An isolated solid conducting sphere of radius \(R\) carries a net positive charge \(Q\) in electrostatic equilibrium.

Due to the mutual repulsion of the surface charge, an outward radial force acts on each surface element, producing an outward electrostatic pressure \(P\) across the sphere's surface.

In terms of \(Q\), \(R\), and physical constants, which of the following is a correct expression for the electrostatic pressure \(P\)?