Circuit Analysis - Resistors and a battery

[jendrix]

Circuit Analysis -- Resistors and a battery

Homework Statement

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.

Homework Equations

V=IR

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

 

[phinds]

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

 

[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.

 

[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

 

[phinds]

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

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.

 

[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.

 

[phinds]

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.

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

 

[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.

 

[NascentOxygen]

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

What answer do you get?

 

[jendrix]

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

What answer do you get?

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

 

[NascentOxygen]

1.18mA ✔

Looks right.