# Homework Help: Kirchoof's rules problem

1. Mar 11, 2008

### jcpwn2004

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

http://img245.imageshack.us/my.php?image=physicsef4.jpg

2. Relevant equations

The change in Voltage = 0.

3. The attempt at a solution

Ok I have no clue where the loops go from. I want to say something like the current for I1 starts at the 10V battery in the middle and the current would look something like this:

8ohm + 2ohm +15v +I1 +3ohm - I2 = 0.....

I just really don't understand these problems and could use some help thanks.

2. Mar 11, 2008

### mysqlpress

First, making use of Kirchhoff's first rule :
The algebric sum of current at a junction is zero.
We have I3 = I1+I2.
Then , by considering two loops of the circuit,(there are a total of three circuits, choose two you like)

Setting up equation
20-I3(10)-8I2+10-I3(4)=0
20-10I3-I1(2)+15-3I1-4I3 =0

30-22I2-14I1=0
35-19I1-14I2=0
and solve it.

3. Mar 11, 2008

### jcpwn2004

Thanks, so do I just do substitution from there?

4. Mar 11, 2008

### mysqlpress

Yes.
and I3=I2+I1.
all currents are solved