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Homework Help: Find magnetic dipole moment of coiled wire

  1. Aug 9, 2015 #1
    1. The problem statement, all variables and given/known data
    A 45-m long wire is coiled so that it makes a coil containing 100 circular loops, one on top of the other. If the wire carries a current of 13 A, what is the magnetic dipole moment of the coil?

    21 A·m2

    6.7 A·m2

    3.3 A·m2

    2.6 A·m2

    1.2 A·m2

    2. Relevant equations
    B(z) = u0/2pi x u/z^3
    Where u0 is the permeability constant and u is the magnetic dipole moment

    3. The attempt at a solution

    I can get as far as B(z) = (2 x 10^7 u)/z^3
    I know I need to find u.
    It doesn't seem to mention where exactly z is, thus can't work out magnetic field. Do you think there is enough information provided in this question to solve it?

  2. jcsd
  3. Aug 9, 2015 #2
    Would this be a solenoid and thus use equation B = u0 x i x n ?
  4. Aug 9, 2015 #3
    I'm curious - how would finding B help ?

    Use simply the formula - M = n*i*A , where n is no of loops , and i the current passing through the loops , and A the area of each loop .

    Also , here's a hint : M due to n circular loops would be the same as due to one large , single loop of current - carrying wire . This might help .
  5. Aug 9, 2015 #4
    Oh I couldn't find that equation.

    But solving for this equation gives me 2.07 x 10^6.
    I guess I need to convert this to A/m^2, but I am not sure the conversion and couldn't find it on Google anywhere?
  6. Aug 10, 2015 #5
    Anyone got any idea?
  7. Aug 10, 2015 #6
    Firstly ,
    Unfortunately , I have made a mistake here . Discard this .
    Secondly ,
    Try the working again now . You will get the answer . Also , I believe you mean Am2 .

    Hope this helps .
  8. Aug 10, 2015 #7
    Ok, let me show you my working.

    n is 100
    I is 13
    A = Pi R^2, which is 506.3. However, I am inclined to take this as one big loop. Thus 45m divided 100 loops.
    A = 0.159?

    M = 206.7
    or M = 2.07 if take n as 1.

    So don't think any of my answers are right?
  9. Aug 10, 2015 #8
    I said I made a mistake - you shouldn't do this .

    Find area of each individual loop , and use the formula M = n*i*A .
    Check your area .

    Hope this helps .
  10. Aug 10, 2015 #9
    Thanks qwerty.
    Was stupidly using circumference instead of radius. Got it now.
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