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My textbook says that no electric field is present inside a metallic cavity.

Let us consider a hollow metallic shell inside another. And if I give a charge (-q) to inner shell, a charge (+q) develops on inner surface of outer shell.

With this much information ,there definitely exists eletric field between the two shells. How do I account for zero electric field inside a metallic cavity when i actually do find an electric field existing?

Am i missing out on any of the other electric fields?

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The setup you have described is completely fine and there will definitely be an electric field between the two shells.

But what your textbook means by metallic cavity is that a single metal piece/block with a cavity in it. And metals are conductors, so they will have no charge on their inner surface, all charge would reside on the outer surface only.

Since the charge enclosed in the cavity is 0, by Gauss' law, electric field is also 0. Notice that electric field is also 0 for the inside of any metal object, cavity or not; since charge is only on outer surface.

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  • $\begingroup$ Can I say that if electric field exists between outer surface of inner shell and inner surface of outer shell, then an equal & opposite electric field exists because of charges on outside and inside of outer shell ? $\endgroup$
    – Shahbaaz1104
    Jan 1, 2017 at 15:29
  • $\begingroup$ @Ali I am not sure what you are implying here. In your setup, there will be an electric field between the inner shell and outer shell ; NO electric field between the INNER SURFACE and OUTER SURFACE of outer shell ; an electric field outside the outer shell. $\endgroup$
    – Sagar
    Jan 1, 2017 at 15:53
  • $\begingroup$ okay fine, but even a varying potential exists between the two spherical surface . And then we say that potential inside a metallic shell must be constant? $\endgroup$
    – Shahbaaz1104
    Jan 1, 2017 at 23:58
  • $\begingroup$ @Ali That is true only if there is only one spherical shell, not when there is a shell inside a shell. Note that this will be valid for inner shell but not just between the two shells. $\endgroup$
    – Sagar
    Jan 2, 2017 at 10:36
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If you look at the outer shell , whatever be the charge on it there will be no electric field inside it while the inner shell exerts an electrostatic field everywhere outside it.

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