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Homework Help: Magnitude of current induced

  1. Mar 30, 2015 #1
    1. The problem statement, all variables and given/known data
    . A circular coil of radius 5.0 cm and resistance 0.20 Ω is placed in a uniform magnetic field perpendicular to the plane of the coil. The magnitude of the field changes with time according to B = 0.50e-20t T. What is the magnitude of the current induced in the coil at the time t = 2.0 s?

    2. Relevant equations

    Φ = BAcos(0) = BA
    emf = -dΦB /dt = -d(BA)/dt = -A * d(B)/dt

    3. The attempt at a solution

    I differentiated that magnetic field and got -10e-20t, then multiplied that times pi*(.052), and ultimately divided by .2 Ω. My answer is 1.66*1018, which is nowhere near any of the answers. I feel like I'm following a logical path but obviously this isn't working, so I don't know what else to do.
  2. jcsd
  3. Mar 30, 2015 #2


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    Homework Helper
    Gold Member

    Your mistake is that you dont take into account the self inductance L of the coil. It will be [itex] E+L\frac{dI}{dt}+IR=0[/itex] where E(t) exactly as you calculated and R=0.2Ohm. You still have to calculate L from the geometrical data of the coil and then solve the differential equation. In doing that becareful that E is not constant but varies with time as of course the current I(t) do so also.
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