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Cancellation of Electric Fields

  1. Jan 12, 2010 #1
    I was having some trouble understanding how an electric dipole creates an electric field around it? Shouldn't the two oppositely charged particles in the dipole totally cancel out their electric fields?

    Well, I figured out that the two particles only exert a force on eachother, but they cannot cancel out their fields. Because, physically speaking, an electric field spreads out in all directions around the particle, and it exists as long as the particle, which is charged, exists. And, the presence of the oppositely charged particles for eachother do not remove the other particles existence, and hence it's electric field?

    But, if that is so, then how does an atom, or the vast number of molecules around us not create an electric field around themself. Do the sub-atomic particles in the atoms and molecules cancel out eachother's electric fields?
     
  2. jcsd
  3. Jan 12, 2010 #2

    Born2bwire

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    The fact that the opposite charges are not colocated ensures that the field is never perfectly cancelled out. However, the general effect is to diminish the "range" of the field. For example, a monopole (one charge) drops off in field as 1/r^2. A dipole drops off as 1/r^3. Higher multipoles drop off with increasing powers of 1/r. So you can see that while the field is not perfectly cancelled out, a general configuration of equal numbers of oppositely charged particles will give rise to a field that falls off very very quickly.

    Molecules can however create fields around them, some molecules have permanent dipole moments. Other molecules, though lacking in a permanent dipole moment, will have a dipole moment induced by an applied electric field. The permanent and induced dipole moments give rise to the London-Van der Waals forces that govern the force of attraction and repulsion between molecules.
     
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