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Homework Help: Calculating the induced emf in a solenoid, have worked on it, can't get answer

  1. Oct 29, 2009 #1
    1. The problem statement, all variables and given/known data
    A solenoid of length 3.7 cm and diameter 0.95 cm is wound with 160 turns per cm. The current is decreasing at a rate of 38.0 A/s.
    (a) What is the induced emf in one of the windings?

    (b) What is the induced emf in the entire solenoid?

    2. Relevant equations

    emf = -dΦ/dt = μ0N²(A/L) di/dt
    μ0 is the permeability of free space = 4π × 10-7 H/m
    N is the number of turns
    A is the area where the magnetic flux passes through = π(d/2)²
    d is the diameter of the solenoid
    L is the length of the solenoid
    di/dt is the rate of change in the current
    For the first part I set N=1 and for the second I set N=160
    3. The attempt at a solution
    4pi * 10^-7 (1)^2 (pi(.95)/2)^2/3.7 (36)
    I get 9.15 E-6 which is not the correct answer and for B when I plug in the second set of numbers I get 234 milliamps or .234 amps
  2. jcsd
  3. Oct 29, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to PF :smile:

    What is B for a solenoid? Note that the expression is in terms of the turns per length ... do not simply use N=1 to calculate B.
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