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Magnet Flux + Fields Problem - Please help!

  1. Aug 19, 2008 #1
    1. The problem statement, all variables and given/known data
    A coil of radious 4cm has its plane perpendictular to a uniform magnetic field of strenght 2 mT directed into the page as in the following diagram


    The magnetic field is then switched off is such a way that it takes 1ms to drop to zero. When switched on again it also takes 1 ms. Switching off the field results in a momentary current of 4ma flowing through the milliameter from X to Y.

    a) What is the change in magnetic flux through the coil when the field is turned off?

    b) What is the rate of change of flux through the coil during this time?

    2. Relevant equations

    3. The attempt at a solution
    a) [tex]\Phi[/tex]=BA
    => ((3.14)x(4*10^-3))^2 x (2x10^-3)

    b) I = [tex]\frac{change in magnetic flux}{change in time}[/tex]
    => (4x10^-3) = (change in magnetic flux)/ (1x10^-3)
    Right = 0.01 Wb <-- How??
  2. jcsd
  3. Aug 19, 2008 #2


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    Homework Helper

    The rate of change of magnetic flux doesn't give you the current. It gives you the induced emf.

    You have found [tex]\triangle \Phi_B[/tex]. You know what is [tex]\triangle t[/tex]. See how to find the rate of change of flux?
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