Kirchhoff's Rules: Circuit Question

1. Apr 7, 2014

navm1

1. The problem statement, all variables and given/known data
If R1 is 3Ω, R2 is 6Ω and R3 is 9Ω, what are the three currents in figure 16-4, in which the batteries are 6.0v and 12.0v?

2. Relevant equations
I1+I2+I3

3. The attempt at a solution

I've got three equations so far

I worked out

+6-3I1-9I2=0

and

+12-6+6I2-9I=0

For the next step I have to find I1 in terms of I2 but I'm not sure how I do that

http://puu.sh/7ZXv9.png [Broken]
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 6, 2017
2. Apr 7, 2014

SammyS

Staff Emeritus
Your subscripts are inconsistent with you equations and/or the figure.

Also, I1+I2+I3 is not an equation. Perhaps you meant I1+I2=I3

Last edited by a moderator: May 6, 2017
3. Apr 7, 2014

navm1

youre right i meant to put I1+I2+I3=0 and

+12-6+6I2-9I1=0

4. Apr 7, 2014

SammyS

Staff Emeritus
12 - 6 + 6I2 - 9I1 = 0 is correct.

For I1+I2+I3=0 to be correct, you should draw I3 in the opposite direction.

You need to correct the other loop equation.

5. Apr 8, 2014

navm1

+6-3I1-9I2=0

is wrong?

I think i was getting mixed up over some wording in my workbook. I've got

2-3I2=I1 now and i'll substitute in the third equation

Last edited: Apr 8, 2014
6. Apr 8, 2014

Staff: Mentor

It looks wrong. Where did it spring from?

Where did that spring from? If you'd explain the laws you applied to arrive at these, it would make the process of checking your steps easier, ditto for finding mistakes.

Perhaps you have some currents and/or resistors labelled differently to how you think you have?

Last edited: Apr 8, 2014
7. Apr 8, 2014

navm1

the question was "If R1 is 3Ω, R2 is 6Ω and R3 is 9Ω, what are the three currents in figure 16-4, in which the batteries are 6.0v and 12.0v?"

and heres the solution, might make it more clear to you what im missing

http://puu.sh/80On3.jpg [Broken]

http://puu.sh/80Oo1.jpg [Broken]

Last edited by a moderator: May 6, 2017
8. Apr 8, 2014

dauto

The problem we're having is that the signs and indices in your currents and/or resistors in the equations don't seem to match what we see in the picture. Your first job is to fix that. Than we can talk about how to solve the equations.