RLC Circuit

1. Apr 9, 2010

uzair_ha91

1. The problem statement, all variables and given/known data

[PLAIN]http://img255.imageshack.us/img255/3048/56893213.jpg [Broken]
Find the voltage of the applied source.

2. Relevant equations

I=V/R
Kirchoff's Voltage Law

3. The attempt at a solution

Applying KVL.
V= 40 + 50 +20
=110

Is this question that simple?
Or am I missing something?

Last edited by a moderator: May 4, 2017
2. Apr 9, 2010

willem2

The voltages across the capacitor and the inductor don't have the same phase, so you can't add them.

3. Apr 9, 2010

uzair_ha91

But the values of resistance and reactance aren't given for capacitor and inductor, so how should I calculate their voltages?
Or I just can't add them anyway? So how to reach the answer?

4. Apr 9, 2010

uzair_ha91

What if I took them as vectors?
VR = 40
VC = -20i
VL = 50i

V=40+30i
The magnitude of this is sqrt (402 + 302) = sqrt (1600 + 900) = sqrt(2500) = 50 volts
Is this correct?

5. Apr 9, 2010

willem2

You don't know what the direction of those vectors is.

Use the equation for a voltage divider

$$V_R = \frac {R} {X}$$

$$V_L = \frac {i \omega L } { X}$$

$$V_C = ...$$

where X is the total reactance of the circuit, a complex number that is a
function of $\omega$ R, C and L

now compute the magnitude of V_L/V_R and V_C/V_R

this will give a relation between R and $\omega L$ and $\omega C$

wich you can substitute in the equation for V_R above.

6. Apr 13, 2010

uzair_ha91

But the values for R and $\omega L$ and $\omega C$ are not given.