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Homework Help: Charge-to-mass ratio of electron/proton

  1. Feb 23, 2010 #1
    1. The problem statement, all variables and given/known data
    The Helmholtz coil current = 2.45 A, the magnetic field = 1.91 x 10^(-3) T, the accelerating voltage = 295 V, and the beam radius = .03m.

    a) Calculate the e/m ratio of an electron. Show that e/m = 2V / r²B²

    b) During the initial use of this experiment, physicists also knew the charge-to-mass ratio of a hydrogen ion, approximately 9.58 x 10^7 Coulombs/kg. If you had little knowledge of atomic structure, what two interpretations could you place on the differences between the two ratios?

    2. Relevant equations
    PE = Ve
    KE = 1/2mv²
    Fmag = evB
    Fcent = mv²/r

    3. The attempt at a solution
    a) Ve = 1/2mv²
    v = √(2Ve / m) ; v² = 2Ve/m
    Fmag = Fcent
    evB = mv²/r
    r = mv/Be

    I can't seem to get past this part of the derivation. I've tried subbing in √(2Ve / m) into r = mv/Be but it turns out too messy.

    b) I'm not too sure about the second part at all.. in what kind of context should I be thinking about the ratios?
  2. jcsd
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