Heat transfer | Thermodynamics | High school physics | Khan Academy

All right, so I don't know about you, but I feel like talking about pizza. It's pizza night over here. I am smelling pizza as it's in the oven. It's on my mind, and I know we're supposed to be talking about heat and thermal equilibrium, but I think we can make this work.

Now, if you're impatient like me, you probably put a slice of pizza on your plate as soon as it comes out of the oven, and you've probably noticed that the bottom of the plate warms up. So, let's think about what's going on here. We have the temperature of the pizza (T_pizza), which is warmer than the temperature of our plate (T_plate). Since the temperature of the pizza is higher than that of the plate, the plate starts to warm up. The pizza's temperature, on the other hand, will actually start to lower. This will happen anytime the temperature of object 1 isn't the same as the temperature of object 2.

We know temperature is how we express if something is hot or cold. What does hot or cold mean? Temperature is actually the measure of the average kinetic energy of the particles in an object. So let's digest that for a second and break it down.

So, temperature is the average kinetic energy of particles, and kinetic energy, as we know, is one half mv squared. When the pizza is warm, its particles are moving faster than if it was cold. When particles in objects are moving, they're actually vibrating around. So, what's happening in the hotter object is that the particles are vibrating faster than when the object is colder. That's what it means to be hot or cold. This type of kinetic energy, the motion of particles, is also known as thermal energy.

Now, you might be wondering, if temperature is the measure of kinetic energy in objects and we have the pizza and the plate with their temperatures changing, does this mean there's some sort of energy transfer going on here? And you would be spot on. Heat is the transfer of energy between objects at different temperatures. This is a really important concept, so let's write that definition out: heat is the transfer of energy between objects at different temperatures.

As you can see with our pizza and plate, heat is transferring from the hot pizza to the cold plate, from the hot object to the cold object. Why is that? Well, let's go ahead and break down our problem and really dig into what's happening with everything in the system. In real life, a lot of energy is also going to be transferred from the pizza into the air around it, but for this problem we're going to simplify it to just look at the pizza and the plate.

So, we said that the higher the temperature, the more all the particles that make up the object are moving and vibrating. So let's go ahead and draw that for the pizza particles. We're going to use yellow to represent the pizza, and I'm going to give the pizza particles a longer velocity vector than those of the plate. I'm going to assume that the particles in the pizza and the plate have the same mass; that way we can ignore the mass part of kinetic energy and simplify the situation.

Now we can just focus on the velocities of all the particles in the system. When you put the slice of pizza on the plate, the particles at the surface of the objects will come into contact. So let's look at what happens when these faster particles from the pizza collide with the slower particles on the surface of the plate. When the particles collide, kinetic energy transfers between the particles. Since the hotter object's particles, in this case the pizza, have more energy, they're able to give some of that energy to the colder object's particles, which is why heat always transfers from the hotter object to the colder object.

When the hotter object's particles transfer some of their energy to the colder object's particles, the hotter object particles have now lost some energy and slowed down. So we can show that by using a smaller velocity vector. At the same time, the colder object's particles have gained energy and sped up. So now this particle in the pizza is at a different velocity than its neighboring particle in the pizza.

When they collide, guess what? We transfer energy again! The faster particle will give some energy to the slower one, and in the process, it slows down. This domino effect carries through to all the particles in the system, in the plate and in the pizza, until all the particles have the same kinetic energy. When the particles in both objects have the same kinetic energy, energy will no longer transfer between the two objects, and the system has reached a state of stability.

We call this thermal equilibrium. You know when you leave your pizza on the counter too long, you come back and it's cold? If you were to actually measure the temperature of the pizza and the plate, they’re going to be the same. We now know that heat transfer only occurs when the temperature is different between objects. So when the temperature is the same, no heat is transferring, and this is what is defined as thermal equilibrium.

So, thermal equilibrium occurs when there is no heat transfer in the system. It turns out pizza is actually pretty good for learning about heat transfer and thermal equilibrium. And if you don't mind, that pizza in the other room is calling my name, and personally, I prefer to eat it before it reaches thermal equilibrium with my plate.