18.3 Electric force between point charges

For a spherical conductor in electrostatic equilibrium, excess charge resides on the surface and the external field acts as if total charge Q were at the centre.

Use centre distance r and remember the field inside the conductor is zero in electrostatic equilibrium.

A charged metal sphere produces an external field decreasing as 1/r², even though the charge is spread over its surface.

The point-charge equivalence does not describe the field inside the conductor or an irregular charged object without symmetry.

Two point charges exert forces of magnitude F=|Q₁Q₂|/(4πε₀r²), along their joining line; like charges repel and unlike charges attract.

Use centre separation r and treat the force as a vector when multiple charges act. Include signs to determine direction.

Doubling one charge doubles force, while doubling separation quarters it.

The inverse-square separation is not the distance from a charge’s surface, and equal force magnitudes act on both charges.