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Some Basic questions

  1. Apr 19, 2007 #1
    Here is some very fundamental things troubling me. It follows......
    How we can cancel
    (1) the magnetic field produced by a magnet(so that there will be no magnetic field)
    (2)the electric field produced by a charge (so that there will be no electric field)

    Does same numbers of positive and negative charges cancel the field of each other in such a way that we have no electric field at all? Sounds silly but atoms are electrically neutral that way. If it is not that way then what is the definition of electrically neutrality? Is it not about having no net electric field?
    In the same way, how two electrons in the same orbital having opposite spin cancel out the magnetic field of each other?

    please explain in simple and basic terms (m An O level student) .
  2. jcsd
  3. Apr 19, 2007 #2
    (1) You can enclose the magnet in a magnetic shield. That is, a closed "box" formed by a high magnetic permeability alloy (as mu-metal) thick enough. How much thick depends on the magnetic flux that the shield has to "short-circuit".

    (2) You can enclose the charge in a Faraday cage. That is, a closed conductive box. Any thickness work.

    Positive and negative charges do not cancel unless they have the same total value and they have the good distribution in space. One "good" distribution is all positive or negative charge in the center surrounded uniformly by the charge of opposite sign (as an atom).

    Some molecules (as water) are electrically neutral as they have as many electrons as protons. But the charges are not distributed in the "good" way, and they produce an electric field.

    Full orbitals have each level filled with electrons of opposite spin, which cancel each other. These orbitals do not contribute to the magnetic field of the atom.
  4. Apr 21, 2007 #3
    How does a magnetic shield work (in the simillar way a faraday cage does, I hope!)?
  5. Apr 21, 2007 #4
    No. A magnetic shield is something that "conducts" the magnetic field much better than air, vacuum, etc. Lines of field "prefer" to pass through this material instead of traversing it, make a tour in the other side, and traverse the shield once again. But if you put a sheet of shield between the two poles of a horseshoe magnet, the field lines will happily traverse the shield to go from one pole to the other.
    P.S. Do not tell anyone that I used the terms "conduct" and "prefer". It is not very scientific https://www.physicsforums.com/images/icons/icon12.gif [Broken]
    Last edited by a moderator: May 2, 2017
  6. Apr 21, 2007 #5
    Thx a lot. :)
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