Find current at high/low frequency

  1. 1. The problem statement, all variables and given/known data
    find the rms current delievered by the 45v (rms) power supply when
    a) the frequency is very large
    and b) the frequency is very small.

    answer: a) 225mA, b) 450mA

    http://yfrog.com/jucurrentqg

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    2. Relevant equations
    w= angular velocity (supposed to be omega)
    L= inductor
    C= capacitor
    j= complex coefficent = sqrt(-1)
    Zl= resistance of inductor
    Zc= resistance of capacitor
    Ztot= total resistance
    R= resistance of resistor
    P= Power

    Xl=wL
    Xc=1/wc
    Zl=j*Xl
    Zc=-j*Xc
    Ztot= R + Zl + Zc = R + j(Xl-Xc)



    3. The attempt at a solution

    with frequency just being high, how am I supposed to get these exact numbers without letters/symbol for I? o.o I tried the calculation and omega did not cancel out either.
     
  2. jcsd
  3. The key idea is that in the case of very high or very low frequency, you can take the impedance (resistance) of the inductor and capacitor to be ~infinite, respectively. Does it make sense why that's true (as an approximation at least)?

    In effect, this eliminates one of the two loops in the circuit for each case.
     
  4. ah yes it does, at first I thought the resistance would skyrocket to infinity / 0
    thank you
     
    Last edited: Oct 4, 2010
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