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Homework Help: Find the current induced by a magnetic field, and the power

  1. Nov 10, 2012 #1
    1. The problem statement, all variables and given/known data
    A technician wearing a brass bracelet enclosing area 0.005 00 m2 places her hand in a solenoid whose magnetic field is 5.00 T directed perpendicular to the plane of the bracelet. The electrical resistance around the circumference of the bracelet is 0.020 0 Ω. An unexpected power failure causes the field to drop to 1.50 T in a time of 20.0 ms. Find (a) the current induced in the bracelet and (b) the power delivered to the bracelet.

    2. Relevant equations
    ΔB = 5.00T - 1.50T = 3.5T (which I will use)
    A = 0.005 m2
    R = 0.02Ω
    t = 0.02s
    Flux = BA
    ε = Flux/t
    I = ε/R
    P = ε2/R

    3. The attempt at a solution
    Flux = 3.5T X .005m2 = 0.0175 Wb
    ε = 0.0175 / 0.02s = 0.875 V
    I = 0.875V / 0.02Ω = 43.75A


    P = 0.8752/0.02 = 38.28w

    My question is: Did I handle correctly the difference in Magnetic field by using 5-1.5?
  2. jcsd
  3. Nov 10, 2012 #2


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    Homework Helper
    Gold Member
    2017 Award

    Yes. The induced emf is equal to the change in flux divided by the time interval. ε = [itex]\Delta[/itex] [itex]\Phi[/itex]/[itex]\Delta t[/itex]. So, you are correct to use the change in B to find the change in the flux.
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