1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Unit confusion

  1. Oct 24, 2011 #1
    I just derived a little formula of protons moving in a magnetic field. With the symetry etc I have I get this:

    p = qrB

    Now I want to calculate the angular as a function like this:

    p = constant * B

    Where p is Gev/C, r is in meters.

    I don't know what numbers or better what form B and q should have to get all numbers right. Anyone care to shine some light on this..?

    Edit, my field is 1.74T r in meters.
    Last edited: Oct 24, 2011
  2. jcsd
  3. Oct 24, 2011 #2


    User Avatar

    Rule of thumb for such cases - if you are confused by units, convert everything to SI.
    You need to convert GeV into Joules, and express proton charge in Coulombs and speed of light in m/s. Google for those values!
  4. Oct 24, 2011 #3

    Something like this?

    [tex]\displaystyle p = 1.602 * 10^{-19} * 5.609*10^{35} r * 1.74 =1.56*r \frac{eV}{c^2} \frac{m}{s}[/tex]

    Edit, I see that I've confused more than the units. I wanted to compute it like this p = constant * r
    Last edited: Oct 24, 2011
  5. Oct 24, 2011 #4


    User Avatar

    OK, I see I must do it for you...
    [tex]p_{[{\rm kg\,m\,s^{-1}}]} = q_{[{\rm Q}]}r_{[{\rm m}]}B_{[{\rm T}]}
    = 1.602\cdot 10^{-19}{\rm C}\,r_{[{\rm m}]}B_{[{\rm T}]}[/tex]
    [tex]p_{[{\rm GeV}/c]} = \frac{p_{[{\rm kg\,m\,s^{-1}}]}\cdot 3\cdot 10^8{\rm m\,s^{-1}}}
    {1.602\cdot 10^{-10}{\rm kg\,m^2\,s^{-2}\,GeV^{-1}}} =
    \frac{1.602\cdot 10^{-19}{\rm C}\,r_{[{\rm m}]}B_{[{\rm T}]} \cdot 3\cdot 10^8{\rm m\,s^{-1}}}
    {1.602\cdot 10^{-10}{\rm kg\,m^2\,s^{-2}\,GeV^{-1}}} =
    0.3\, \frac{{\rm GeV}}{c}{\rm\,\,m^{-1}\,T^{-1}}\cdot r \cdot B

    Or - in other words - easier to remember and imagine - 1GeV particle makes circles of 1m radius in 3T field.
  6. Oct 25, 2011 #5
    Thanks for taking the time to help me!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Unit confusion Date
I Confusion with units : MeV/C2 Apr 24, 2016
Confusion regarding SI base units Mar 8, 2013
Units confusion- help needed Dec 3, 2012
Unit conversion confusion Sep 21, 2012
Units of Force Confusion lb/in Mar 11, 2012