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Homework Help: Rate of change of radius through a a circular wire loop?

  1. Apr 19, 2009 #1
    A circular wire loop of radius r = 19 cm is immersed in a uniform magnetic field B = 0.690 T with its plane normal to the direction of the field. If the field magnitude then decreases at a constant rate of −1.0×10^-2 T/s, at what rate should r increase so that the induced emf within the loop is zero?


    I tried this and I got an equation of 0=(Bcos0)(dA/dt)+(Acos0)(dB/dt)
    and I plugged it in to get dA/dt=.0016m^2/s and then i solved for the radius in this case which is .0228m and my answer is supposed to be in mm/s so i got 22.8 mm/s as my answer.
  2. jcsd
  3. Apr 19, 2009 #2


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    Homework Helper

    Area = πr^2.
    hence dA/dt = 2πr*dr/dt.
    Now find dr/dt.
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