Energy of relativistic particle in LHC

[Saitama]

Homework Statement

(see attachment)
The proton charge is $1.6 \times 10^{-19} C$ and the speed of light is $3 \times 10^8 m/s$. The proton's mass is not necessary in this problem.

Homework Equations

The Attempt at a Solution

The particle revolves in a circular path, hence
\frac{mv^2}{R}=qvB
mv=eRB
(e is the charge of particle)
Since p=mv (momentum)
p=eRB
I don't understand how the mass of proton is not necessary for the problem. Is my expression for p correct?

Any help is appreciated. Thanks!

 

[mfb]

Your equations do not use the proton mass, how could it be relevant? You can use the approximation $E=pc$ to get the energy, as the mass of the proton is negligible relative to its momentum.

Your formulas are good for nonrelativistic particles only, you cannot use this approximation for protons in the LHC.
The result is the same with relativistic formulas, but that is just a coincidence here.

The sketch has wrong positions for LHCb and ALICE :(.

 

[Saitama]

Your equations do not use the proton mass, how could it be relevant? You can use the approximation $E=pc$ to get the energy, as the mass of the proton is negligible relative to its momentum.

Your formulas are good for nonrelativistic particles only, you cannot use this approximation for protons in the LHC.
The result is the same with relativistic formulas, but that is just a coincidence here.

The sketch has wrong positions for LHCb and ALICE :(.

Thanks a lot mfb for the help!

 

[rude man]

1. I don't understand how the mass of proton is not necessary for the problem. Is my expression for p correct? p=eRB

Yes it is, even relativistically, if you remember p is the relativistic momentum = mv = γm₀v.