Thermal energy and heat transfer

0:00- [Instructor] You get heat from the sun or from a campfire, and you notice how the beach sand feels hot under your feet, but the ocean feels refreshingly cool. But what exactly is heat? (laughing) That's what you wanna try and figure out in this video. Now, for thousands of years, people had all kinds of theories.

0:14Some thought that it was some kind of a magic power. Others believed that it was a fundamental element, a fundamental building block of the universe, like air, water and earth. By the 1700s, people started believing that heat was some kind of an invisible fluid that flowed from a warmer object to a colder object.

0:30In fact, they called this the calorific fluid. The word calorie comes from the Latin word for heat. But eventually, (laughing) all of these theories got replaced by what we understand today. Today we know heat has something to do with the movement of particles, the kinetic energy of the particles.

0:45But again, to be very clear, we need to first define something else, all right? So let's go on a side quest over here. Let's first introduce the idea of thermal energy. So what exactly is thermal energy? Thermal energy is the sum of all the kinetic energy of all the particles in a system.

1:01For example, if you wanna calculate the thermal energy of the ocean, just add up all the kinetic energies of all the particles, meaning all the molecules, like the water molecules and molecules of other substances that might be there in the ocean, add up all the kinetic energies of that, and that total kinetic energy is what gives you the thermal energy of the ocean.

1:15Similarly, thermal energy of the air in the room is the sum of all the kinetic energies of all the molecules that make up the air in your room. Now, to be precise, we need to also add potential energies because kinetic and potential energies can convert into each other.

1:30But for our purposes, we don't have to worry too much about that. For our purpose, it's just thinking in terms of adding up all the kinetic energies, and that's that, that will be your thermal energy. But here's a quick question. If something has a lot of thermal energy, does it mean it is hotter?

1:42Well, let's see. If you were to take a glass of this same ocean water, right, then you would agree that it would have the same temperature as of the ocean, right? So for example, 22 degrees Celsius. It won't be any hotter or colder than the ocean itself. But what about the thermal energy?

1:57Well, it has much fewer molecules compared to the ocean. There are little number of molecules, and therefore if you add up the kinetic energies over here, it'll be much, much smaller than what you'd get for the ocean. In other words, look, this has much smaller thermal energy compared to the ocean, but it's not hotter or colder compared to the ocean.

2:15In other words, they have the same temperature. So thermal energy is not the same thing as the temperature. The ocean has a lot more thermal energy compared to this glass of water, even though they are the same temperature. So that brings up the next question then, (laughing) what exactly is temperature?

2:29Well, temperature is a measure of average kinetic energy of the particles in the system. To make sense of this, let's take some simple numbers, okay? So if you want to calculate the thermal energy over here, we have to add up the kinetic energy of all the molecules over here.

2:43Let's assume that there are only 100 molecules. Then the thermal energy would be the sum of the kinetic energy of all those 100 molecules. And for the sake of simplicity, let's say that happens to be 500. Again, these are (laughing) not actual values, okay? We're just taking some numbers to help us understand the concept.

2:58Okay, now what would be the thermal energy over here? Well, since there are a lot more molecules, let's assume there are 10,000 molecules. Now, now thermal energy of the ocean would be the sum of kinetic energy of all those 10,000 molecules. And let's say that just happens to be 50,000.

3:11So you can clearly see thermal energy of the ocean is more than that of the glass of water. But what about temperature? Well, for temperature, you remember, you have to find the average kinetic energy, which means you have to divide by the total number of molecules.

3:23So over here we divide by 100, and over here we'll divide by 10,000. But when we do that, look what we get, 500 divided by 100 is five, and even 50,000 divided by 10,000 is five. So look, right in front of our eyes, we can see the thermal energy is not the same, but the temperature is exactly the same.

3:40Another quick question, can you think about whether the thermal energy and temperature are extrinsic or intrinsic quantities? Pause and think about it. All right, let's see. Thermal energy, does it depend on the amount of stuff? Yes, we can see over here, if we take a little bit of that stuff, then there'll be a little bit thermal energy, a lot of stuff gives you a lot of thermal energy.

3:57So since it depends on the amount of stuff, it's an extrinsic or extensive property. But what about temperature? Well, look, temperature does not depend on the amount of stuff. If you take a little bit of it or you take a lot of it, the temperature over here is the same.

4:09So since temperature does not depend on the amount of stuff because you're averaging it out over here, it's an intrinsic or an intensive property. So when you're measuring the temperature of something, let's say when you say that this coffee is very hot, it has a higher temperature, what we really mean is that the average kinetic energy over here, that is very high.

4:27And we say, for example, this ice is very cold, it has a low temperature, that means the average kinetic energy of all the molecules over here, that is very low. That's the meaning of the word temperature. So now we're ready to understand what heat is, okay? So if you're to keep a hotter system in contact with a colder system, you know that energy flows from a hotter system to the colder system.

4:46For example, if you take a very hot rod with a very high temperature and dip it in water which is relatively at lower temperature, relatively colder, then you know that energy gets transferred from the hotter rod to the colder rod. This happens until the temperature equalizes, right?

5:02In other words, we say this happens until we reach thermal equilibrium. Thermal equilibrium is a fancy word for saying that the two systems have the same temperature. All right, now, the energy that flows from a hotter system to the colder system until thermal equilibrium is reached, that is what we call heat.

5:22Let's take some more examples. If you're to let a ice sit on a table, for example, then it's colder compared to the surroundings, which is hotter. And so energy starts flowing from the surrounding into the ice, and eventually that's why the ice melts. This energy, this thermal energy that's flowing, that's heat.

5:39Similarly, if you were to let a hot coffee sit and its heat will start flowing, thermal energy starts flowing outwards, right? Again, that thermal energy that's flowing, that is heat. So heat is the thermal energy that's being transferred from the hotter system to the colder system.

5:58That's what heat is. In our day-to-day life, we like to say things like, "Hey, there's a lot of heat in this coffee," but as far as science is concerned, that is not heat. What the coffee has is thermal energy. We only say it's heat when it's flowing from a hotter system to a colder system.

6:16Think of a bank account, for example. The money that you have in your bank account, that's like the thermal energy, but when you're doing a transaction, you're doing a debit or a credit. That amount of money that's flowing from one bank account to another, that is the heat.

6:31But finally, we understood what heat is, what about cold? What is cold? Well, cold is not really a new thing. You can think of cold or coldness as loss of heat. So for example, when you're touching ice, you feel cold because heat is moving away from your body.

6:44So loss of heat is what we perceive as cold is not a new thing. All right, so to put it all together, when we say that, "Hey, that coffee has a lot of heat in it," what we're really talking about is the thermal energy, which is the sum of all the kinetic energy of all the molecules and the atoms that make it up.

6:58And then if we average it out, if we find the average kinetic energy of all the particles of our system, that is the measure of temperature, a measure of how hot or how cool something is. And finally, the thermal energy that flows from a hotter system to colder system until the temperatures are equalized, until thermal equilibrium is reached, that is what we call heat.

7:15Heat is thermal energy that's being transferred.