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Conceptual Question on Current

  1. Feb 13, 2008 #1
    Consider a wire, with current flowing through it.
    A small light insulator is brought close to the wire. The insulator has positive charge on its right end and a negative charge on its left end. But there is no net charge, it is neutral.

    What happens? Does the electrons in the current attract the insulator?
  2. jcsd
  3. Feb 13, 2008 #2
    A current will produce a magnetic field around it. The direction of the magnetic field is given by the right hand rule. The magnetic field will only produce an EMF if it is changing. Thus if the current is AC then the current will induce a sinusoidal EMF in the insulation. The elections in the insulator will only flow in large numbers if that EMF exceeds the break down voltage of the insulator.
  4. Feb 13, 2008 #3
    Hmm okay. Well that is a much deeper explanation. But this was the original question. And we assume it is DC circuit

    You have a current going into the page. You have a magnet close to the wire, and a insulator with + and - Q at either ends, and you have neutral copper metal near the wire.

    The choices were:

    a) North pole of magnet faces magnetic field, insulator is attracted, copper dont' move
    b) North pole of magnet faces magnetic field, insulator doens't move and copper doesnt' move either.

    I said insulator moves, because the wire can be thought of as an infinite charged plane, thus creating a small electric field powerful enough to move a light insulator
  5. Feb 14, 2008 #4
    The wire is certainly not similar to a charged infinite plane, since it is one dimensional. An infinite charged line would be a better aproximation. The electric field of a charged line is propotional to 1/r, much different than constant field of a plane.

    But in your case the wire is not even charged. If electrons travel through, it does not mean that the net charge is negative: the are still stationary protons there and their positive charge exactly compensates the electrons' charge.

    A conductive material can theoreticly have a non zero net charge (if it has a finite capacity and voltage relative to infinity), but a thin wire has virtually no capacity: if you integrate the electric field 1/r, you find out that integral (ln (r)) diverges when the starting radious aproaches zero.
  6. Feb 15, 2008 #5
    The answer is that the charged insulator will also move. Proved experimentally. The prof also gave a explaination:

    The charged insulator rod will move towards the wire because it will polarize the wire, resulting in an attractive force between the wire and the charged rod. The bar magnet will rotate counterclockwise to align along a tangent to a circle whose centre is at the wire. North points in the direction of the magnetic field vector.
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