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Okay, I've got a little bit of a layman's question here.
We're doing a bit of spring cleaning in our office and we've found a cabinet with boxes upon boxes of stored wires, so naturally, this discussion arose...

Picture a normal, bog-standard wire, with a plastic outer coating. Now, quite often when these wires are stored, they will wrapped up and twisted, to effectively make a coil.

I was just wondering what the effects of this type of storage would have.

What if you had a 15m wire and only used the each end to cover about a single meter (leaving 14m still twisted and wrapped in the middle), what the effect of the electrical current running through this have?

Thanks for helping us settle a mild dispute!

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4 Answers

up vote 3 down vote accepted

You are talking about the inductive effects of the coil of wire. Essentially a wrapped up coil of metal with electrons running through it creates a linear magnetic field since moving electrons through a wire creates a redial field and if you approximate the coil to have infinite loops the field becomes liner.

But, this effect would be very, very small for the wires you are talking about since (a) the coils are not very densely packed and (b) not very much current is flowing through them

Here is the wiki on inductors:


The simple relationship between voltage($v$), inductance($L$), and current($i$) is:

$$V(t) = L \frac{di(t)}{dt}$$

One last thing to consider, magnetic field drops off with distance fast (so as you move away from the source it gets really weak). The plastic protective covering around your wires are a relatively similar thickness to the wires themselves and would buffer anything nearby to most the magnetic effects (which would be weak to begin with)

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Thanks for this, my argument was that there would be magnetic effects but not as they are plastic coated, because that would cancel it out –  Dan Hanly Dec 22 '11 at 11:59
Welcome to Physics.SE! We have the MathJax rendering engine active on the site which means that you can format mathematics with LaTeX-alike syntax as Nic has done for you in the edit. –  dmckee Dec 22 '11 at 15:13
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A magnetic field will be generated around the coils of wire if you pass a current through them...

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Sorry for necromancing but someone on the internet is wrong.

Nothing major happens with regard to magnetic fields since the cable hold a wire pair carrying equal and opposite current, thus creating two magnetic fields that almost completely cancel each other out.

The most notorious feature of loaded coiled cables is that they potentially generate a lot of heat in a tight space. In most cases it's not an issue, but at high load with little cooling such a coil could be a fire hazard.

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It depends on a number of factors. If you run DC current through it, you will establish a strong axial, static magnetic field inside the coil (that is, in the region around which the wires are wrapped). Running an AC current through it will generate an oscillating magnetic field (which, by the way, can induce fields in other loops of wire if they're oriented correctly).

@AtomicCharles's suggestion that the field drops off rapidly only holds for single current-carrying wire, but for a coil this is not the case.

See: http://en.wikipedia.org/wiki/Helmholtz_coil for further details.

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