# Homework Help: AC circuit voltage homework

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1. Feb 14, 2017

### aryan pandey

1. The problem statement, all variables and given/known data
the problem is given in the attachment

2. Relevant equations
basic knowledge of ac circuits and phasor diagrams

3. The attempt at a solution
for point A , i calculated the potential easily. and for point B , i calculated the potential using phasor relationship .. now after doing VA - VB, i got the relation that its modulus will first decrease and then eventually increase
but in the answer given is that modulus of VA - VB will remain unchanged.

2. Feb 14, 2017

### Staff: Mentor

You'll have to show the details of how you got your relation. Can't fix what we can't see

3. Feb 14, 2017

### aryan pandey

truely speaking , i am confused how to proceed this question and hence not able to provide my approach .
through this question i want to grasp the concept of the question.

4. Feb 14, 2017

### cnh1995

Well, could you post the phasor diagram that you've drawn?
What can you say about the locus of VB from the fact that R1 and R2 are equal?

5. Feb 14, 2017

### Diegor

You should think about the extreme cases. What hapen when R=0 and when R tends to infinity maybe that helps as a start. Anyway you have to get an equation for Vab in function of the variable recistance.

6. Feb 14, 2017

### Staff: Mentor

Start by writing expressions for the potentials at points A and B. You referred to phasors, and that's a great idea. You can work graphically or algebraically (complex value phasor representation). The complex value method employs complex impedances.

Hint: Since the AC frequency is not changing the reactance of the capacitor will not change: It will be a constant. So just assign an arbitrary name to the capacitor impedance, say -jZ. (You do recall that in terms of phasors the impedance of a capacitor has a negative imaginary value, right?)