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Q & A: blocking a wire with a field

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Most recent answer: 09/27/2013
Q:
Is there any way to stop voltage from flowing through metal without an insulator? Like in a piece of rail, can you create a field or attach something to the side to cause the voltage to stop flowing at a certain point?
- Chris Rewczuk (age 37)
Omaha
A:

In an ordinary piece of metal, you can't really block the current with some applied magnetic field. Applying an electric field wouldn't quite make sense, precisely because it's metal, so a current will flow to build up some charge to cancel that field. However, most metal has positive magnetoresistance, meaning that applying a magnetic field will make it harder for current to flow, although not enough to turn the metal into an insulator. The simplest way this works isn't hard to explain. As an electron starts to move in response to an electrical field, its path bends due to the magnetic field. That makes it carry less current downstream in the direction of the electrical current. Actually, in an ordinary piece of metal, the electrons are already zooming around very fast in all directions even before a voltage is applied. If there's a big magnetic field at right angles to the wire, the electrons will make little loops around the field direction, not going very far along the wire. When a voltage is applied, there won't be as much current response as there would if they weren't looping.

There are special materials, for example those used in some read-heads on computer disks, in which the resistance is extremely sensitive to magnetic fields. (You can search for the names "giant magnetoresistance" and "colossal magnetoresistance". ) Most often, though, these have reduced resistance in big fields. One reasonably standard material with large positive magnetoresistance at room temperature is the semi-metal bismuth. Carefully deposited films of bismuth can have resistance increase of a factor of 3.5 even at room temperature in a standard laboratory magnetic field.

Mike W.


(published on 09/27/2013)

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