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Below is a copper coil, presumably forming an electromagnet. From my understanding the electrons travel around the coil to produce a magnetic field. But why don't the electrons jump the wires and take the shortest path?

Below I tried to draw the path that would make sense (for me) for the electrons to take:

Peter Mortensen
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rrswa
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  • Can you also post original image? Isn't the coil insulated? – User323693 Aug 21 '19 at 04:23
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    The wire is coated with an insulating layer. – chamod Aug 21 '19 at 04:24
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    Are such coils always insulated? Even those really small wires forming a coil? – rrswa Aug 21 '19 at 04:28
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    @Ruan Especially that so-called "magnet wire". – DKNguyen Aug 21 '19 at 04:41
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    @Ruan I'd say they're always insulated because when we put a coil in a system we want the electricity to have to flow through it rather than taking a shortcut. I only hesitate with "always" because there may be some super-exotic situation where you wanted the short. Perhaps there's a time where you want a solid ring of copper connected to something, but due to some mechanical properties you want to make it out of wire rather than one solid chunk. I can't think of any cases of that happening, but electrical engineers are a clever bunch! – Cort Ammon Aug 21 '19 at 15:51
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    @Ruan coils *can* be made from bare wire, but they have to have loose turns (not touching), and can't handle high voltages. There aren't many commercial situations where this is useful, but you might see it in a hobby project. – hobbs Aug 21 '19 at 16:25
  • Huh, my magnet wire growing up had a red tint to it. – Jacob Krall Aug 22 '19 at 01:26
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    Distance-*shimistance*.... [Path of least resistance](https://en.wikipedia.org/wiki/Path_of_least_resistance) – Mazura Aug 22 '19 at 01:47
  • If I'm correct, the point is that these are not elections which travel (they do, of course, but they are not the main thing that is happening) - it's the field that propagates through conductor material which makes the difference - if wire is not insulated and is stretched so that layers are farther away, the propagation still happens through the wire more likely than through the air (unless potential gradient is high enough). – Askar Kalykov Aug 22 '19 at 18:39
  • @Mazura that's not really how it works. Put one resistor \$R_1\$ and one \$R_2 = 2\cdot R_1\$ in parallel, and _not_ all of the current will go through \$R_1\$ but only \$\tfrac23\$ of it. Moreover, if the lower-resistance one goes around a transformer core, that will actually induce a potentially much higher voltage than the resistive one, so even if the winding has a much lower resistance than the air gap between the windings, you may still get arcing between them. – leftaroundabout Aug 19 '20 at 15:40
  • I've wondered about this since high-school physics; it never made any sense and they never mentioned the insulation. Great question. – iono Jul 14 '21 at 06:08

2 Answers2

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This type of wire, used for making coils, is commonly called "magnet wire". https://en.wikipedia.org/wiki/Magnet_wire

It looks like it's bare copper, but it's actually coated with a very thin layer of transparent insulation. Otherwise, you're absolutely right -- if the wire were really bare, the coil wouldn't work because the current could cut straight across from one lead to the other.

Glenn Willen
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    That makes sense, for some reason i couldn't find an answer online, thanks – rrswa Aug 21 '19 at 04:31
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    The windings are layered carefully, particularly on high voltage devices, so that adjacent layers do not have large potential differences. This reduces the stress on the insulation layers so it doesn't matter if they are thin or suffer damage. – rolinger Aug 21 '19 at 13:54
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    And if you've ever made the mistake of thinking this is just bare wire, you find very rapidly that you can't solder to it because the insulation burns and then gets in your way, preventing adhesion. When soldering such wire, one typically has to chemically strip the end so that it isn't insulated first. – Cort Ammon Aug 21 '19 at 15:53
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    And accidental damage to the insulation (or cracking caused by heat or corrosive environments or excessive inductive motion of the coil or ....) causes shorts in which the current does short-cut along the way which makes the magnet less effective. – dmckee --- ex-moderator kitten Aug 21 '19 at 22:43
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    @CortAmmon, I've scraped off the insulation on these ends before. It's a pain/annoying, so taking it off chemically makes sense if you're doing it more than twice. – computercarguy Aug 21 '19 at 23:53
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    @CortAmmon Depending on the thickness one can strip the insulation off the end by just dipping it in molten solder (on the tip of your soldering iron) for a few seconds. You'll see it smoking which is the lacquer burning off – slebetman Aug 22 '19 at 00:45
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    Or you can burn the insulation off with fire. I once worked in a place where the assemblers had little alcohol burners for stripping magnet wire. – Jeanne Pindar Aug 22 '19 at 01:20
  • I have used a combination of butane cigarette lighter, knife blade, and abrasive paper. – Michael Harvey Aug 22 '19 at 16:25
  • Pulling the end a few times through a folded piece of emery paper works also. The insulation tends to pull away with the friction, so you end up removing surprisingly little copper--not enough to matter matter electrically or mechanically, at least at my amateur level. Don't do this on spacecraft, life support systems, or if your boss is looking. – Wayne Conrad Aug 23 '19 at 15:16
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Sometimes they do take the shortest path, when they are not supposed to. As others have said, the wires are normally insulated. However, if a current flowing in the magnet is suddenly interrupted by (say) an open circuit the voltage will rise until those electrons "get out" - either by sparking across an air gap or breaking through the insulation.

Dirk Bruere
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  • Think about the water in your pipes. The water flows in the pipes as long as the pressure is not to great. But at sufficiently high pressure, the water gets out and eventually flows to the lowest point available. This could happen if a very high voltage is suddenly applied to your inductor. Of course the inductor can fail in other ways: excessive current for too long will cause heating that destroys the insulation and you will have a smoking problem... – richard1941 Aug 30 '19 at 15:22