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Homework Help: Show what the magnitude of induced emf

  1. Nov 19, 2011 #1
    Someone who knows what they are talking about: Show what the magnitude of induced emf

    1. The problem statement, all variables and given/known data

    Consider a magnetic field B = K(x3z2,0, -x2z3)sinωt in the region of interest, where K and ω are positive constants and t is variable time. Show that the magnitude of the induced emf around a circle R in the plane z = a with its center at x = 0, y = 0, z = a is:
    ε = (K/4)∏a3R4ωcosωt

    2. Relevant equations

    Fluxb = ∫B . dA

    3. The attempt at a solution

    Since the normal vector points in the k direction, we only have to worry about Bz.

    ∫Bzdydx. So -∫∫(sinwt)x2a3dydx.

    The make the change to polar:

    -aK3∫∫(sinwt)(rcosθ)2r dr dθ = -(K/4)a3R4∫cosθsin(wt) dθ.

    This doesn't get me anywhere. I'm not really sure what I'm supposed to be integrating over, which is probably why I'm stuck.
    Last edited: Nov 19, 2011
  2. jcsd
  3. Nov 19, 2011 #2

    I like Serena

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    Homework Helper

    Hi auk411! :smile:

    Let's start with Bz.
    Can you write it down separately?
    It seems you did not copy it correctly.

    Secondly you did not bring the constant out of the integral properly.

    Furthermore in polar coordinates you would integrate r from 0 to R, and theta from 0 to 2pi.

    And for the tip: rewrite (cosθ)2 using cos2θ.
  4. Nov 19, 2011 #3
    (cosθ)2 using cos2θ.[/QUOTE] .... huh, what trig identity are you using.

    this still doesn't answer the most pressing question. t varies, theta varies and r varies. we have 3 varying variables in a DOUBLE integral. I see no way to reduce them to two. How do I get around this.
  5. Nov 19, 2011 #4

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    First things first.
    You appear to have skipped my question, so I'll answer it myself:

    Bz = -Kx2a3sinωt

    This is not what you used.

    cos(2θ) = 2 cos2θ - 1

    No, you have 2 variables.
    You appear to be thinking spherical coordinates, but you should be thinking in cylindrical coordinates.

    z is constant at z=a.
    Only the other 2 vary.
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