Sunday, November 28, 2010

Feynman and Fire

Today I tackle fire.

Feymman's book “Six Easy Pieces” is a primer on everything in physics, so I decided to read it. Last time I got stuck on page 7 and found something wrong, this time I got stuck on page 16. I'm not saying he got this wrong too, I'm just saying I don't understand it. Here's what he said:

Now, for example, one of the oxygen molecules can come over to the carbon, and each atom can pick up a carbon atom and go flying off in a new combination-- “carbon-oxygen”-- which is a molecule of the gas called carbon monoxide. It is given the chemical name CO. It is very simple: the letters “CO” are practically a picture of that molecule. But carbon attracts oxygen much more than oxygen attracts oxygen or carbon attracts carbon. Therefore in this process the oxygen may arrive with only a little energy, but the oxygen and carbon will snap together with a tremendous vengeance and commotion, and everything near them will pick up energy. A large amount of motion energy, kinetic energy, is thus generated. This of course is called burning; we are getting heat from the combination of oxygen and carbon. The heat is ordinarily in the form of the molecular motion of the hot gas, but in certain circumstances it can be so enormous that it generates light. That is how one gets flames.

He's explaining chemical reactions and how you get fire. He says that you get light and flame when carbon and oxygen atoms smash together to form a new molecule. He says this agitates the molecules around them and that causes heat. But I don't see how.

I could see it if say, fire was caused by atoms flying OFF a molecule. They could hit molecules around them with great force, which would make those hit others which in turn hit more. Fast moving molecules cause heat. Lots of heat is one of the things you need to make fire.

But atoms imploding? I don't get that. Maybe it's like two people seeing each other across a crowded room? Only instead of walking through the crowd, they run, smashing into everyone that's in the way? But he said the others don't get agitated until after the carbon and oxygen meet.

Again, I could see it if the collision had pieces break off, flying everywhere with such force that these pieces bumped the molecules around them, but that doesn't happen at the molecular level, so I just couldn't get it.

So I decided to check into it.

I looked up fire, burning, combustion, exothermic reactions, endothermic reactions and numerous other things and got nowhere nearer finding an answer to the question, so I asked my husband.

He said that when two things collide, you have two principles involved, the conservation of energy and the conservation of momentum. Let's say you have 2 atoms that are the same weight and size which are going in the same direction. One is going 3 meters per second and it hits the other one which is going at 1 meter per second. What happens if they stick together once they've hit? The law of conservation of momentum says one would slow down and the other one would speed up and they'd end up going the average of their two speeds. This change in speed would change the amount of energy stored in the atoms. (The amount of energy stored in something is called kinetic energy).

So at first you have 1 squared units of kinetic energy (which is 1x1=1) and 3 squared units of kinetic energy (which is 3x3=9). Add them together, you get 10.

But after the collision, they are both going two meters a second so 2 units of kinetic energy squared (2x2=4) plus 2 kinetic energy squared (2x2=4) you get 8.

So you went from 10 energy to 8 energy which is less. Having less energy would mean things get colder, not hotter, which you need for a flame. If the atoms hit head on, things would be even worse because they would slow down even more. Which backs up what I said.

So obviously something is wrong with the model. Earlier in the paragraph Feynman says that the carbon came off a solid crystal such as graphite or diamond. Maybe that makes a difference?

I'll try and figure it out for the next post.
Six Easy Pieces by Richard P Feynman, Penguin Goup, Copyright California Istitute of Technology 1963,1989,1995 Quote is on page 16.