# What is the rms potential difference across the resistor?

Hello every9one i'm stuck on finding the first part:
A series RLC circuit is driven by an alternating source at a frequency of 380 Hz and an emf amplitude of 90.0 V. The resistance is 20.0 , the capacitance is 12.1 µF, and the inductance is 24.2 mH.

What is the rms potential difference across the resistor?
Am i t hinking this right?
Vab = Vmax sin(wt)
Vab = iR
i = 90/20 = 4.5, but thisn't isn't i max is it?
do i have to take i/sqrt(2) to get i max?
then with i max can i find the rms across the ressitor?
I found a formula that syas:
Vab = Vmax*sin(wt) but time isn't given at all, netiehr isVmax!

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Integral
Staff Emeritus
Gold Member
You must set up an equation for the current involving ALL of the circuit components. You cannot get the answer by considering the resistance alone.

Astronuc
Staff Emeritus
This is not correct {i = 90/20 = 4.5 A}.

As Integral indicated, the 90 volts applies to the R + L + C in series.

R is a non-reactive component so the voltage drop will be in phase with the applied volatage. L and C are reactive components so the voltage drops will be out of phase with the applied voltage.

So VR has same shape, but different amplitude, than V.