# Voltage & Resistors

1. Sep 16, 2009

### xpack

http://img44.imageshack.us/img44/6898/83724998.png [Broken]

How do I begin to work this kind of problem?

Last edited by a moderator: May 4, 2017
2. Sep 16, 2009

### Mr.Green

I am not well familiar with the rules, but I think you should write down your attempts so that someone can help you in solving this problem.

Hint: Use Ohm's law.

3. Sep 17, 2009

### CEL

What are the currents through the series R1-R2 and through R3-R4?

4. Sep 17, 2009

### xpack

series r1-r2=39 k ohms
series r3-r4=31 k ohms
???

5. Sep 17, 2009

### Mr.Green

OK. Great. Now, you have the series resistance values and you know the voltage across them. You should know the current. Afterward the knowledge of the voltage across each single resistor will help in finding Vx.
Try it. It is easy.

6. Sep 17, 2009

### xpack

But because series r1-r2 and series r3-r4 are parallel does the voltage split between them? or is that the current?

7. Sep 17, 2009

### Mr.Green

The current is splitted between the parallel resistances and they have the same voltage. In other hand R1 and R2 are in series and the same current will flow through them. The same is true for R3 and R4.

The current through R1 and R2 id given by (let's call it I1)

I1 = V / (R1 + R2)

The current through R3 and R4 can be obtained in the same way.

As soon as you get the two currents you can find the voltage drop across R2 and R4. Now all values are known. Finally,

Vx = V2 - V4 ( why not V4-V2; because the + sign is drawn on V2 and the - sign on V4 )

8. Sep 17, 2009

### xpack

I know this may be asking alot but can you show the math, then I can clearly see what your plugging in where. This is not my hw by any means this is the practice for the actual assignment and I'm still confused a bit.

9. Sep 17, 2009

### Mr.Green

OK. But afterward you should do it yourself so that you deeply understand it.
Remember that you can't make muscles looking me lift weights :) .

I1 = V/(R1 + R2) = 0.076 mAmp
I2 = V/(R3 + R4) = 0.096 mAmp

V2 = I1 * R2 = 1.368 V
V4 = I2 * R4 = 2.016 V

Vx = V2 - V4 = - 0.645 (It is negative. What does this mean? )

Last edited: Sep 17, 2009
10. Sep 17, 2009

### xpack

Oh okay I understand know I know that V=IR but i wasnt entirely sure what R to use. But this helps me understand. Thank you again you have been great help.