A point charge +q is trapped inside an empty cavity of an initially neutral metal block. Which statements are necessarily correct in electrostatic equilibrium?
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Question

A point charge +q is trapped inside an empty cavity of an initially neutral metal block. Which statements are necessarily correct in electrostatic equilibrium?

G1 Physics · Accepted Answer

Take a Gaussian surface entirely inside the metal and enclosing the cavity. Since E=0 in the metal, q + Q i n n e r Q_inner Q i ​ nn er = 0, so Q i n n e r Q_inner Q i ​ nn er = -q. Because the conductor was initially neutral, Q o u t e r Q_outer Q o ​ u t er = +q. The inner surface distribution is generally nonuniform unless special symmetry exists, so D is false.