Posted by **Heather** on Saturday, March 24, 2012 at 2:53pm.

I need to calculate the voltage through the resistor and the voltage through the inductor of an RL series circuit for different frequencies. The voltage of the source is 2.5 V. The inductor is at the top and has an inductance of .200 H and the resistor is at the right with a resistance of 5.1 kilo ohms.

To calculate VR and VL I was going to use VR=Z2/(Z2+Z1) and VL=Z1/(Z2+Z1).

I read that Z for the resistor is equal to R and i don't know what Z for the inductor is. i know that the total Z is sqrt(R^2+w^2L^2) so i don't know if Z for the inductor (or Z1) will be Ztot-ZR.

I want to know if I'm approaching this correctly. What I did for VL when the frequency was 1000 Hz was:

Z2= Ztot-Z1

Z2=3.91

VL= Z2/(Z1+Z2)=1.92 mV.

Am I doing this correctly?

How can the angles for Vr be calculated?

- Circuits -
**bobpursley**, Saturday, March 24, 2012 at 3:23pm
Zi=Xi= j2PIf*L where j is a phasor

R+Xi=5100+j w.200

Now, when you compute that total impedance, notice that it is a complex number.

Z=R+Xl=5100+j6280*.2

mag Z= sqrt (5100^2+1256^2)=5252ohm check that. For the angle, theta=arctan(1256/5100), so I will be at that angle with respect to source voltage.

Now, you have to consider angles on the inductor.

Vl=Z2/(R+Xl) but Z is at 90, and the denominator is at theta, so Vl is at 90-theta.

The angle of Vr is at theta, it is in phase with the current.

Vr and Vl do not add to Vs in magnitude, however, as vectors, they will add to Vs

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