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Homework Help: Solenoid Inductors problem

  1. Jun 13, 2014 #1
    1. The problem statement, all variables and given/known data

    An 85 mH solenoid inductor is wound on a form 0.80 m in length and 0.10 m in diameter. A coil is tightly wound around the solenoid at its center. The coil resistance is 8.1 ohms. The mutual inductance of the coil and solenoid is 27 µH. At a given instant, the current in the solenoid is 540 mA, and is decreasing at the rate of 2.5 A/s. At the given instant, the induced emf in the solenoid is closest to:
    A) 170 mV
    B) 150 mV
    C) 210 mV
    D) 230 mV
    E) 190 mV

    2. Relevant equations

    emf =-N (ΔΦ/Δt)

    3. The attempt at a solution

    i've tried to think of other equations to use such as ωBA sin θ where it would be 90 degrees because that's the peak. but that doesn't work.
    so i though i might be able to use the above equation but it seems like i am missing too many variables.
    i know the equation for self inductance is ( -L * ΔI/Δt )
    but it is in the given instant and not over a range of time..
    any small guidance would be much appreciated; i'm not sure where to go from here.
    Last edited: Jun 13, 2014
  2. jcsd
  3. Jun 13, 2014 #2
    would i just go about it by perhaps multiplying the 85 mH of the sole by the rate of decrease which is 2.5 A/s?
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