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.
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Six Easy Pieces by Richard P Feynman, Penguin Goup, Copyright California Istitute of Technology 1963,1989,1995 Quote is on page 16.

Thursday, November 25, 2010

Feynman: What Was He Thinking?

In his book “Six Easy Pieces" Richard P Feynman said this:

“In figure 1-2 we have a picture of steam. This picture of steam fails in one respect: at ordinary atmospheric pressure there might be only a few molecules in a whole room ....”

Last post I showed how actually, there'd be about 143,000,000,000,000,000,000,000,000 of them in that 40 foot square lecture room.

Feynman was too great a physicist to make an error of that magnitude- so I'm guessing there is some kind of a misunderstanding.

Earlier in the book he said you'd have to increase the size of a drop of water a billion times before you could see the water molecules in it. According to Wiki Answers a water molecule is about .0000000003 m big, so each molecule would be 30 cm. big (about a foot).

So think of a magnifying glass looking at a thousands of dots on a page. With each magnification, the dots are bigger, but you see less of them. Eventually you magnify it so much you can only see a couple of the dots at a time. So maybe that's what he meant.

So what if you shrunk that room so it got smaller and smaller. At half the volume, it would only hold half the molecules, at a quarter it would only hold one quarter of the molecules, so at a billionth volume it would only hold one billionth of the molecules. What you are doing is the same as with the magnifying glass, only in 3 dimensions, not just 2.

So let's divide the number of steam molecules in the room by 1 American billion. To do this you'd take the 143,000,000,000,000,000,000,000,000 and knock off 9 zeroes from the end. But that's just shrinking it down a billion times lengthwise. You also want to shrink it down 1 billionth of its height and 1 billionth of its width. So move the decimal point another 18 places to the left. Doing that you get .143 molecules in that room. Put another way, you'd only have a molecule in there about 1/6 of the time.

So it still doesn't agree with Feynman. Of course I might have got the height of the ceiling in the lecture room wrong, so let's double it. That's .286 molecules.

Maybe he meant that when the air is fully saturated. Wikipedia says that the most you can get is 30 grams of water in a square meter of air. I was calculating it with 12. Two and a half times 12 is 30 so let's multiply the number of molecules in the shrunken room by two and a half.

You get .715 molecules. Meaning you'd get a molecule in there about 70% of the time. It's still not what Feynman said. So I think he got it wrong. My husband who understands physics very well (and has an Honours degree in Mathematics) agrees with me.
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Six Easy Pieces by Richard P Feynman, Penguin Goup, Copyright California Istitute of Technology 1963,1989,1995 Quote is on page 7.

Monday, November 22, 2010

I Tackle Feynman

In my previous blog post I took on Steven Holtzner, the author of Physics Workbook for Dummies. I've found four mistakes now, the earliest being on page 10. When I told my 18 year old son about them, he gave me a bunch of other, better physics books to read, one being Six Easy Pieces by Richard Feynman. This is a series of lectures he did for first and second year college kids at Caltech in 1961-62. These lectures were written down, then carefully edited for publication.

In his obituary, Richard Feynman was described by the New York times as “arguably the most brilliant, iconoclastic and influential of the postwar generation of theoretical physicists.” He won a Nobel prize for his work on quantum electrodynamic theory. Referring to this book, Feynman said “ I tried very hard to make all the statements as accurate as possible.”

So surely there wouldn't be any errors in HIS book.

Yet I found one. On page 7! There he says:

“In figure 1-2 we have a picture of steam.



This picture of steam fails in one respect: at ordinary atmospheric pressure there might be only a few [water] molecules in a whole room ....”

To me that's blatantly wrong. You can SEE steam. In order to see it, there must be a huge number of water molecules in it, thus the statement must be false. So I decided to find out how many molecules of steam there really are in a room.

First of all, what IS steam? Wikipedia says it is water molecules in the air, commonly known as water vapour.

To make things simple, I figured I'd first work out how many grams of water there'd be in a cubic meter of air and work from there. For those who don't know metric, a meter is about a yard, and a gram is about .04 ounces.

Wikipedia says:

If all the water in one cubic meter of air were condensed into a container, the mass of the water in the container could be measured with a scale to determine absolute humidity.... Absolute humidity on a volume basis is the quantity of water in a particular volume of air.

That's exactly what I'm trying to work out.

So, how to you find out what this number is?

Well, it depends on the temperature and the relative humidity of the air, as you can see from this chart.

At the moment, my room is at 21 degrees C and the relative humidity is 68%. Caltech is in Pasadena California, so this would have been normal for them in the first weeks of October 1961 when Feynman would have given this lecture. So, using this chart, making the temperature to be 20 degrees and the humidity to be 70%, you get that the absolute humidity is about 12 g/m3. So in every cubic meter of air in my room there are 12 grams of water.

So how many water molecules are there in a gram of water? Yahoo Answers says:

If you have 1 g of water, you therefore have 1/18 of a mole, or 3.34E22 molecules.

E just means “times ten to the power of” . To make things simple, let's call 10 to the power of 22 a muvillion (a word I made up).

So, you have 3.34 muvillion molecules in a gram of water. But I have 12 grams in a cubic meter of air, so that makes 12 x 3.34 which equals 40.08 muvillion molecules in a cubic meter of air.

Now Feynman said on a previous page that the average [lecture?] room was 40 feet by 40 feet. He didn't say how high the ceiling was, so I'll say it's the height of our ceiling, which is 2.4 meters. 1 Foot = 0.3048 Meters, so 40 feet is 12.192 meters, so the calculations can begin.

12.192 m (length of room) x 12.192 m (width of the room) x 2.4 m (height of the room)= 356.75 cubic meters in a room that size. Now multiply that by the 40.08 muvillion molecules in a cubic meter of air and you have the number of water molecules in a room that size.

The answer comes to 14298.5 muvillion. Written out and rounded off that comes to 143,000,000,000,000,000,000,000,000 steam molecules in that room. That's a heck of a lot more than three!

So what was Feynman thinking? That's my next post.

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Physics Workbook for Dummies, by Steven Holtzner, Wiley Publishing Inc 2007

Six Easy Pieces by Richard P Feynman, Penguin Group, Copyright California Institute of Technology 1963,1989,1995 The quote is on Page xxiv.

chart: http://www.tis-gdv.de/tis_e/misc/klima.htm

Saturday, November 20, 2010

I Tackle Physics

I haven't posted for a while, and in that time I decided to ditch the study of religion in favour of studying science, specifically, physics.

I gave up on religion because whatever religion you are checking out, its followers expect you to accept whatever they say as truth. On further investigation you find out that what they say doesn't agree with their own Holy Books because most of them haven't even read them. And yet they teach.

Surely the study of science is going to be way less frustrating. So I went and bought myself Physics Workbook for Dummies, by Steven Holzner, PhD because I figured that way I could start right from the beginning and work my way up.

First section is measurements and conversions. Straight forward right? No way you could disagree with the stuff that's at the most basic beginning right?

Well, I was wrong. I've only gotten to page 15 and I've already found 3 errors. Heaven knows how many there are going to be when I get to the more advanced section.

For instance, on page 12 it says that 1 meter is 39.37 inches. But when they calculate how many inches in two meters, they tell you the answer is 78.6. So obviously they rounded it down to 39.3, though any 4rth grader could tell you that you should round up if the last digit is a 7. Besides, they never said you should round. That's in the next lesson. So the correct answer, is 78.74, not 78.6 which they said. But hey, that's a minor thing compared to this question:

Question 15. How many centimeters in a kilometer? For those of you who aren't metric, there are 100 cm in a meter and a thousand meters in a kilometer, so the answer should be 100,000, or 1.0 x 10 to the power of 5. However, the answer they give is 1.0 to 10 to the power of -5, or 0.00001. Hey, they are only wrong by a factor of 10,000,000,000!