Two identical spheres each of radius R are kept at centre-to-centre spacing 4R as shown in the figure. They are charged and the electrostatic force of interaction between them is first calculated assuming them point like charges at their centres and the force is also measured experimentally. The calculated and measured forces are denoted by ${\mathrm{F}}_{\mathrm{c}} \mathrm{and} {\mathrm{F}}_{\mathrm{m}}$ respectively. (${\mathrm{F}}_{\mathrm{c}} \mathrm{and} {\mathrm{F}}_{\mathrm{m}}$ denote magnitude of force) 

When they carry charges of the same sign ${\mathrm{F}}_{\mathrm{c}}<{\mathrm{F}}_{\mathrm{m}}$ and when they carry charges of opposite signs ${\mathrm{F}}_{\mathrm{c}}>{\mathrm{F}}_{\mathrm{m}}$ irrespective of their material. 

When they carry charges of the same sign ${\mathrm{F}}_{\mathrm{c}}>{\mathrm{F}}_{\mathrm{m}}$ and when they carry charges of opposite signs ${\mathrm{F}}_{\mathrm{c}}<{\mathrm{F}}_{\mathrm{m}}$ only when they are insulators

When they carry charges of the same sign ${\mathrm{F}}_{\mathrm{c}}>{\mathrm{F}}_{\mathrm{m}}$ and when they carry charges of opposite signs ${\mathrm{F}}_{\mathrm{c}}<{\mathrm{F}}_{\mathrm{m}}$ irrespective of their material.

When they carry charges of the same sign ${\mathrm{F}}_{\mathrm{c}}>{\mathrm{F}}_{\mathrm{m}}$ and when they carry charges of opposite signs ${\mathrm{F}}_{\mathrm{c}}<{\mathrm{F}}_{\mathrm{m}}$ only when they are conductors