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What effect would a copper ring around a current carrying conductor have on the current in the conductor? How can I go about mathematically modelling the current in the conductor?

Details : The conductor in question is a electrode roughly 2 meters in diameter. It slides into the copper ring roughly the same diameter but very small in length compared to the electrode and is therefore a ring as you stated and not a tube. By half efficiency I mean the current in the electrode is much less than what it normally runs at. The ring was originally stainless steel and was then replaced by this copper version. After this and some other changes the decrease in current was noticed. It was suspected that the copper is having some form of coupling effect or transformer action with the electrode thus limiting the current. The ring was then divided into 6 parts and rejoined this time insulted from one another inhibiting current flow in this ring. The reduced current was still observed in the electrode. This to me is a pretty clear indication that the copper ring in fact has no effect and I merely want to confirm this hypothesis. I'm assuming capacitive effects are not of a concern either as the ring is small compared to the conductor and the AC power supplied to the electrode is of low frequency. Another observation that is convincing me that the copper is having no effect is that if a transformer action or some form of coupling was occurring between the electrode and the copper, the current induced in the copper would definitely be visible in the form of temperature because at such high currents, even with low resistance, the heat in the copper should be very high.(I2R). Would you agree with this?

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  • \$\begingroup\$ A coaxial cable is a conductor surrounded by a long conducting ring. Maybe look up the theory on this first to see if it suits what you are trying to achieve. \$\endgroup\$ – Andy aka Jun 24 '13 at 8:16
  • \$\begingroup\$ Andy has given a good starting point. For the length and inner diameter of the 'ring' it would act like a very short coaxial cable if the ring was grounded. See rfcafe.com/references/electrical/coax.htm \$\endgroup\$ – JIm Dearden Jun 24 '13 at 8:59
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    \$\begingroup\$ -1 for not mentioning that you are dealing with 70 kA currents until after people spent time trying to help you out by writing answers. \$\endgroup\$ – The Photon Jun 24 '13 at 15:40
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A conducting ring around a wire going thru its center won't have much effect at all on the impedance of the wire.

I think you are confusing the electric and the magnetic fields. Current thru a wire makes a magnetic field going around the wire. If you put a ring around the wire made of a magnetic material, like iron or ferrite, then it will effect the impedance of the wire. The ring is then in line with the magnetic field around the wire. The magnetic material will have a lower magnetic permeabiliy than air, which in effect increases the inductance of the wire for a short distance near the ring.

There are devices which operate on this principle. Sometimes you see a lump in the power cable shortly after coming out of some device. That lump is exactly such a ferrite ring. This effectively adds series common mode inductance to the cable, which helps block RF produced from the device getting onto the cable where it can radiate or conduct the RF onto the power line.

Since copper doesn't have significantly different magnetic permeability from air, putting a copper ring around a wire doesn't do much of anything.

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Very little effect.

If the ring had a relative permeability much different than 1, then the ring of copper would increase or decrease the inductance seen by the conductor at that section. But, the relative permeability of copper is very nearly 1.

If you have AC currents of a very high frequency in the conductor, the current will couple into the ring capacitively and migrate from the center conductor to the ring by skin effect. But, "ring" implies that this ring isn't very long (otherwise, I'd call it a "tube"), so the total capacitance between the ring and the conductor is small, so the total energy coupled will be very small, and only then to the extent that you have very high frequency currents in the conductor.

Consider that many buildings are constructed with mains wiring inside metal conduit. The only noticeable effect here is added physical robustness, and this is a "ring" that extends the entire length of the cable.

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  • \$\begingroup\$ Your answer closely correlates to my suspicions. It is indeed a ring and not a tube of about cm in height. Something I must mention however is that the conductor, although at low frequency, is carrying very high current upwards of 70kA and is not a conventional copper conductor but an electrode. Does this change anything? \$\endgroup\$ – Jason Bonarius Jun 24 '13 at 13:55
  • \$\begingroup\$ @JasonBonarius If that's DC, no, not really. If it's AC, then the higher current will mean higher \$\dfrac{di}{dt}\$ also, which will mean more eddy currents. Is that significant? Guess it depends on the frequency and geometry of the ring and conductor. \$\endgroup\$ – Phil Frost Jun 24 '13 at 13:59
  • \$\begingroup\$ #ring of about 40cm in height \$\endgroup\$ – Jason Bonarius Jun 24 '13 at 14:03
  • \$\begingroup\$ By taking that into consideration my question would then have to be, would the electrode run at only half efficiency regardless of the geometry of the ring and electrode? This seems a bit too significant and the problem might lie elsewhere due to non electromagnetic effects? \$\endgroup\$ – Jason Bonarius Jun 24 '13 at 14:09
  • \$\begingroup\$ @JasonBonarius half efficiency? Half of what? Why? Maybe you should have included some more detail in your question, if you didn't want just a general answer. \$\endgroup\$ – Phil Frost Jun 24 '13 at 14:19

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