
#1
Aug906, 10:47 AM

P: 6

Hi, this is my first post, and I'm sure this is a very basic question: Suppose two electrons collide, and thus momentum is conserved. Momentum, as we all know, is defined as the product of mass and velocity. But the means of transferring momentum between these electrons is a photon, which is massless. So, it seems there must be some point at which momentum is being transferred, but at which the magnitude of the momentum is zero, and thus is not conserved. So, I'm confused: just how is momentum transferred via massless photons while also being conserved? Thank you very much!
Joe 



#2
Aug906, 11:34 AM

Mentor
P: 40,876





#3
Aug906, 11:52 AM

P: 6

Thanks for the response. So, how is momentum, when it's carried by a photon, defined?




#4
Aug906, 12:02 PM

Mentor
P: 40,876

Momentum Transference[tex]E^2 = p^2c^2 + m^2 c^4[/tex] For photons, m = 0, so: [tex]p = E/c = hf/c[/tex] 



#5
Aug906, 12:04 PM

Emeritus
Sci Advisor
PF Gold
P: 9,789

[tex]P = \frac{h}{\lambda}[/tex] Where P is momentum, h is plank's constant and [itex]\lambda[/itex] is the wavelength of the photon. Edit: Doc Al beat me to it 



#6
Aug906, 01:04 PM

P: 6

Excellent. Thank you very much!



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