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Homework Help: Magnetic Field of Toroid

  1. Feb 20, 2009 #1
    You are going to wrap a toroid with 273 m of copper wire that can carry a current of I = 1.7 A. The toroid has radius R = 16 cm and cross sectional diameter D = 1.2 cm. How large a magnetic field (T) can you make at the average toroidal radius?

    I have been using the formula B = ([tex]\mu[/tex]*N*I)/2*[tex]\pi[/tex]*R
    where [tex]\mu[/tex] = 4[tex]\pi[/tex]*10^-7, N = number of windings of the coil, and I is the current...I have no idea how to figure out N though. Any help would be greatly appreciated.
     
  2. jcsd
  3. Feb 20, 2009 #2

    Doc Al

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    Staff: Mentor

    You know how long the wire is and the size of the toroid. How many turns can you make with that much wire?
     
  4. Feb 20, 2009 #3
    well L = 272m, and Im given R and the cross sectional diameter, and I know I need to combine those to get the size of the toroid, but I dont know how to combine them. Ive attached a picture that associated with the problem.
     

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  5. Feb 20, 2009 #4

    Doc Al

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    Hint: What's the circumference of the toroid cross-section?
     
  6. Feb 20, 2009 #5
    Well, circumference = diameter time pi, so would the cross sectional circumference = pi * 1.2cm ? Even so, Im still really confused how to incorporate that into finding the total area of the circle. I understand I should essentially be thinking of the toroid as an inner circle and outer circle, but I can visualize how to incorporate both into the total area...
     
  7. Feb 20, 2009 #6

    Doc Al

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    Good. So how many times can you wrap the wire around that circumference?
    You don't need the area of the circle, just the circumference.
     
  8. Feb 20, 2009 #7
    Thank you so so much! It makes much more sense to me now. Another student in my class explained it in a way that implied needing the area of the circles, which is what confused me. Thanks again
     
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