# Homework Help: Find the total resistance

1. May 12, 2013

### MJay

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

9.15
a) Each box has the restistance 1 Ohm. Find the total resistance between 1) A-B 2) A-D 3) B-C
b) Find I (electric current(?)) from A-D if you put a battery from B-C
c) Find I on the Ammeter and the direction (the lower picture)

http://sphotos-h.ak.fbcdn.net/hphotos-ak-prn1/v/954585_10151577257929660_1441841441_n.jpg?oh=536e2a504401f9beebed972302e56839&oe=51920295&__gda__=1368521085_3193921ab59c05a6069ec6b2519d2702
9.126
a) Use Kircchofs 2nd law on ABED to show that I3 ) 0,5A
b) Explain how I2 + I5 = 2A. Use Kirchhoffs 2nd law on BEFC to find another equation for uknown I2+I5
c) Find I1 and I4
d) Find the resistance R.

2. Relevant equations

Kirchoff's circuit law is relevant

3. The attempt at a solution
9.15
a) 1) Thought it would be just 1 ohm but it's supposed to be 0,63 2) I used 1/Rtotal = 1/R + 1/R + R which means 2+1 = 3 which is wrong while it's supposed to be 0,5.
This makes the rest pretty complicated to solve.
9.126

I used Kirchoffs law and found following formulas but not sure if it's supposed to be solved as
5 unknowns while we've only been into 3.

2. May 12, 2013

### tiny-tim

Welcome to PF!

Hi MJay! Welcome to PF!
You need to use all the paths from A to B
erm

calculate it again!

3. May 12, 2013

### MJay

Hey thanks :)

I tried a different method now:

which gives 2/3 then I insert it in 1/R= 1/(2/3) + 1/(AB+BD) Am I on the right way?

4. May 12, 2013

### tiny-tim

Hi MJay!
(this is part 2)

Correct

(But your notation is terrible, and I suspect is misleading you …

you can't just call everything R, and expect not to make mistakes )
nooo

think again

5. May 12, 2013

### MJay

Yeah understand but it get's so messy while I'm doing this in the way we learned it in school which might get confusing also, I'm not sure what way I should put it. But I have no clue or book to help me out, as this is the toughest exercise (marked with 2**).