# Kirchoff's Laws

1. Jan 28, 2008

### andy s

Hey guys, im really struggling with a question on Kirchoff's Laws. Ive attempted it 6 times, and each time have got a different answer. Please help.

See attached file.

p.s Apoligies if ive posted this in the wrong thread, i wasnt sure where to put it so i though "general physics" would be the best.

#### Attached Files:

• ###### Kirchoff's Laws.doc
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2. Jan 28, 2008

### Mindscrape

Just as a pointer for the future, a lot of people probably did not respond because they were afraid that your attachment may have a virus, so if you want to post a picture of something in the future then try and do it as a jpg image (in fact maybe you want to do it now). Macs are immune to most viruses, so I came along. :)

Also the resistor in the bottom left is not labeled, may not have helped your cause. Furthermore, the policy here is to show some of the work you have done so we can tell where you went wrong.

I'll be nice and give you a start. Let's say I want to find the equation for the voltage loop in the top half, going counter-clockwise. I go through and look at all the potential drops, and then add them up such that they are all zero. For example

$$-12V + I_1*5 + 9 + (I_3)15 = 0$$

There are two more loops to look at, what did you get for them? Can you tell me what the mysterious resistor is also?