A thin, metallic spherical shell contains a charge Q on it. A point charge q is placed at the centre of the shell and another charge q₁ is placed outside it as shown in the figure. All the three charges are positive. The force on the central charge due to the shell is

Step-by-step solution

According to the properties of conductors, the electric field inside the cavity of a conducting shell due to any external charges is always zero (electrostatic shielding). The external charges in this case are the point charge q₁ and the charge distributed on the outer surface of the shell. Therefore, the electric field at the centre due to the outer surface charge ( E _ outer ) and the point charge q₁ ( E _ q₁ ) must sum to zero: E _ outer + E _ q₁ = 0 E _ outer = - E _ q₁ The positive charge q at the centre induc