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Calculate the current in the coil

  1. Jun 28, 2009 #1
    1. The problem statement, all variables and given/known data

    A circular coil of 394 turns and area 0.78 m2
    is in a uniform magnetic field of 0.8 T. The
    maximum torque exerted on the coil by the
    field is 0.0059 N · m.
    Calculate the current in the coil. Answer in
    units of A

    2. Relevant equations
    torque= i*A*B



    3. The attempt at a solution

    what i did is that i solved for the current using the above equation i= torque/(A*B) but i keep getting the wrong answer .. i think because of the number of turns but i dont know were to include it

    i need ur help thanx in advance
     
  2. jcsd
  3. Jun 28, 2009 #2

    Doc Al

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    Staff: Mentor

    Torque = μ X B, where μ = nIA.
     
  4. Jun 28, 2009 #3
    so is it
    i = torque /(number of turns *A*B)
     
  5. Jun 28, 2009 #4

    Doc Al

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    Staff: Mentor

    Yes.
     
  6. Jun 28, 2009 #5
    Assume the 394 turns of wire are used to form
    a single-turn coil with the same shape but
    much larger area.
    What is the current if the maximum torque
    exerted on the coil by the field is 0.0059 N · m?


    this a second part of the question but i really didnt understand the difference
     
  7. Jun 28, 2009 #6

    Doc Al

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    Staff: Mentor

    You need to figure out the area of this larger coil. Hint: What's the length of the wire?
     
  8. Jun 28, 2009 #7
    i tried to figure out the length where i came up with this equation but still i couldnt figure what to put for the F

    length = F/(i*B)

    im kind of lost
     
  9. Jun 28, 2009 #8

    Doc Al

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    Staff: Mentor

    Figuring out the length of the wire is a geometry problem. You had a coil of a given area and number of turns. What was the circumference of that coil?
     
  10. Jun 28, 2009 #9
    C= 2*pi*r
     
  11. Jun 28, 2009 #10

    Doc Al

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    Good. Use the known area to find the radius.
     
  12. Jun 28, 2009 #11
    ok i did so know
    1 loop = 2*pi*r (i just found)
    394= X
    so do i do the cross multiplication to solve for X which is the new area
     
  13. Jun 28, 2009 #12

    Doc Al

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    Do this:
    Use the area of the original coil to solve for the radius, using A = pi*r².
    Use the radius to find the length of each loop, using the circumference formula.
    Find the total length of the wire by multiplying the length of each loop by the number of loops.
    That becomes your new circumference of your big coil. Work backwards to find, its radius then area.
     
  14. Jun 29, 2009 #13
    ok thanx alot i got it know .. sorry if i wasted your time
     
  15. Jun 29, 2009 #14
    i got for the larger area = 121084 m^2

    i plugged in this eqs i = (.0059)/(394*121084*0.8)

    but i got the wrong answer =(
     
  16. Jun 29, 2009 #15

    Doc Al

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    Staff: Mentor

    That looks OK.

    When used to make the giant coil there's only one turn.
     
  17. Jun 29, 2009 #16
    thanx again i got the right answer
     
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