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Homework Help: Magnetism and Centripetal force question

  1. Jun 12, 2007 #1
    1. The problem statement, all variables and given/known data

    One method for determing masses of heavy ions involves timing their orbital period in a known magnetic field. What is the mass of a singly charged ion that makes 7.0 revolutions in 1.3 x 10^-3 seconds in a 4.5 x 10^-2 T field.

    a) 2.1x10^-25 kg
    b) 1.3x10^-24 kg
    c) 6.5x10^-23 kg
    a) 5.0x10^-20 kg

    2. Relevant equations

    I'm not sure of which equation to use. Maybe....
    Fc = Fb
    m4(pi^2)r/T^2 = QvB

    3. The attempt at a solution

    m4(pi^2)r/(1.3x10^-3s/7)^2 = Qv(4.5x10^-2)

    This might be the wrong formula.
  2. jcsd
  3. Jun 12, 2007 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    It is the right formula.

    If [itex]mv^2/r = qvB[/itex] where [itex]v = 2\pi r/T[/itex] then:

    [tex]qB = m2\pi/T[/tex]

    [tex]m = qBT/2\pi[/tex]

    Just plug in the numbers.

  4. Jun 13, 2007 #3
    One caution:

    In the previously cited equation [tex]m = qBT/2\pi[/tex], T is the orbital period = (1.3 x 10^-3)/7 sec
    Last edited: Jun 13, 2007
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