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Homework Help: Kirchoff's Laws

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data
    The circuit in the figure is composed of two batteries (e1 = 8 V and e2 = 6 V) and four resistors (R1 = 110 W, R2 = 40 W, R3 = 30 W, and R4 = 50 W) as shown.

    [PLAIN]https://wug-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?cc/DuPage/Phys1202/fall/homework/Ch-21-DC-Circuits/kirchoff/cir06.gif [Broken]

    2. Relevant equations

    Kirchoff's Junction/Loop Rule

    3. The attempt at a solution
    This problem has been giving me nightmares! I have very little understandings of these 2 rules, even though I read my book. It doesn't show clear examples. But anyway here's my poor understanding of this problem. As I understand we need to draw a loop inside the first box that increases its potential in a clockwise direction. Now its asking for the current through the first resistor, so the current I1 will pass through R1 meet the junction, split into currents I2 and I3. This is all I know. Can anyone please show what's the next step from here. And Please please make it easy for me, as Im having a really hard time with this problem. Thank you in advance!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 19, 2010 #2


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    So far you've got: I1 goes through R1 and then splits up into I2 (going down through R2) and I3 (going right through R3). Okay, that's a good place to start.

    Now what does Kirchoff's current rule say about these three currents?

    Also, what is the current flowing through R4? And why?
  4. Sep 19, 2010 #3
    That they all add up to zero? Also I think I1 goes through R4, but I think that I'm wrong.
  5. Sep 19, 2010 #4


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    You need to know this, not be asking us. Please get your class notes or textbook, and look up what it has for the kirchoff current rule (or junction rule). What does it say?

    You are not wrong. But you can be sure by applying the junction rule to the lower junction.

    PS: I have to leave now. Perhaps someone else will be around to help.
  6. Sep 19, 2010 #5
    Goku, as I mentioned before, I did read the book and I do have my notes in front of me. And I wasn't really asking, I was just confirming, because that's what the book says about Junction Rule, all the currents at a Junction add up to zero.
  7. Sep 20, 2010 #6


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    Okay, good. Let's start from there.

    Another way to think of the Junction Rule is this: the sum of all currents entering a junction must equal the sum of all currents leaving it.

    Out of the three currents (I1, I2, I3), which ones enter and which leave the top junction?

    Can you translate the words of the Junction Rule into an equation using the variables I1, I2 and I3?
  8. Sep 20, 2010 #7
    Ok, I was able to finally get the 3 equations, but now I'm having problems with the algebra, this system looks really difficult to solve.

    Junction Rule: I1+I2 + I3= 0
    Left Loop: V-I1R1+I2R2-V2-I1R4=0
    Right Loop: E2-I2R2-I3R3=0

    Can you tell me if these are right, and where do i go from here? Thanks.
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