Property of conductor

Question

So I read from a book that if a charge is injected anywhere in the conductor it must come over to the surface.
But when I read another paragraph from the same book,  it was given that when a charge +Q is placed at centre of a spherical conducting shell, a charge -Q will appear on its inner surface and +Q on its outer surface.
But according to the property it must come directly to the surface then what was the need to mention -Q?
Also, does it change the definition to, "Net charge inside a conductor is 0 and if a charge (let's say +Q) is injected anywhere inside then another charge here, -Q devlops in the inner surface and due to this +Q appears on the outer surface?
Please help.

BowlOfRed · Answer

So I read from a book that if a charge is injected anywhere in the conductor it must come over to the surface.

"Within" here meaning "inside the conductive material", not "surrounded by the conductive material".  Conductors have mobile charges within.  A net charge inside the conductor would cause charge migration until all the excess charge was on the outer surface.

But when I read another paragraph from the same book, it was given that when a charge +Q is placed at centre of a spherical conducting shell,

This is a conductive shell, so the inside of the shell is not a conductor.  You can imagine the interior to be air.  The charge here is merely surrounded by a conductor.  Because the interior is not a conductor, the charge there cannot move.
If it were a solid ball of metal, then you would be correct (and there would be no "inner surface" where the charge would be induced).

Property of conductor

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