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Confused about solenoid radius from this site

  1. Sep 8, 2013 #1
    So I was reading a thread on PF talking about the radius of a solenoid and Bob S answered with a link:
    On that site, it states that the Radius = the outer radius - the inner. It doesn't make sense. What happens when there is only one layer of coil and the outer radius (OR) is basically the inner radius (IR). That would create a larger magnetic field than if I had multiple layers of coils and the OR > IR which would then create a smaller one. I don't understand why.
     
  2. jcsd
  3. Sep 8, 2013 #2

    mfb

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    Staff: Mentor

    Where?
    Even in that case, your cables have a finite width.

    You can probably consider this equation in the limit r1 -> r2, and get a simpler equation for the case where the difference between both is not significant.
     
  4. Sep 8, 2013 #3
    In the equation where the denominator is 2(r2-r1). So I'm assuming that the more layers of coils you have, the greater r2-r1 is and therefore less magnetic fields. Which I don't get. I thought the more coils you have, the greater the magnetic field.
     
  5. Sep 8, 2013 #4

    mfb

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    Careful, the radii are used in the logarithms as well. It is not clear which effect will dominate if you don't calculate it.

    "More coils" would also increase n (more windings) or I (more current), cancelling this effect. More coils give a higher field strength.
     
  6. Sep 8, 2013 #5
    Oh....Whoops. Thank you. I can't believe I forgot about that.
     
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