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Homework Help: J j thomson's previous experiment

  1. Mar 10, 2004 #1
    j j thomson's "previous" experiment

    I thought I was going to like this book, but it's defying me.

    I'm staring at pg 123 of Tipler's Modern Physics text (3rd ed.) This is talking about an experiment he says Thomson did before the "famous" J J Thomson experiment. It goes like this:

    That's all he says about that experiment, except for a brief note later on that the measurements obtained from that one were actually more accurate than the measurements he obtained from the later, and more well-known, method.

    Anyway, on the preceding page, he talks a bit about the circular path followed by charged particles in a magnetic field and gives

    [tex] quB = \frac{mu^2}{R}[/tex]

    Obvious enough. (Here, little q is of course the charge on an individual particle, so it corresponds to e, not Q, in the other equations.)

    But I don't see how that helps in getting to equation 3-3 above. All I get from the two preliminary equations is
    [tex]\frac{e}{m} = \frac{Qu^2}{2W} [/tex]
    To get from that to his equation 3-3 I have to say
    [tex]u^2 = \frac{4W^2}{B^2R^2Q^2}[/tex]
    and I don't see anything that lets me say that.

    Any ideas?
  2. jcsd
  3. Mar 10, 2004 #2
    I'm not familiar with Thomson's "previous" experiment, but if you look at the two equations:

    [tex] quB = \frac{mu^2}{R}[/tex]
    [tex]\frac{e}{m} = \frac{Qu^2}{2W} [/tex]

    If you solve for u in the first equation you get

    [tex]u = \frac{eBR}{m}[/tex]

    which you can then substitute into the second equation to get the desired equation.

  4. Mar 10, 2004 #3
    Hmmm...I hate to admit to how long I didn't see that.

    Thanks for providing the requisite smack in the head.
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