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Don Beegles
2007-08-16, 01:32 PM
It feels wierd that my first post after my long silence (which wasn't over anything important, if anyone had wondered at all; I merely had no internet for a while, was at camp, and I'd rather talk to my girlfriend than most) is to ask a homework question, but here it goes anyway;

I'm taking AP physics this year, and the summer assignment is to read a chapter of the book and answer the questions at the end. I think I got the general idea on most of them, which is all he expects, but I have no idea on the very last one. It is "Use conservation of momentum to explain why photons emitted by hydrogen atoms have slightly less energy than predicted by Eq.27-10." Eq.27-10 is Bohr's hf=Eu-El.

I really have no idea at all, and was hoping that someone here would be able to give some clue.

PlatinumJester
2007-08-16, 01:46 PM
Physics is all fiction.

Telonius
2007-08-16, 01:58 PM
I'm not at all certain, but here's my guess. Photons do not have mass (m=0), but they do have momentum. In conservation of momentum, you have several points where the calculation is based on mass * vector. Setting mass at zero for those points may give some odd results.

mudbunny
2007-08-16, 02:00 PM
Been a long time since I did physics, but think of it this way:

Assume that the momentum of the Hydrogen starting out is X.
You end up with two particles with their own momentums.
Momentum must be conserved.

Ted_Stryker
2007-08-16, 02:13 PM
Been a long time since I did physics, but think of it this way:

Assume that the momentum of the Hydrogen starting out is P (symbol changed from X in the original post).
You end up with two particles with their own momentums.
Momentum must be conserved.

Yeah, that's basically it. You can even set P = 0 so that you are in the rest frame of the hydrogen atom.

The transition of an electron into a lower energy state releases a certain amount of energy.

Most of that energy goes into the photon, but not all of it, because of conservation of momentum. Essentially, there's a tiny bit of recoil that kicks the H atom into motion in the opposite direction of the photon's motion (EDIT -- Which is necessary to keep the total momentum = 0). This energy has to come from the energy released by the electron's transition, so the resultant photon has a tiny bit less of total energy and momentum than you would predict from the formula.

Don Beegles
2007-08-16, 05:55 PM
Thanks a lot everyone.

That's simpler than I thought. It's actually almost exactly what I was thinking, but it didn't seem right to me. Apparently it is and I was overthinking it.

Scatman
2007-08-17, 12:50 AM
Physics is all fiction.
Truer words have never been spoken.

Xuincherguixe
2007-08-17, 06:12 AM
Physics is all fiction.

Any Ninja could tell you that.

nagora
2007-08-17, 06:15 AM
Physics is all fiction friction.

An easy mistake to make.

PlatinumJester
2007-08-17, 06:21 AM
No mistake. I won't believe in Gravity until I see it with my own eyes. Neuton was such lier. He probably got crapped on by a bird BUT said it was an apple to stop himself from looking stupid.

Friction is also made up as is Radiation, electrical current and Al Gore.

Charity
2007-08-17, 06:43 AM
Tell you what if you don't belive in gravity, just lean out of the window of a tall building, say to about 90 degrees... see how you feel about it then shall we?

RandomNPC
2007-08-17, 10:24 PM
just write down that a wizard did it, you should get credit for trying.

also, gravity sucks.

gravity was the science cover of magic, see we're all pulled to the earth by magic, thats why we can't fly, it's just been around so long everyones accepted it.