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Homework Help: Ratio of charges in thomson expt

  1. Sep 20, 2013 #1
    1. The problem statement, all variables and given/known data

    three particles having charges 1:2:3 produce the same point on the photographic film in the thomson experiment. The masses are in the ratio of a) 1:2:3 b)3:2:1 c) 2:3:1 d) 1:3:2

    2. Relevant equations

    a = F/m = Eq/m

    3. The attempt at a solution

    Since all the three particles are striking at the same point on the screen, they should be subjected to same acceleration. Assuming that same electric field acts on all the three particles, then,
    q1/m1= q2/m2 = q3/m3 which implies q/m is constant. Hence, q is directly proportional to m. So the masses must be in the ratio of 1:2:3 .

    I want to confirm whether this solution is correct. Does this problem contain all the relevant information needed? I will be thankful if any body clarifies my doubt.
  2. jcsd
  3. Sep 20, 2013 #2


    User Avatar

    Staff: Mentor

    Presumably the experiment you're referring to involves J.J. Thomson's mass spectrometer? If so then you've reasoned correctly that the charge to mass ratio needs to be the same to produce the same accelerations.
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