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Heat transfer
Uncontrolled systems always evolve toward more stable states—that is, toward more uniform energy distribution (e.g., water flows downhill, objects hotter than their surrounding environment cool down). Created by Khan Academy.
Want to join the conversation?
Wouldn't we just get the velocity of the particle of the pizza and then just use the weight of the pizza (since it's going to be tedious to do it each particle)and calculate the Kinetic Energy?(3 votes)
But it would be much simpler if the plate's particles are the same mass, as they would cancel out.(4 votes)
is good heat transfer to ohio and michigian(1 vote)
Video transcript
- [Instructor] 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. So, 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. And this will happen
anytime the temperature of object one isn't the same as the temperature of object two. We know temperature is how we express that 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, temperature is the average kinetic
energy of particles. And kinetic energy, as we
know, is 1/2 mv squared. So when the pizza is warm, its particles are moving
faster than if it was cold. And when particles and 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, and 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 on 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 gonna use yellow
to represent the pizza, and I'm going to give the pizza particles a longer velocity vector
than those of the plate. And 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 he 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 slow 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. So 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. And 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. And 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, and we call this thermal equilibrium. You know when you leave your
pizza on the counter too long and 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. And 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, equilibrium occurs when
there is no heat transfer in the system. So it turns out pizza
is actually pretty good for learning about heat transfer
and thermal equilibrium. And if you don't mind, that pizza and the other
room is calling my name, and personally, I prefer to eat it before it
reaches thermal equilibrium with my plate.