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- [Voiceover] So if you're in a room of some kind

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I encourage you to try a little experiment right now.

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So look around the room

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and see if there's something in the room

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that's made out of wood, or maybe paper, or cloth,

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and it's been in the room for some time,

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so hopefully it has the temperature of the room.

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And then find something else that's made out of metal

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that's also been sitting in the room for awhile

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and it doesn't have its own source of energy,

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so don't use your computer.

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It should just be something that's been

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passively sitting in that room for awhile,

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it's not too hard to find,

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and touch them both.

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And what you will see, is that even though

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they've both been sitting in that room for awhile,

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the metal is going to feel a lot colder.

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The metal is going to feel colder.

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And this is a bit of a conundrum

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because they've both been sitting in this room

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for a while, so they both should take on

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the ambient temperature.

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So let's make this a little bit more concrete.

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Let's say that the temperature of the room

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is 70 degrees fahrenheit.

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And the key for this feeling colder,

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is that the ambient temperature of the room

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is less than your body temperature.

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Your body temperature is going to be

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roughly 98.6 degrees fahrenheit.

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So let me write this, this is body temperature.

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And the temperature at the surface

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of your skin might be a little bit different than this,

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but let's just assume that it's

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roughly 98.6 degrees fahrenheit.

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And so, what's happening is, this metal one,

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this metal surface, isn't actually colder.

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It doesn't actually have a lower average

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kinetic energy than the wood surface.

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They've both been sitting in this room for a while,

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they're both going to have the ambient temperature

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of 70 degrees fahrenheit.

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So what just happened?

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Why, to your skin, and to your brain,

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does the metal actually feel colder?

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And the simple answer is, it's better

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at taking the heat away from you.

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So why is it better at taking the heat away from me?

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Well, let's just imagine, let's say that these

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here are the atoms on the surface of my skin.

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So these are the atoms on the surface of my skin,

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the bottom of my skin, let's just say my hand

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is touching a surface like this.

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That's my thumb right over there,

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and I'm touching the surface.

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And it's going to have an average kinetic energy

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that would be in relation to a body temperature

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of 98.6 degrees fahrenheit.

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So these things are going to bounce around,

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or vibrate around.

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And maybe the covalent bonds between

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the carbon atoms and the other atoms

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on my skin that keeps them from breaking free fully,

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but they're going to be kind of oscillating around,

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bouncing around a little bit.

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And they'll even kind of push on each other,

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and this could be kind of the electrostatic forces doing it.

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But they're going to have some average kinetic energy.

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And let's say my hand is touching both of these

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surfaces at the same time.

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So I have the wood surface, I'll do that in yellow.

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So I have the wood surface, right over there.

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That is wood.

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And I have the metal surface, I'll do that in white.

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So I have the metal surface, right over here.

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And this metal surface, we already talked about,

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is going to feel colder.

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Let me draw the rest of my hand, actually.

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So the rest of my arm, you get the idea.

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So what's going on here.

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So let's just think about it at a microscopic level.

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So the wood, first of all, its surface

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is going to be uneven.

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So you're going to have atoms up here,

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but then you're going to have gaps,

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there's going to be air here.

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Let me actually scroll down a little bit.

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So it's going to be like this,

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so you're going to have gaps like that.

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And it also has internal gaps, like that.

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So this would be the wood,

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while the metal is much denser.

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And the surface is actually much smoother.

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So the metal, let me do the metal in that white color,

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the metal atoms are much more closely packed.

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It is much denser, the surface is smoother,

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it won't have any internal air pockets,

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it's not going to have any internal air pockets in it.

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And so what's going to happen?

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Well, we've always said, you're going to have

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a transfer of heat from the higher temperature system,

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or the higher temperature thing,

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to the lower temperature thing.

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And so, they're already going to have some kinetic energy,

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these things are going to have an average kinetic energy

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that's consistent with 70 degrees fahrenheit.

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So, let me just draw a couple of these arrows.

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Same thing over here, they're going to have

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the same average kinetic energy.

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So these things are all jostling around,

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bouncing around and pushing on each other

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with the electrostatic forces.

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So, hopefully this gives you an idea of things.

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But, my hand is warmer,

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my hand has a higher average kinetic energy.

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And so the atoms and molecules of my hand

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are going to bounce into

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the atoms and molecules of the wood,

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and they're going to transfer the kinetic energy.

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But we realize in the wood is,

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I'm making less contact.

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Because, first of all, the surface of the wood isn't smooth,

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so I'm making less contact.

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So this one over here might just bump into

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another air particle, it actually won't

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bump into a wood particle.

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But some of the wood particles will start to

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take some of the kinetic energy away from me.

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And I will sense that as being a little bit cool,

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so maybe that takes a little kinetic energy,

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that bumps into this guy.

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So the kinetic energy does get transferred down.

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But it's going to be transferred down a lot slower

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than what would happen in the metal.

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Because, one, I don't have as much surface contact

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between my hand and the wood,

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because of these gaps.

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I also have air pockets in the wood, like this.

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And in general, the wood is less dense.

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So, there's going to be less collisions

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and it's going to take more time

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for that kinetic energy to be transferred

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away from my hand.

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And the metal, on the other hand,

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as soon as this atom bumps into this one,

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that's going to bump into that one,

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that's going to bump into that one,

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that's going to bump into that one.

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And that kinetic energy is going to be

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very quickly transferred down the metal.

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So it's going to be able to take more heat away from me.

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So this molecule over here is going to get

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some kinetic energy from a molecule in my hand.

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But then, it's going to bump into it's neighbor

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and transfer that kinetic energy.

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So it's going to lose it's kinetic energy quite quickly,

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and so it's ready to be bumped into again

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by another molecule from my hand,

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and take on more kinetic energy.

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So it's going to sap the heat away from me faster.

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So you have faster heat transfer,

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than you have with the wood.

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And, from your body's point of view,

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this heat being sapped away from you faster,

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even though the two surfaces are actually

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the same temperature,

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you perceive this, your body perceives this

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right over here as being colder.