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Homework Help: Finding Current of a Circuit

  1. Jul 16, 2008 #1
    1. The problem statement, all variables and given/known data
    A circuit consists of a 215 resistor and a 0.200 H inductor. These two elements are connected in series across a generator that has a frequency of 120 Hz and a voltage of 235 V.

    (a) What is the current in the circuit?

    (b) Determine the phase angle between the current and the voltage of the generator.

    2. Relevant equations

    3. The attempt at a solution
    I used the equation to get 150.796 for Xl, then i plugged that into the equation Irms=Vrms/Xl to find current, but that gave me 1.558, which was not the right answer
  2. jcsd
  3. Jul 16, 2008 #2


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    Staff: Mentor

    Your second equation in #2 above is incomplete. The resistor and inductor are in series, so you must use their total impedance in the V = I * Z equation. Does that fix it for you?
  4. Jul 16, 2008 #3
    I'm still not getting it. I used V=IZ. I have V, which is 235 volts right? still, i did not have I or Z. So i used Z=(square root of)R^2 +(Xl-Xc)^2, and I got .89 for Z. Then plugging back into V=IZ, (235v)=I(.89)=262.49A for current?
  5. Jul 17, 2008 #4


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    Homework Helper

    Hi kdrobey,

    For these series RLC problems the impedance is

    Z=\sqrt{R^2 + (X_L - X_C)^2}

    and so the impedance cannot be smaller than the resistance, so something is wrong there. What were the actual numbers you used to calculate Z?
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