Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Algebra help!, Kirchoffs rule problem.

  1. Feb 27, 2007 #1
    Hey guys, I'm having a little trouble with some basic algebra. I'm working on solving for currents using kirchoffs rule.

    4E/R - 9I3/2 = I3

    Trying to solve for I3... I should get:
    8E/2R -9I3 = 2(I3).. and then 8E/2R = 11(I3).. and then I3= 4E/11R

    However I am looking at my teachers solution and her answer was:

    4E/R - 9I3/2 = I3
    4E/R = 2I3/2 + 9I3/2 = 11I3/2 . How did she get the 2I3/2? It doesn't make sense to me. Once again, I am trying to solve for I3.

    Please help, I really appreciate it. Thanks.
  2. jcsd
  3. Feb 27, 2007 #2
    Look at your first equation again. You did two things in that equation. There is your error.
  4. Feb 27, 2007 #3
    I don't understand..

    How did I do 2 things?
    If i'm left with 8E/2R = 11(I3), then finding I3 would be dividing both sides by 11, which would not give me the correct answer.

    Also, that doesn't explain where she got the 2I3/2 on the right side. Thats what I'm concerned about.

    Still stuck, thanks for the help, but I don't quite understand it.
    Last edited: Feb 27, 2007
  5. Feb 27, 2007 #4
    How did you go from - 9I3/2 to -9I3?
  6. Feb 27, 2007 #5
    I multiplied both sides of the equation by 2.
  7. Feb 27, 2007 #6
    Did you multiply each term by 2?
  8. Feb 27, 2007 #7

    4E/R - 9I3/2 = I3

    x 2

    8E/2R - 9I3 = 2I3
  9. Feb 27, 2007 #8
    I"m stupid.. I figured it out.

    Thanks man.
  10. Feb 27, 2007 #9
    4*2=8. right?
  11. Feb 27, 2007 #10
    no problem friend
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook