# Homework Help: 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.