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Homework Help: Finding RMS current in an AC solenoid

  1. Aug 2, 2011 #1
    1. The problem statement, all variables and given/known data
    A 2000 turn solenoid of length 1.50 m and diameter 5.00 cm has a dc resistance of 3.0. It is
    connected to a 50 Hz, 40.0 Volt (rms) ac supply. Find the rms current in the solenoid

    2. Relevant equations
    Vind = L di/dt
    Vind = Induced EMF
    Ohm's Law
    V-Vind = IR
    L = (4pi x 10^-7)n^2 Al

    Where L=self inductance and l = length

    3. The attempt at a solution
    This is my first time using this website and i am not sure how to get in symbols, I will do my best to be clear

    I started finding the inductance.

    I then used Ohm's Law: V-Vind=IR

    After mathematical manipulation I got I = V/R[1-e^(Rt/L)]

    My question is as regards the frequency. It seems to have no effect on the system if you use my method yet if I try to find the induced E.M.F. using Faraday's Law on the magnetic flux you will find:

    Vind= 0.05 Sin(theta) I(t)
    Vind= 0.05 Sin (2 pi (f) t) I(t) from theta=wt

    So I was wondering if someone could explain both the concept and the calculations of the effect of the frequency. Thank You.
  2. jcsd
  3. Aug 2, 2011 #2
    Just to clarify I am aware that the current is periodic and that the frequency can help in calculating max current and other such things however, I was wondering if it was of any use in this specific question.
  4. Aug 2, 2011 #3


    User Avatar

    Staff: Mentor

    You've effectively got a resistor and inductor in series, and it's driven by a 50Hz AC supply. If you were to calculate the effective impedance (Z) of the resistor+inductor at the given frequency, you could apply Ohm's law to find the current: I = V/Z. The magnitude of I is what you're after.

    For an inductor the impedance varies in direct proportion to the frequency.

    This presumes that you've been introduced to the concept of complex impedance...
  5. Aug 2, 2011 #4
    Thanks very much I hadnt covered impedance well. I have since gone through and understand it.
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