# Phase Difference between Voltage and Current

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1. Feb 27, 2015

### Sylvia

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

The circuit above has a 29 resistor and a 11.1 mH inductor. Calculate the theoretical phase difference between the current CPA and voltage VPB if the circuit is driven by a sinusoidal voltage source with a peak amplitude of 3.5 V at the following frequencies:

20 Hz
420 Hz

2. Relevant equations
tan(x) = (XL / R), XL = (omega)(L)

3. The attempt at a solution

I tried substituting into the given relevant equations, and for 20 Hz, I got 2.75 degrees. However, this is the wrong answer. Please help!

Last edited by a moderator: Apr 27, 2017
2. Feb 27, 2015

### Staff: Mentor

Your relevant equation would give you the phase angle of the overall impedance, and that would in turn tell you the phase angle of the current with respect to the source voltage. But the question is asking you to compare the current's phase to that of the voltage across the inductor. You'll need to find expressions for the current and for VPB.

Last edited: Feb 27, 2015
3. Feb 27, 2015

### Sylvia

So current = 3.5 V divided by Ztotal, and VPb = I * L * omega?

4. Feb 27, 2015

### Staff: Mentor

That's the idea. Don't forget the "j" in the inductor's impedance.

5. Feb 27, 2015

### Sylvia

Im confused about where to proceed from there (my professor has done a horrible job of teaching this subject and the textbook isn't making much sense to me unfortunately)

6. Feb 27, 2015

### Staff: Mentor

Calculate the current and the voltage VPB. Put the results in polar form (magnitude and phase angle).

7. Feb 27, 2015

### Sylvia

Uhhhh sorry I'm in the non-calculus based class...

8. Feb 27, 2015

### Staff: Mentor

??? No calculus involved. Just circuit rules like Ohm's law.

9. Feb 28, 2015

### Staff: Mentor

When you look at this closely, you can see that the question being asked is quite basic: what is the phase difference between current through an ideal inductor and the voltage across that ideal inductor.

No calculations are needed!

BTW, anyone know what is supposed to be represented by that box with a + and - on it??