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Another question on Lenz's Law

  1. Feb 21, 2005 #1
    For the situation shown in the figure below, the magnetic field changes with time according to the expression B = 2.07t^3 - 3.95t^2 + .808 and r2 = 2R = 4.96 cm.
    Figure: http://east.ilrn.com/books/sepsp06t/pse6e.31.32p.e.jpg

    (a) Calculate the magnitude of the force exerted on an electron located at point P2 when t = 1.92 s.

    (b) At what time is this force equal to zero?

    So, F = qvB and v = (qBr)/m, where m is the mass of an electron.
    I got the velocity formula by using F = ma, and substituting F as qvB and a with v^2/r. Then I solved for v. I used the two formulas to calculate the force, but is it incorrect.

    Am I doing this right or am I totally wrong?
    Any help would be great! Thx!!!
  2. jcsd
  3. Feb 21, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Who says the electron is moving?

    Use Faraday's law to find the induced electric field due to the changing magnetic flux. Hint: Apply Faraday's law to a circle of radius 2R.
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