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Homework Help: Finding rate of magnetic field PLEASE HELP!

  1. Feb 24, 2008 #1
    1. The problem statement, all variables and given/known data
    A 102-turn, 3.8-cm-diameter coil is made of 0.59-mm diameter copper wire. A magnetic field is perpendicular to the coil. At what rate must B increase to induce a 3.4 A current in the coil? The resistivity of copper is 1.7·10-8 Ω·m.


    2. Relevant equations



    3. The attempt at a solution
    A = (0.00059/2)^2 * pi = 2.734 x 10^-7
    I = 3.4 A
    I don't what to do???
     
  2. jcsd
  3. Feb 24, 2008 #2
    Is there an answer to compare to at the back of the book?
     
  4. Feb 24, 2008 #3
    no because its an even number :(
     
  5. Feb 24, 2008 #4
    B=µoNI/2r, B is magnetic force, µo is magnetic constant, N is number of turns, I is current, r is radius

    I think that's it but that R is unused in the problem, and it concerns me that I missed something.
     
  6. Feb 24, 2008 #5
    is r the radius of the coil or th wire??
     
  7. Feb 24, 2008 #6
    well i did the radius of the coil and its wrong but thanks for trying
     
  8. Feb 24, 2008 #7
    How do you know its wrong?
     
  9. Feb 24, 2008 #8
    its my homework problem and our homework is on lon-capa....so we have like 4 tries for each problem and tells you when you enter the answer.
     
  10. Feb 24, 2008 #9
    oh my gosh this problem is driving me nuts I can't figure it out!!! I tried using all sorts of formulas like...
    R = pL/A to get the resistance but I don't have L
    So then I tried...
    B = U*I*(N/L) to get L but I don't have B!!
     
  11. Feb 24, 2008 #10

    hage567

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    Yes you do have L because you know the diameter of the coil and how many turns there are.
     
  12. Feb 24, 2008 #11
    Isn't the L for B=UIN/L the length of the solenoid not the length of the wire?
     
  13. Feb 24, 2008 #12
    oh yea...ugh I hate this problem!!!
     
  14. Feb 24, 2008 #13

    hage567

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    I was referring to the L in the resistivity equation that Bubbles was asking about.
     
  15. Feb 24, 2008 #14
    Oh I see. What's the step after finding R?
     
  16. Feb 24, 2008 #15
    How do I find L with having the diameter and number of turns??
     
  17. Feb 24, 2008 #16
    I was going to find E using I = E/R then use that in the equation...
    E = A * B/t to get the rate
     
  18. Feb 24, 2008 #17
    Well, circumference=pi*diameter, lol 102 turns = 102 circumferences
     
  19. Feb 24, 2008 #18
    oh lol....ok so my length is 9.8696 the when I use R = pL/A is it the area of the wire or coil??
     
  20. Feb 24, 2008 #19

    hage567

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    This is what I would try.
     
  21. Feb 24, 2008 #20

    hage567

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    The wire, since that is what you are finding the resistance of.
     
  22. Feb 24, 2008 #21
    Area of wire, cross section area.
     
  23. Feb 24, 2008 #22

    hage567

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    The question did not actually state that this was a solenoid. It said it was a loop of 102 turns with a magnetic field perpendicular to it (so going through it). The magnetic field is not being produced by the coil. So I don't think the B = U*I*(N/L) equation is what you are looking for here, IMO.
     
  24. Feb 24, 2008 #23
    and the length would be the length of the wire not the coil right
     
  25. Feb 24, 2008 #24

    hage567

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    Yes, that's right.
     
  26. Feb 24, 2008 #25
    Oh I see. Thanks for setting me straight. I'm still learning and your opinion is very helpful to me.
     
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