A spherical metal body has +Q charge, uniformly distributed over its outer surface. The body has a cavity at its centre. Select proper procedure from those given below which will change the charge on the outer surface of this body from +Q to –Q.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Charge –2Q is inserted in the cavity and is kept insulated from the inner surface of the cavity
Inside the metal of a conductor the electric field is zero, so by Gauss's law a charge \( q \) in the cavity induces \( -q \) on the inner surface; the total charge is conserved, so the outer surface carries the rest.
With \( -2Q \) held insulated in the cavity:
\( \text{inner surface: } +2Q \)
\( \text{outer surface: } +Q - 2Q = -Q \)
Option (2) Charge –2Q is brought closer to the body externally and kept at some small distance away from it — Incorrect. An external charge only redistributes the charge; the outer surface still carries \( +Q \).
Option (3) Another metal sphere having total charge –2Q distributed on its surface is kept in contact with the given spherical body for some time and then the two are separated — Incorrect. On contact the total \( -Q \) is shared between the two spheres, so the body keeps only a part of it.
Option (4) A metal body having charge –Q is inserted in the cavity and kept touching the inner surface of the cavity — Incorrect. The charge \( -Q \) passes to the body and its outer surface becomes \( +Q - Q = 0 \).