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Homework Help: Deflection and Magnetic Fields in Thomson's Experiment

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data

    Calculate the magnetic field required to produce a deflection of .20 radians in Thomson's experiment. Assume the potential across the deflection plate is u=200V, the length of the deflection plates is x=5.0 cm, and their separation d=1.5cm. Compare this value of B to the Earth's magnetic field (B=.5 G = .5 * 10^-4 T).

    2. Relevant equations

    e/m(e) = u theta / B^2 * x * d

    e/m(e) = 1.7588 * 10^11 C/kg

    3. The attempt at a solution

    1.7588 * 10^11 C/kg = (200V) (.2 rad) / (B^2)(.05m)(.015m)

    B^2 = (1.516*10^-6 T^2)(.2 rad)

    Now I can't figure out how to convert theta from radians to make this make sense. Do I just multiply by .2? Then I get that B=.55*10^-3 T, but I think the right answer is closer to the Earth's magnetic field.
  2. jcsd
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