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Homework Help: Question on Kirchoff's Laws

  1. Feb 9, 2012 #1
    I was hoping someone could help out, I am working out a problem with Kirchoff's Laws and cannot seem to get the math to work out properly, at least I think the math is not working properly.
    I was hoping to run it by someone here.

    1. The problem statement, all variables and given/known data

    The circuit is as follows:
    A battery of 24 V on one end connected to two parallel resistors with values of 5 Ω and 7 Ω.
    Mesh one will be the loop between the battery and the 5 Ω resistor, which has a current of I1.
    Mesh two will be the loop between the 5 Ω resistor and the 7 Ω resistor, which has current of I2.

    2. Relevant equations
    The equations I set up are as follows:
    0 = -24 + 5*(I1 - I2)
    0 = 7*I2 + 5*(I2 - I1)

    3. The attempt at a solution
    After calculating the values I obtained these numbers:
    I1 = 8.229 A
    I2 = 3.429 A

    I tried verifying these answers by using V = I*R
    Finding Req to be 2.917 Ω.
    Therefore I = V/R and I = 8.228 A and since the current from I1 is the same value it seems that the values obtained from Kirchoff's Laws do not match the Ohm's Law current.

    I am uncertain if I am not realizing a concept but this does not make sense to me.

    Any help is appreciated.
    Thank you,
  2. jcsd
  3. Feb 9, 2012 #2


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    Homework Helper

    I think your work is all correct. V =IR finds the current through the resistors as 4.8 and 3.42 representing I1 - I2 and I2, so I2 = 3.42,
    I1 - I2 = 4.8
    I1 = I2 + 4.8
    I1 = 3.42 + 4.8 = 8.23
  4. Feb 10, 2012 #3
    thank you for your help Delphi, sorry I posted in the wrong area.
    I think I understand now,

    A. B.
  5. Feb 10, 2012 #4


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    Homework Helper

    Most welcome! This is the correct area for your question.
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