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Homework Help: Concentric Circles & Magnetic fields

  1. Nov 19, 2008 #1
    1. The problem statement, all variables and given/known data

    Figure a below shows two concentric circular regions in which uniform magnetic fields can change. Region 1, with radius r1 = 1 cm, has an outward magnetic field 1 that is increasing in magnitude. Region 2, with radius r2 = 2 cm, has an outward magnetic field 2 that may also be changing. Imagine that a conducting ring of radius R is centered on the two regions and then the emf around the ring is determined. Figure b gives emf as a function of the square R2 of the ring's radius, to the outer edge of region 2. The vertical axis scale is set by s = 24 nV.

    (a)What is the rate dB1/dt?

    (b)What is the rate dB2/dt?

    (c) Is the magnitude of 2 increasing, decreasing, or remaining constant?
    B2 is increasing
    B2 is decreasing
    B2 is remaining constant

    2. Relevant equations
    I don't think there are any equations for this problem.

    3. The attempt at a solution

    I know that once I find part b, if it is 0, then for part c B2 is remaining constant. If B2 is positive then the anwser for part c is that B2 is increasing. Finally, if B2 is negative in part B, then part c will be decreasing.

    I think for part A it is just a matter of looking at the graph and counting how many nV there are at each distance of the ring. I'm not really sure where else to go from there.

    Any help is appreciated, thanks.
  2. jcsd
  3. Nov 19, 2008 #2


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    Staff Emeritus
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    Homework Helper

    The equation (yes, there is one) relevant to this problem will be in your physics textbook. Look for "induced emf" or "faraday's law" or "induction", either in the contents or index.
  4. Nov 19, 2008 #3
    Okay great, I found Faraday's Law (E = -d(Magnetic Flux)/dt).

    I will try to work with this and see if this gets me any closer to a solution.

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