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Homework Help: Elliptical Solenoid

  1. May 19, 2008 #1
    1. The problem statement, all variables and given/known data
    I have to find an expression for the magnetic field of an elliptical solenoid. This is an actual solenoid sitting on the desk next to me, but even an infinite/very long solenoid approximation would be a wonderful start.

    2. Relevant equations
    Biot-Savart law

    3. The attempt at a solution
    Therein lies the problem... I've not the slightest clue where to start. I just started working in a lab at my school and the grad student here left me this problem while he's away on vacation. I've only seen Biot-Savart from my Intro to Electromagnetism class, so I'm not to savvy with it yet. A poke in the right direction would be greatly appreciated!
    Last edited: May 19, 2008
  2. jcsd
  3. May 19, 2008 #2

    Dr Transport

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  4. May 30, 2008 #3
    I tried looking up elliptical cylindrical coordinates and when I saw all of the sinh's and cosh's I got kinda scared... but I did come across something else. I found that for an elliptical loop of current, the magnetic field is:

    u[0] = 4*pi*10^(-7)
    I = current
    a = semi-major axis
    b = semi-minor axis
    k = sqrt(1-a^2/b^2)

    B = (u[0]*I)/(pi*a)*E(k), where E(k) is a complete elliptical integral of the second kind.

    E(k) = int( sqrt(1-k^2*sin^2(theta)), theta = 0..pi/2).

    Given the specs that I have, k=0.826, I found an integral table (CRC standard mathematical tables, 18th edition) and found that E(k) = 1.372.

    Plugging in my other numbers (this is assuming 1 amp of current), I got the field to be:

    B = 1.065 gauss.

    This seems to be somewhat correct since the current for a circular loop of about the same size is pretty small too. Now I need to find a way to incorporate the number of turns (in this case, 1010 turns) into my equation so that I can get an idea of how strong the elliptical solenoid will be. Any ideas?
    Last edited: May 30, 2008
  5. May 30, 2008 #4

    Ben Niehoff

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    I'm fairly sure that the magnetic field inside an ideal (infinite) solenoid is independent of the shape of its cross section. What matters are the cross-sectional area and the sheet current density.
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