Hi. It’s Mr. Andersen and this is AP Physics essentials video 84. It is on heat which is
the transfer of energy from an object that has a higher temperature to an object that
has a lower temperature. And right now I am a higher temperature object sitting in a folding
chair. I am going to touch it in two points that I am not actually sitting on. One that
is metallic and one that is non-metallic. Now which one is colder? The metal feels colder
to me, but it is not. They are both room temperature. This chair has just been sitting in this room,
and so why does it feel colder? It is because I am transferring more energy. Metals are
good at conducting that energy through heat and so it feels colder. And so any object,
there are really in physics three ways that we can get energy into and out of that object.
And the first way is to push on it. If I do work on an object, if I apply a force or a
given distance, I can add energy to it. I could remove some of the material and therefore
the energy and put it back. And then the last thing I could do is I could heat it up. I
could transfer energy from a higher temperature object. And so when we have a higher temperature
and a lower temperature in contact, energy is going to be transferred from hot to cold.
And that is called heat. Now there really are only three ways that we can transfer that
heat, through convection, conduction and then finally through radiation. Conduction requires
you to actually touch that object. So when you touch a flame or touch hot metal that
is going to be conduction. It requires direct contact. Convection is going to occur through
the movement of fluids. Those fluids could be gas. Those fluids could be liquid. And
then finally radiation. It is going to transfer that energy through electromagnetic radiation.
That is how the energy is hitting me from these lights and from the sun. Now we are
going to use a PHET simulation to look at this. We have a solid object inside this canister.
And I am going to add heat to it. I am going to add an energy into the system using this
flame. And watch what happens to the molecules inside. They are going to start to vibrate.
They are increasing their kinetic energy. And I have converted it from a solid to a
liquid to a gas. Now let’s say I wanted to remove some of that heat, how could I do
that? I could just simply use ice. And what is going to happen? I am pulling energy out.
And as I do that those molecules and the kinetic energy they have, the temperature that they
have in now being transferred into the ice. What is happening? They are going from a gas
into a liquid and now into a vibrating solid. And so conduction, convection, radiation,
how do those differ? They are all energy transfer remember, but conduction requires direct contact.
So when you are touching a hot tea kettle it is vibrating the molecules inside your
finger. So what is going on? Look at this simulation. You could thing of your finger,
the molecules are moving not so quickly. But when we attach it to that tea kettle where
the molecules are moving more quickly there are collisions along that contact and we are
transferring some of that energy. Collisions between the two transfers the energy from
the hotter temperature to the colder temperature. And what is the temperature at the end? It
is going to be the same exact temperature. How is convection different than that? It
is exactly the same. We are going to have molecules colliding. So it is this whole idea
of kinetic theory. But there has to be some kind of a fluid present. And so inside that
tea kettle, as we heat up the fluid on the bottom, so the water on the bottom is getting
faster. And so it is becoming less dense and it is moving up to the top of the tea kettle.
Eventually at the top it is getting a little bit colder and then it is moving down again.
So he have this convection current. Now it does not have to just be a liquid, it could
be the air above the burner that is being heated through convection as well. And then
finally we have radiation. So the light coming off an infared light bulb or even the sun
itself is transferred to the object through electromagnetic radiation. When it hits the
object it gets the molecules excited and so we are transferring heat from one to another.
And so did you learn that energy can be transfered through heat? And it could be through conduction,
convection and radiation? I hope so. And I hope that was helpful.