Richard Feynman was an inspiring science communicator. In one of his television broadcasts [embedded below], he talks about a log burning on an open fire and provides one of the most eloquent explanations of the physical chemistry of wood combustion [full video and transcript below].
Feynman explains that throughout its life, a tree accumulates a great amount of potential energy in the form of carbon stripped by photosynthesis from its preferred molecular partner oxygen thanks to energy from the sun – that at a tree grows from the air, and not from the ground. It is only once it is provided with a source of ignition that carbon atoms in the tree and oxygen atoms in the air gain enough energy to reunite, a process that releases a great amount of energy, sustaining high temperatures and producing a chain reaction that is a fire. As Feynman put it, “the light and heat that’s coming out, that’s the light and heat of the sun that went in. So it’s stored sun that’s coming out when you burn a log!”
An interpretation of Feynman’s discussion is that combustion re-balances the imbalance caused by photosynthesis. The carbon that was taken from the atmosphere, and the energy that was taken from the sun by the tree, is quickly released back to the atmosphere during a fire – a closed cycle, where the regrowth of a new tree begins the process once again. If we extend this interpretation to the burning of ancient and fossilised carbon stores, such as peat, coal, oil, and gas; we can appreciate that the energy released comes from sunlight that once shone down on our planet many thousands or millions of years ago.
In a separate post on ‘Why wildfires fuel climate change’, I discuss how this natural component of the carbon cycle has been tipped far from equilibrium by human activity.
[Richard Phillips Feynman (May 11, 1918 – February 15, 1988)]
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 to 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, it 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 if it starts to climb the hill and then rolls away again. But if you made it go fast enough, it will fall into the hole.
And so, if you set something like wood and oxygen, there’s carbon in the wood from the tree, and the oxygen comes and hits it, the carbon, but not hard enough, it just goes away again, the air is always coming, nothing is happening.
If you can get it faster, by heating it up somehow, somewhere, or 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 them 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 are happening, they’re perpetual, once they get 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, with all that activity of the atoms there and I put a cup of coffee over that, massive wood, that’s going this, it’s going to get a lot of jiggling. So that’s what the heat of the fire is. And then of course, if; you see this is what happens when you start it, like it just go on and on.
Wonder where, how do they get started, why is that the wood has 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, and 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. The plants grow out of the ground. But if you ask where the substance comes from? You find out where does it come from, 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 that 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 that getting there 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 and we know that oxygen and carbon stick together, very tight. How is it that 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 this is 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 leave in the carbon and water and stuff to make the substance of the tree. And then we take the substance of the tree and stick it in the fireplace. And the… All the oxygen made by these trees and all the carbons 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 those nice light and everything comes out, and everything is being undone, you’re going 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 that the sun is so jiggly, so hot? I got to stop somewhere. I leave you something to imagine.