Find current at high/low frequency

[Acuben]

Homework Statement

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

http://img714.imageshack.us/img714/1592/currentq.gif

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Homework 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)

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.

 

[zhermes]

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.

 

[Acuben]

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.

ah yes it does, at first I thought the resistance would skyrocket to infinity / 0
thank you