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Homework Help: AC Theory

  1. Apr 12, 2007 #1
    A o.3micro farad capacitor is connected across a 15V, variable frequency ac power supply. Find the current in the circuit when the frequency is a) 100herts (b) 100kilohertz

    c) solve this question if thee 0.3 microfarad capacitor is replaced by a 40 ohm resistor.

    My solution
    I = wCV
    a) 2*∏(pie)*100*o.3*10^-6*15
    = 2.82*10^-3 A

    b) 2*∏*100*103 *o.3*10^-6*15
    = 2.82 A

    Xc = 1/2πfC
    therefore C = 1/2πfXc

    C = 1/2*π*100*40
    = 7.9*10-5

    = 15*2*π*100*(7.9*10^-5)
    = 0.74A

    b) I=wVC
    = 15*2*π*100*10^3*7.9*10^-5

    did i do this correct? please let me know if im going right.
  2. jcsd
  3. Apr 12, 2007 #2


    User Avatar
    Science Advisor
    Gold Member

    This looks right.
    You used this already above when you wrote "my solution"
    (V=Xc*I --> I=V/Xc).
    You're on the wrong track here. Use Ohms law to relate I, R and V.
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