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Why is the magnetic field around a piece of wire circular?

  1. May 22, 2012 #1
    I am trying to understand how exactly electric and magnetic fields work.

    So far this is my understanding. A particle which has charge has an electric field around it. If the charged particle moves, it forms a magnetic field with its force lines perpendicular to direction of the movement.

    If this charged particle were an electron and it were to orbit a nucleus of an atom, then it would create a magnetic field perpendicular to the line between the electron and the nucleus. Now, in a normal piece of matter, there is not just one electron and there is not just one nucleus for those electrons so generally they tend to cancel out. Except for ferromagnetic materials where the electrons in orbit around different nuclei somehow line up to create a macro magnetic field. (I understand this is because these materials have partially filled outer shells which mean that due to Hund's rule, the electrons are more likely to have same spin direction. But I don't understand why materials above this proton number (still with partially filled shells) cannot have ferromagnetic property?).

    If there's anything wrong there, please point out.

    Moving on.

    What I don't understand: when we apply a voltage across a conductor, the charged particles within it tend to move to the end which negates the voltage. OK, but why does this movement create a magnetic field which is CIRCULAR and not RADIALLY outward? Since the electrons are generally moving in 1 direction at a macro level, the magnetic field induced should be perpendicular to this direction in macro level. No?

    This is obviously not true and I am trying to find out why it is not true. What am I missing?
  2. jcsd
  3. May 22, 2012 #2
    A charged particle is not particle, it is a wave/particle. The moving charge particle/wave has a wave function. This wave function has a momentum associated with it. This momentum creates a momentum field called the vector potential field (Maxwell). The vector potential is in the same direction as the particle/wave velocity. This momentum field influences other charges near the moving particle. This influence is seen as the magnetic field. The magnetic field is defined as the Curl of the vector potential, and as the vector potential gets smaller away from the wire, the magnetic field appears perpendicular to the wire.
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