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Homework Help: Magnetic fields and induced EMF

  1. Jun 11, 2008 #1
    1. The problem statement, all variables and given/known data
    An coil has 4000 turns of wire and a mean area of cross-section 25cm^2. A magnetic field of flux density 0.16T passes through it. When the magnetic field is reduced steadily to zero in 0.0050s, emf is induced across the terminals of the coil. Calculate the value of this induced emf.

    2. Relevant equations

    E = N D Φ /Dt

    3. The attempt at a solution

    E = 4000 X (0.25 X 0.16) ÷ 0.0050 = 32 X 10^3V?

    Or heres another solution
    Ɛ = - N∆Φ / ∆t

    25 cm² = 25 x 10^-4 m²

    Φ = NBA = 4000 turns x 0.16 T x (25 x 10^-4 m²) = 1.6 Wb

    Ɛ = - (4000 turns x 1.6 Wb) / 0.0050 s = - 1.28 x 10^6 V or - 1.28 MV

    Please help. The second one seems more logical however I'm not sure its right. Thanks
  2. jcsd
  3. Jun 11, 2008 #2


    User Avatar
    Homework Helper
    Gold Member

    Your initial equation can be used to solve the problem:


    You are making an error though. You are not converting the area units correctly.


    not 0.25m^2
    Last edited: Jun 11, 2008
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