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i would have thought that because the electric and magnetic fields oscillate, the charge could be positive or negative between 0 and 1 inclusive at any one point in time. i cannot see any explanation for the charge being 0 unless they cancel each other out.

what is the reason for this?

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  • $\begingroup$ charge is not the same as electric field $\endgroup$
    – garyp
    Commented Apr 14, 2015 at 12:20

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The oscillation is not an oscillating charge but an oscillating dipole, and a dipole has zero net charge.

NB this doesn't mean a light wave is actually a pair of equal and opposite charges. It just the electromagnetic field that is produced from a pair of oscillating charges.

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    $\begingroup$ I'm not sure what you mean. A solitary electrical charge can create an electromagnetic wave - it seems like you're saying there must be an oscillating electric dipole to create an electromagnetic wave. Could you clarify? $\endgroup$
    – Brionius
    Commented Apr 14, 2015 at 12:42
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    $\begingroup$ @Brionius: An isolated charge can't oscillate - that would violate conservation of momentum. If you see a charge oscillating there is always something else present to provide the potential well in which it is oscillating, and that something else is overwhelmingly likely to be a (net) positive charge. I suppose an electron could oscillate in a gravitational potential well, though offhand I can't think of an experimentally accessible system where the emission would be measurable. $\endgroup$
    – John Rennie
    Commented Apr 14, 2015 at 12:53
  • $\begingroup$ Actually an electron in a synchrotron will emit EM even though there aren't any positive charges nearby. So while this isn't an isolated charge I guess it means you don't have to have a pair of charges. $\endgroup$
    – John Rennie
    Commented Apr 14, 2015 at 13:11
  • $\begingroup$ There are lots of circumstances in which a charge will radiate that don't involve an electric dipole. For example, an electromagnetic wave passing a solitary charge will radiate. Or, a solitary charge moving in a magnetic field will radiate. Or as you said, a solitary charge moving in a gravitational field. I think it would be misleading to say that these situations involve dipoles. $\endgroup$
    – Brionius
    Commented Apr 14, 2015 at 14:54
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Electromagnetic waves constitutes from changing ELECTRIC AND MAGNETIC FIELD.. There is no transfer of charge here.. It is the oscillation of electric and magnetic FIELD.. Field isn't defined as positive or negative.. Why should the consideration of charge would come to picture at all when only field line propagates..

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