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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


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    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


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    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!
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