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Q & A: magnets and currents

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Most recent answer: 10/22/2007
Q:
How does a current carrying conductor produces magnetic field around it? I need detail answeer.
- ayesha (age 14)
islamabad, pakistan
A:
We actually have no ’detailed’ answer to the ’how’ question. We know exactly how to calculate what the fields are, using Maxwell’s equations, but the early attempts (by Maxwell, for example) to find some detailed underlying mechanism by which fields propagate didn’t turn up anything useful. Nowadays we have quantum mechanical versions of the equations, but they really don't add much for the type of situation you're asking about.

So the result is that the magnetic fields loop around the wire. If the wire has a long straight stretch, the strength of the field falls off inversely with the distance from the wire. You can figure out the amount of field from some other wire pattern by adding fields from each little part. The strength of a field from a little part of a wire is proportional to the current in the wire and inversely proportional to the square of the distance from that part. The fields have directions to them, so they have to be added as vectors. That means that if there are two pieces of wire near each other with currents flowing opposite directions, the fields they produce far away will cancel each other out.

To really get the equations, you could look up a text on elementary electromagnetism.

Mike W.

Lee H

(published on 10/22/2007)

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