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As the title says, I am wanting to calculate the minimum proximity for a magnet to be near a AC line for no electrical impedance.

My scenario is this: I have a theoretical device like MagSafe (Apple's magnetic DC connector for laptops) however with one main difference, it would be with AC instead of DC. My understanding is that a magnet could cause the Proximity effect and create an electrical impedance.

To avoid the impedance but still use a magnet in close proximity in a "MagSafe" like device, I could simply move the magnet further away from the current. The part that I don't know is how to calculate the minimum distance required. I gather that I would need to determine the magnet's strength but I am unsure what other parts I need for the calculation (does the AC voltage matter for the proximity? can the proximity be affected by shielding the AC line somehow?).

The AC voltage in my case would follow the AS60038 standard (230v, +10% to –6% variation)

If this was a real device (rather than theoretical), I gather that I could simply measure the AC voltage etc and move a magnet around it to find out however I am curious how one could do this theoretically.

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  • \$\begingroup\$ Looks at larger profile photo. If that's you then I'm sure you know who I suspect people have said you look like :-). \$\endgroup\$ – Russell McMahon Jan 10 '15 at 14:47
  • \$\begingroup\$ Yep that's me in the photo though to be honest, no one has mentoned to me who I look like (though I really want to know now!). They do mention I look like a 12-16yo. ;) Thanks for your help by the way, have learnt a few new things. \$\endgroup\$ – Turnerj Jan 10 '15 at 22:35
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    \$\begingroup\$ One may have to squint somewhat to see a resemblance - Harry Potter came to mind :-) - I think that would usually be a compliment. \$\endgroup\$ – Russell McMahon Jan 11 '15 at 4:18
  • \$\begingroup\$ Ahhhh ;) Only thing missing is the scar. \$\endgroup\$ – Turnerj Jan 11 '15 at 4:21
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You have a degree of misunderstanding re proximity effect - it is effectively irrelevant in this application as regards magnet proximity to conductors.

I could write more BUT that's a complete answer for practical purposes.

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  • \$\begingroup\$ Ahhhh, that doesn't surprise me that I misunderstood that. With it being irrelevant in this case, would there be little-to-no electrical impedance by simply having a magnet next to an AC line or are you saying that just my understanding about the proximity effect being a cause of impedance in this case was wrong (ie. a magnet would still affect an AC line)? \$\endgroup\$ – Turnerj Jan 10 '15 at 10:13
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    \$\begingroup\$ Minimal effect - you are lucky :-). If you wound wires in a coil around the magnet you'd get interaction. That article is mainly about wires wound in a coil so that the magnetic fields of nearby turns interact. For one wire near one magnet you'd need vast currents (10's to 100's of amps, to get anything like significant results. As an example - some olde analog meters were "moving iron" where a coild made a magnetic field and attracted a iron pole piece to move a pointer. I long ago saw a meter to measure car cranking curreny which sat on an insulated lead and used the magnetic field .... \$\endgroup\$ – Russell McMahon Jan 10 '15 at 14:44
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    \$\begingroup\$ .... to move an iron piece (or magnet) attached to a pointer. Currents measured were 10's to 100's of amps. You can do the same thing with a Hall sensor and a single wire at much lower currents BUT they are far more sensitive. \$\endgroup\$ – Russell McMahon Jan 10 '15 at 14:45

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