# Kirchoffs law

Discussion in 'Homework Help' started by sunny1982, Dec 27, 2012.

1. ### sunny1982 Thread Starter Member

Dec 27, 2012
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0
Hi people I was just wondering if someone can check my assignment questions I have attempted and see if I am correct. I would like to know I am going in the right direction before I continue all the other questions. All help greatly appreciated. In circuit one my task is to find the current flowing through the 30 ohm resistor using kirchoffs law. In the second circuit my task is to find the current flowing through 1 ohms resistor using kirchoffs law.

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2. ### WBahn Moderator

Mar 31, 2012
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A few suggestions:

1) Track your units! Don't just tack them on at the end based on what units you want the final answer to have. This would have caught the fact that the expression in your very first equation is wrong, since the units don't work out. You have

7 = 10 I1 + 12 I2(I1-I2)

The should have been written as

7V = 10Ω I1 + 12Ω I2(I1-I2)

The left hand side and the first term on the right have units of V, but the second term on the right has units of VA.

2) Your second line is correct, but only because it does not follow from your first equation. While this almost certainly reflects that, in your head, you had the right concepts in play, it could also be described as being too sloppy to make the effort to check your first line to ensure that it is even self-consistent and then being to sloppy to ensure that your work in consistent from one line to the next. Sounds harsh, but that's how it will be presented to the jury after such sloppiness causes a planeload of people to die. So learn to track units and check your work as you go now, when the only thing at stake is a few points on an assignment.

3) Learn to check your own answers -- and do so whenever and wherever possible. You came up with an answer of 0.474A for I2. Is that consistent with the problem? What would the voltage between the top node and bottom nodes be? What does this mean that I1 is? What does this mean as far as the current in the middle branch? Is this consistent with top-bottom node voltage? When I am working a problem, where it is a homework assignment or a part of a design on a million dollar IC, I always try to follow the answer with at least one check computation that supports the conclusion that the answer is, in fact, likely correct. In an amazingly high fraction of problems, either homework or real world, the correctness of the answer can be verified from the answer itself. Don't just throw that away (always think about what the jury will hear).