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Finding the current in a circuit

  1. Apr 2, 2010 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img156.imageshack.us/img156/2420/physicsm.jpg [Broken]

    sorry for the image, i made it very quickly

    in the following circuit find

    a) the current in R
    b) R
    c) Emf
    d) if we cut the circuit in X, then what would be the current that goes through the 28V battery

    2. Relevant equations

    V = I*R

    3. The attempt at a solution

    a)
    i used the first kirchhof law and i found that

    I3 = I2 - I1 = 6 - 4 = 2A

    b)i used the second law of kirchhof and found that

    Emf - 18 - 24 = 0 => Emf = 42 V

    c)again i used the second law and found that

    28 - 42 + 24 - I3*R = 0 => R = 5 Ω

    d) i dont know how to do it, please help
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Apr 2, 2010 #2
    If you cut the circuit at X then there will be no current in that branch of the circuit, and you are just left with the top and bottom branch. A much simpler problem because you have already calculated R.
    (I haven't checked your answers to the other parts of the question.)
     
  4. Apr 2, 2010 #3
    hi, thanks for the answer

    but if we have only the bottom then it will be

    28 - 3*6 - I*5 = 0 => 28 - 18 - I*5 = 0 => 10 = 5I => I = 2

    but the book has 3.5 A :frown:
     
  5. Apr 2, 2010 #4
    The current through the bottom resistance is no longer equal to 6A after you
    cut the circuit at X.
     
  6. Apr 2, 2010 #5
    Why are you using a current of 6A in the 3ohm resistor, and an unknown current of I? There is only one current now and this is the new current you need to find.
     
  7. Apr 2, 2010 #6
    ok I understand it now, thanks everyone

    so the answer will be

    Vab = I*(5+3)

    but E = Vab

    so I = 28/8 = 3.5

    thanks again :)
     
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