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A Five Wire Junction

  1. Feb 1, 2008 #1
    1. The problem statement, all variables and given/known data

    The information about the magnitudes of the current density and the diameters for wires 1, 2, 3, and 4 is given in the table. Some of the values are unknown.
    Wire Current density (A/mm^2) Diameter (mm) Total Current (A)
    1-------- 1.6 -------- 2.0 -------- ???
    2 -------- ??? -------- 3.0 -------- 2.0
    3 -------- 3.0 -------- 1.1 -------- ???
    4 -------- 0.8 -------- ??? -------- 4.0

    What is the current at the 5th wire?

    [​IMG]

    2. Relevant equations

    I know you need to use the conservation of current, but I'm just not sure what formula to use.


    3. The attempt at a solution
     
    Last edited: Feb 1, 2008
  2. jcsd
  3. Feb 1, 2008 #2
    study kirchhoff's junction law...
    J = I/pi*r^2 ; pi = 3.14...
    Hope this helps... good luck.
     
  4. Feb 1, 2008 #3
    would I need to convert the current density and diameter to metres before calculations?
     
  5. Feb 2, 2008 #4
    it I5 is going to be-9.84
     
  6. Feb 2, 2008 #5
    Yep, that's the answer I got, just wasnt sure if it was right. Thanks.
     
    Last edited: Feb 2, 2008
  7. Feb 2, 2008 #6
    yes... always go meters for length unless otherwise specified!
    and yess the answer is -9.9
     
  8. Feb 2, 2008 #7
    in this case, I didnt convert; I just left it as it
     
  9. Feb 2, 2008 #8
    That is because the diameter is in (mm) so is the current density :)
     
  10. Feb 2, 2008 #9
    Ah, good to know. I probably would have done this on the test without noticing. You probably saved me a few marks, Thanks.
     
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