higher temperature, lower temperature. Temperature is proportional to average kinetic energy. So these molecules, they're gonna be vibrating around, they're gonna be bumping around. They're gonna have, each of them are gonna have kinetic energy and if you average them, that's gonna be proportional to temperature. So let me depict each of this individual molecule's kinetic energy. Maybe this one is doing that. Maybe this one is doing that. Maybe this one is going in this direction. Maybe that one is going that direction. That one is going in that direction. This is going in that direction. That is going in that direction. So, notice, they all have different directions. And the magnitude of their velocity can be different. They all have different speeds. They all might have different speeds. So they have different speeds right over here. And they're all bumping into each other. Transferring their kinetic energy, transferring their momentum to from one particle to another. But when we talk about temperature we talk about the average kinetic energy or what's proportional to the average kinetic energy of the system. Well this one, each of these molecules are also gonna have some kinetic energy. But on average it's gonna be lower. Maybe this one is doing something like this. This one is doing something like this. This is doing something like this. This is doing something like this. So they're different, but on average they're gonna be lower. So, hopefully you see that this magenta arrows are bigger than these blue arrows that I'm doing. And they don't all have to be, for example this one might have a lot of kinetic energy. But when you average it out, the average here is gonna be lower than the average here. So, just like that. Now, if this is our initial state, what do we think is going to start happening? Well, before our different groups of gases were colliding with itself or the magenta was colliding with the magenta. The blue is colliding with the blue. But now they're gonna start colliding with each other. And so you can imagine when this molecule right over here, collides with this molecule, it's gonna transfer some kinetic energy to it. So after the collision, this one might be going. After the collision. So lets just say they just bounced in to. So this is right before, and lets say they just finished bouncing into each other. So right after they finish bouncing into each other, this one might ricochet off. So this one is going to go this way. Let me do this in a different color. So it might hit this one, bounce off, and then transfer some of its kinetic energy and then it bounces off in this direction. While this one, after the collision, after the collision is going to is maybe going to move much faster in this direction. And so notice, you have a transfer of energy. Just with that one collision you had a transfer of kinetic energy from this molecule to that molecule. And this is going to happen throughout the system. That the faster molecules, the ones with more kinetic energy as they collide you're gonna have transfer of energy. So you're gonna have a transfer of energy from the higher temperature to the lower temperature. Transfer, transfer of energy. And this transfer of energy. And you can consider this transfer of thermal energy, we're talking about temperature here. So, the things that are related to temperature, we would say thermal. So these, this is transfer of thermal energy. Transfer of thermal energy. The amount, so you're gonna have, if you're gonna start with higher energy here. You have higher average kinetic energy. You have lower average kinetic energy here. But this is going to transfer energy from the magenta to the blue. It's gonna go from higher temperature to the lower temperature. And that energy that's being transferred, that energy that's being transferred, we call that, and this is a word that you have probably heard many times in your life, we call that energy that's being transferred, we call that heat. That literally this hotter gas over here is heating up. Is heating up this cooler gas. And the way that this transfer of thermal energy is happening where it's through the collision of the particles, the transfer of kinetic energy to the collision of particles that's transferred momentum, we call this conduction. We call this thermal conduction, or I'll just call it conduction. Let me write thermal conduction. I'll do it in a new color. So this that is being described is thermal conduction. Which is a way that many times you have experienced heat being transferred. For example, you probably have had the experience of, if you take a, I don't know, you take a pot. Lets say you take a pot like this. Lets say it's a cold pot at first. So its particles, the particles in the pot have a lower kinetic energy. So I'm not gonna be able to do all of them. And then you put it over a fire. You put it over a fire. Let me see if I can draw it. You put it over a fire. So this is the fire, this is the fire. And we're talking about really just heating up the metal of the pot, I'm not even concerned about what's in the pot right now. So this fire is going to heat up, it's gonna heat up the bottom of this pot first. And it's actually gonna do it primarily through thermal conduction, because fire is nothing but super hot air particles and those super hot air particles are gonna bump in to the metal particles of your pot. So these metal particles of this pot, they're kinetic energy is going to start going up. So this part of the pot is going to start heating up. And right when you turn your stove on, the top of the pot might still be cool, but the bottom is going to get hot very fast. But if you just wait a few minutes, these metal particles are gonna keep bouncing and vibrating into each other. And so eventually, the top over here is going to get quite hot. Is going to get quite hot. And the way that the top of this metal got hot it was through thermal conduction that the metal of the bottom got hot first, and then they bounced into their neighbors, or vibrated into their neighbors and transferred some of that kinetic energy. And so once again you see this transfer of heat from a higher temperature region, to a cooler temperature or lower temperature region.