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Q & A: Coiling a magnet

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Most recent answer: 10/22/2007
Q:
coiling wire multiplies it magnetic feild what would coiling a magnet do?
- nate (age 17)
az
A:
Actually, the magnetic field from each little piece of wire with a current flowing through it just adds to the field from all the other pieces. Coiling a wire up makes the magnetic field stronger and more uniform over a volume because it brings more of the wire closer in to where the field is desired (inside the core of a solenoid).

You can bend a solenoid around on itself, no problem, and in fact many are constructed in exactly this way. The magnetic field lines travel along the length of a solenoid, and if you bend the ends around, you can get the magnetic field lines to go in a circle inside a torus-shaped coil of wire. You can increase the strength of the field by putting a magnetizable material inside -- a doughnut-shaped piece of iron, for example, will serve admirably.

This is a very useful configuration, because of Faraday's law of induction. If you wrap another coil of wire around the iron doughnut, and flow an alternating current through the first coil, the second coil will see a voltage across it, and the ratios of the two voltages in the coils is given by the ratio of the number of turns in the wire (because each turn adds to all the others). This is the idea of a transformer -- coiled magnets wrapped around toroidal cores can take electrical energy in one set of wires at one voltage and give you electrical energy out of a second set of wires at any voltage you desire, just by choosing the number of turns properly.

You don't have to wrap the coils around into a doughnut, but it's better if you do. The field lines spread out at the ends of an ordinary solenoid, and the filed gets weak outside as the field lines travel to the other end. If you use a magnetizable material as a "flux return" you can avoid all the stray fields leaking outside which would cause radio interference and energy loss.

I guess you can do the same thing with a permanent magnet, but the result is not quite so exciting. Horseshoe-shaped magnets are already almost coiled up anyhow -- I guess by bending around the magnet you can make the poles close together, which increases its ability to pick stuff up.

Tom

(published on 10/22/2007)

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