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Copper Pipe in Solenoid Center

  1. Sep 10, 2010 #1
    I wanted to know how the magnetic field of a solenoid would be affected if a copper/iron pipe section was put into its center. In other words, what would the resulting magnetic field be if I simply wrapped a coil around a metal pipe section without removing the pipe afterward?

    Will there be a stronger field? I was wondering as I came across a copper pipe with a 1cm diameter and am working on a low-power coilgun.

    Thank you.
  2. jcsd
  3. Sep 10, 2010 #2
    The copper pipe will have absolutely no effect on a dc solenoid. It will have a very serious attenuating effect on an ac or pulsed solenoid.

    Bob S
  4. Sep 12, 2010 #3
    Okay, thanks. Why exactly would the magnetic field strength be diminished so dramatically?

    Also, is there a method to amplify the magnetic of a pulsed solenoid without increasing the input current or changing the coil's wrap density?
  5. Sep 12, 2010 #4
    From Faraday's law;

    Vtube = -d/dt[∫B·ndA] = -A dB/dt, where A is the cross-sectional area of the tube, and V is the induced voltage around the circumference.

    Because the copper tube is an electrical short circuit, there is a very large azimuthal induced current in the copper tube that opposes any change in the magnetic flux inside. (Lenz's Law).

    High frequency ferrite will help. Also small-diameter coated soft iron wire, like inside the old style automotive cylindrical ignition coils.

    [Added note] If you cut a narrow slit along one side of the copper tube for the entire tube length, this will eliminate the induced azimuthal (Lenz's Law) current which opposes the pulsed magnetic field inside the tube.There could still be small higher-order effects though.

    Bob S
    Last edited: Sep 12, 2010
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