Well I'm confused. The thing that was implanted to me is that when I hear about conductors, some charge is present and it can move freely. Now what I want to know is that is it possible for a conductor not to have a charge? My question is based on this problem.

  1. A uniformly charged insulating sphere with radius R and charge +Q is enclosed by a spherical conducting shell with inner radius R and outer radius 2R(both centers at origin). Determine the magnitude of the electric field if

a)R < r <2R(inside the conducting shell but outside the insulating sphere)

b) r>2R(outside the conducting sphere)

Since +Q is inside the insulating sphere enclosed inside the conductive shell, this implies that +Q cannot go to the conductor. And according to the problem, it was not mentioned that a charge is present in the conductive shell. So is it okay for me to assume that both electric field in a) and b) is 0 in accordance to Gauss's Law?

  • 1
    $\begingroup$ In a conductor you have free electrons that can move around, and you have ions with an opposite charge. The total charge in this problem is zero, but that doesn't mean that the charge is zero everywhere. Inside a conductor the electric field will be zero (in the static case), because in case of a non-zero field the electrons will just move until the field vanishes everywhere. Gauss Law then implies that there is no charge inside the conductor, and that then means that any charge due to the electrons having shielded the fileds in the interior will end up as surface charges. $\endgroup$ – Count Iblis Aug 26 '15 at 15:53
  • $\begingroup$ Oh, well that clears things up, also, by the way, as you have said, that conductors have free electrons that can move around, does this mean, that normally, conductors have negative charges?? $\endgroup$ – llawliet_78 Aug 26 '15 at 15:58
  • $\begingroup$ They can have both positive or negative charges, because extra electrons can be added or electrons can be removed. $\endgroup$ – Count Iblis Aug 26 '15 at 16:00
  • $\begingroup$ @Ilawliet Well, conductors contain electrons which are electrically negative individually. But as a whole you should rather think of a container as having a negative (net) charge if it has too many electrons, and a positive (net) charge if it has lost electrons $\endgroup$ – Steeven Aug 26 '15 at 16:02

A conductor can have a zero net charge. One way to do this is to have an inner surface with a net charge of $-Q$ and an outer surface with a net charge of $+Q.$

So for instance you can have a surface charge density of $-Q/(4\pi R^2)$ on the surface $r=R$ and you can have a surface charge density of $+Q/(4\pi (2 R)^2)$ on the surface $r=2R$ and this has zero net charge. You can achieve this by having some electrons leave one surface and leave that surface with a net positive charge and have them arrive at the other surface as extra electrons leaving that surface with a net negative charge. Obviously the total charge stays the same.

Why would this happen? If mobile charges are free to move through the body of the conductor and there were an electric field the charges could flow from one surface to the other and keep doing so until the field was zero inside the body of the conductor.

If the field is zero inside that body of the conductor then if you place a Gaussian surface on a spherical shell of radius $r'$ with $R<r'<2R$ then you get a certain flux. That flux is proportional to the total charge enclosed by the Gaussian surface which in this case includes any charge on the insulator (the entire insulator is enclosed by the Gaussian surface) and any charge on the inner $r=R$ surface of the conductor (the $r=R$ surface is enclosed by the Gaussian surface) and the charge (if any) in the body of the conductor from the inner surface all the way to the Gaussian surface $r=r'.$

If the conductor doesn't have a net charge, then the charge on the outside $r=2R$ surface would have to be the same (equal and opposite) as the charge on the $r=R$ surface (though since they have different areas the surface charge would have to be equal).

So based on the electric field in the body and the charge of the insulator you can find out the charge on the inner surface. Based on the total charge of the conductor and the charge on the inner surface you can find the charge on the outer surface. Then based on the charge on the insulator and the two surfaces you can find the electric field outside the conductor. Each time you use Gauss's law and solve for a field or a charge (which ever you don't know) or you use conservation of charge (the total charge of the conductor is the sum of the charge on the two surfaces).


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