Feynman 'Fun to Imagine 2: Fire

The atoms like each other to different degrees.

Oxygen, for instance, in the air,

would like to be next to carbon,

and if they get near each other, they snap together.

If they're not too close, though, they repel, and they go apart.

So they don't know

that they could snap together.

It's just as if you had a ball that

was trying to climb a hill, and there was a hole it could go into,

like a volcano hole, a deep one.

It's rolling along.

It doesn't go down in the deep hole,

because it starts to climb the hill

and then rolls away again.

But if you made it go fast enough,

it'll fall into the hole.

And so if you have something like wood and oxygen,

there's carbon in the wood from the tree.

And the oxygen comes and hits it, carbon, but not hard enough.

It just goes away again.

The air is always -- nothing's happening.

If you can get it faster, by heating it up somehow, somewhere, somehow,

get it started, a few of them come fast, they go over the top, so to speak.

They come close enough to the carbon

and snap in.

And that gives a lot of jiggly motion, which might hit some other atoms,

making those go faster, so they can climb up and bump against other carbon atoms

and they jiggle and they make others jiggle, and you get a terrible catastrophe,

which is one after the other all these things are going faster and faster,

and snapping in, and the whole thing is changing.

That catastrophe is a fire.

It's just a way of looking at it.

And these things are happening.

The perpetual...

Once it gets started, it keeps on going.

The heat makes the other atoms capable of reaching,

to make more heat

to make other atoms and so on.

So this terrible snapping is producing a lot of jiggling.

And if I put...

Wwith all that activity of the atoms there,

and I put a cup of coffee over that mess of wood that's doing this,

it's going to get a lot of jiggling.

So that's what the heat of the fire is.

Then, of course, you see, this is what happens when you start to think.

You just go on and on.

You wonder where -- how did it get started?

Why is it that the wood's been sitting around all this time, with the oxygen all this time,

and it didn't do this earlier or something?

Where did I get this from?

Well, it came from a tree.

And the substance of the tree is carbon.

Where did that come from?

That comes from the air.

It's carbon dioxide from the air.

People look at trees and they think it comes out of the ground,

that plants grow out of the ground.

But if you ask where the substance comes from,

you find out -- where do they come from?

The trees come out of the air?

They surely come out of the -- no.

They come out of the air.

The carbon dioxide in the air goes into the tree and it changes it, kicking out the oxygen.

And pushing the oxygen away from the carbon, and leaving the carbon substance with water.

Water comes out of the ground, you see.

Only -- how did it get in there?

It came out of the air, didn't it?

It came down from the sky.

So, in fact, most of the tree,

almost all of the tree, is out of the ground.

I'm sorry.

It's out of the air.

There's a little bit from the ground.

Some minerals and so forth.

Now, of course I told you the oxygen --

we know the oxygen and carbon stick together very tight.

How is it the tree is so smart,

as to manage to take the carbon dioxide,

which is the carbon and oxygen nicely combined, and undo that so easy?

Ah, life.

Life has some mysterious force?

No, the sun is shining.

And it's the sunlight that comes down and knocks this oxygen away from the carbon.

So it takes sunlight to get the plant to work.

And so the sun all the time is doing the work of separating the oxygen away from the carbon.

The oxygen is some kind of terrible by-product which it spits back into the air,

and leaving the carbon and water and stuff to make the substance of the tree.

Then when we take the substance of the tree and stick it in a fireplace,

and there's all the oxygen made by these trees,

and all the carbon -- would much prefer to be close together again.

And once you let the heat to get it started,

it continues, and makes an awful lot of activity while it's going back together again,

and all this nice light and everything comes out, and everything is being undone.

You're going back from carbon and oxygen back to carbon dioxide.

And the light and heat that's coming out, that's the light and heat of the sun that went in.

So it's sort of stored sun that's coming out,

when you burn a log.

Next question.

How is it the sun is so jiggly?

So hot?

I gotta stop somewhere.

I'll leave you something to imagine.