Anyone who likes quantum mechanics and special relativity... help.

[2Xtreme21]

Yes, it's homework. No, I'm not asking you to solve anything for me.

With that out of the way, can someone explain to me how a particle's momentum can be expressed in terms of eV/c ? And is this the correct equation relating a given energy and a given momentum to a particle's mass: E^2 - (pc)^2 = (mc^2)^2 ? Found it on Wikipedia, but everytime I use it I never get the mass correctly. Though, the problem states that the answer should be expressed in the unit "GeV/c^2"... is that clue enough that I can disregard the c's in the equation?

Thanks in advance for the help.

 

[BD2003]

Originally posted by: 2Xtreme21
Yes, it's homework. No, I'm not asking you to solve anything for me.

With that out of the way, can someone explain to me how a particle's momentum can be expressed in terms of eV/c ? And is this the correct equation relating a given energy and a given momentum to a particle's mass: E^2 - (pc)^2 = (mc^2)^2 ? Found it on Wikipedia, but everytime I use it I never get the mass correctly. Though, the problem states that the answer should be expressed in the unit "GeV/c^2"... is that clue enough that I can disregard the c's in the equation?

Thanks in advance for the help.

What makes you think you can just disregard part of the equation?

 

[ForumMaster]

you might get better responses if you post this in Highly Techincal

 

[quentinterintino]

well, i'm not sure if this is what you're going for but the momentum for a particle is simply p =m*c... of course, if relatvistic effects are taken, the mass m is defined by the equation: m = m0/sqrt(1-v^2/c^2). It's often convenient to express mass as energy (E = 0.5*m*v^2). [v]=m/s



energy can be in units of eV. [eV] = electron volts

 

[2Xtreme21]

Thanks quentinterintino. I guess what I'm getting confused about is if I have p = 4x10^9 eV/c, should I put 4x10^9 = m * c ? Or (4x10^9 / 3x10^8) = m * 3x10^8 ?

 

[quentinterintino]

well, momentum is generally expressed in terms of mass * velocity. your mass is eV, velocity is c. I would think the former answer is correct. If you post the problem in full I can try and give you a better answer.


EDIT: by the way, the latter equation should look more familiar in this form E=mc^2. This guy named albert got famous from it.

 

[mchammer]

42

 

[2Xtreme21]

Originally posted by: quentinterintino
well, momentum is generally expressed in terms of mass * velocity. your mass is eV, velocity is c. I would think the former answer is correct. If you post the problem in full I can try and give you a better answer.


EDIT: by the way, the latter equation should look more familiar in this form E=mc^2. This guy named albert got famous from it.

"A physicist measures the energy of a particle to be 5 GeV and the momentum to be 4 GeV/c. What mass would she measure for this particle (express your answer in units of GeV/c2)?"

I'll try doing 4x10^9 / 3x10^8, but I'm not sure how the units would then correctly be GeV/c^2. Maybe I'm just thinking way too much into it..

I appreciate your help. Thanks.

Edit: Doing that doesn't work. Now I only have 2 more tries to get it right before the system locks me out heh.

 

[quentinterintino]

Do some dimensional analysis... remember 1 eV = 1.6e-19 J. Good luck -- I'm going to sleep

 

[2Xtreme21]

Thanks for your help.

 

[silverpig]

Originally posted by: quentinterintino
well, momentum is generally expressed in terms of mass * velocity. your mass is eV, velocity is c. I would think the former answer is correct. If you post the problem in full I can try and give you a better answer.


EDIT: by the way, the latter equation should look more familiar in this form E=mc^2. This guy named albert got famous from it.

That is not right at all.

E^2 = (pc)^2 + (mc^2)^2

You are given that the energy is 5 GeV and the momentum is 4 GeV/c... just plug those numbers in.

25 GeV^2 = 16 GeV^2 + (mc^2)^2

9 GeV^2 = (mc^2)^2

3 GeV = mc^2

m = 3 GeV/c^2

 

[silverpig]

Oh, and this problem is simpler if you use c=1 units. Physicists often set h(bar) = c = 1 units. Heh, I even had a prof who set 2*pi = 1