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Homework Help: Electron deflected via magnetic field

  1. Jun 8, 2010 #1
    1. The problem statement, all variables and given/known data

    an electron is accelerated through a potential difference from 0v to N kv. It is then ejected through a slit and deflected 10* over a distance r of 2 cm.

    find B ,v_f, and t(time)

    2. Relevant equations

    qdelta V = 1/2Me v_f^2 (solved for vf)

    M(v/r)= qBsin[tex]\theta[/tex] solved for B

    3. The attempt at a solution

    As listed above I solved for the variables listed (assuming I did it right, please correct me if Im wrong)
    but what is the relation of t to this, is it a kinematic or a break down of db/dt?
  2. jcsd
  3. Jun 8, 2010 #2

    Andrew Mason

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    Homework Helper

    You have done the first part correctly.

    However, for the second part you seem to be using the formula for centripetal force, which is not correct. r is not the radius of curvature. r is the distance the electron travels from the slit opening, over which distance it is deflected 10 degrees.

    So this part concerns a deflection of 10 degrees over a distance of 2 cm. after accelerating through the potential difference. How long did it take for the electron to travel that 2 cm.? That is the time that you need to find.

    The force (Lorentz force) multiplied by the time over which it acts gives you the impulse received by the electron (after passing through the slit).

    The change in velocity after passing through the slit is easy to calculate. So you can determine that change in momentum.

    How is change in momentum related to impulse? That will give you the correct answer.

    Last edited: Jun 8, 2010
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