How Do Kirchhoff's Laws Determine Currents in Electrical Circuits?

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Homework Help Overview

The discussion revolves around applying Kirchhoff's laws to determine the currents in an electrical circuit. The original poster presents a circuit diagram and attempts to find the values of I_C, I_B, and I_L using Kirchhoff's first and second laws.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to solve for the unknown currents using Kirchhoff's laws but encounters discrepancies in their results compared to a suggested solution. They question whether they have chosen the correct loops for analysis.
  • Some participants question the formulation of the equations, particularly the signs and directions of the currents, and suggest that reversing the direction of I_B might be necessary.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the circuit and the equations derived from Kirchhoff's laws. There is no explicit consensus on the correct approach yet, but guidance has been offered regarding the direction of current and its effect on the equations.

Contextual Notes

Participants are discussing the implications of current direction on the equations used, and there is a mention of potential changes across resistors that may affect the setup of the equations. The original poster's calculations differ from those suggested by others, indicating a need for further clarification on the circuit analysis.

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Homework Statement



http://img181.imageshack.us/img181/9554/cktpz6.jpg


Find the values of I_C,I_B,I_L

Homework Equations


\sum V=\sum IR
\sum I=0


The Attempt at a Solution



By Kirchoff's first law:
I_C-I_B-I_L=0

by Kirchoff's 2nd law:
0.1I_C+1.2I_L=14

0.011I_B+1.2I_L=12

When I used these to solve for the unknowns I got
I_C=21.25A \ \ I_B=11.35A \ \ I_L=9.9A

Now the solution says to use the first two equations and this one:
0.1I_C +0.011I_B=2

and that gives a different answer to what I had calculated. Did I take the incorrect loops?
 
Last edited by a moderator:
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Your third equation should be -0.011*ib+1.2*il=12. Do you know why?
 
The only way I see that would work is if I reversed the direction of the current I_B which I think should not really matter because when solved if the direction is wrong the answer will be negative.
 
rock.freak667 said:
The only way I see that would work is if I reversed the direction of the current I_B which I think should not really matter because when solved if the direction is wrong the answer will be negative.

What (dau)^4 is saying is that the current I_B is passing through the 0.011 ohm resistor in the same direction that you are tracing currents in that loop. So the change in potential across that resistor will be a "drop" of 0.011·I_B in that equation. (The way you have potential changes traced in your other loop is consistent.)

You'll then find that

0.1I_C+1.2I_L=14
and
-0.011I_B+1.2I_L=12

leads to

0.1I_C +0.011I_B=2
 
Last edited:

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