# Circuit Analysis - Resistors and a battery

1. May 15, 2014

### jendrix

Circuit Analysis -- Resistors and a battery

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

Hello, if you see the attached I am trying to find the current I4.My plan was to find Voltage across R4 and use Ohm's law.

2. Relevant equations

V=IR

3. The attempt at a solution

I have simplified the circuit down(attached pic), I thought I could use the voltage divider equation to find voltage across R4||R6 which would be equal to the voltage across R4 however I'm getting a different answer

I got 50*(3.197/(3.197+4.7)) =20V making I4= 2mA

http://i.imgur.com/vjVbcEi.png
http://i.imgur.com/6FwW7QG.png

Last edited by a moderator: May 15, 2014
2. May 15, 2014

### phinds

So you believe that absolutely no current is flowing through R3? How do you come to that conclusion?

3. May 15, 2014

### jendrix

Hmm that makes sense, I was thinking that going by Kirchoff's voltage law that the sum of the loop on the right would=0 in which case I could use the potential divider equation.

4. May 15, 2014

### jendrix

Though it makes sense as Voltage across R3 would be the same as R2 yet if you did the voltage divider equation then you would get a different solution.

So the only way would be to put R2||R3 then do the voltage divider equation? Or is there another way?

Thanks

5. May 15, 2014

### phinds

This whole "voltage divider" thing is, to me, a brain-dead way of going about it. Don't get me wrong, I know people use it but I just find it a distraction that leads to the kind of error that you got. Just do a full loop analysis. It will take a lot more steps than what you were trying to do but that's because what you were trying to do was wrong.

6. May 15, 2014

### jendrix

It was the way the question was worded, it guided you towards doing it that way.Personally I prefer loop analysis although my university will only let us use nodal, it gets marked incorrect if we do a loop analysis.

7. May 15, 2014

### phinds

Gads, I HATE that kind of crap. I had a prof once who counted my solution to a calculus problem as wrong because I used a simple method out of Thomas and he wanted us to use a more complicated method out of the book he made us use.

Well, node analysis in this case is about the same as loop analysis, so do that

8. May 15, 2014

### jendrix

Thanks, I think you're spot on, I've done it the way he wanted but I'll double check it with loop analysis to see which way is quicker, I think everybody has "their" way of doingthings and that should be acceptable.

9. May 15, 2014

### Staff: Mentor

You can use the potential divider approach, basing it on resistor R3 and all the other resistors combined into a single resistance.

10. May 16, 2014

### jendrix

I did it using the divider approach by combining R2*R3/(R2+R3) This gives 1.45kΩ

Then 50*(1.45/(1.45+4.7)

=11.79V

Then 11.79/10kΩ

=1.18mA

Thanks for the help and suggestions

11. May 16, 2014

Looks right.