With a large, charged sphere in aqueous NaCl solution (large enough to sediment in the field of gravity;; 10m diameter). This sphere is in a vessel made up of the same material as the sphere, and the vessel carries like charges. As this sphere approaches the bottom of the vessel (flat), electrostatic repulsion will prevent the sphere from settling completely. Instead, the large sphere will hover at a distance Zmin above the glass, which corresponds to where the minimum in the potential curve for that sphere near the bottom - that is, by a superposition of gravitational energy and electrostatic repulsion energy.

Assume that Zmin = 120 nm and that Van der Waals interactions may be neglected at this distance. Other: the sphere has a density difference to the solvent of ?? = 0.5 mg/mL, the salt concentration is c = 0.5 mmol/L. The dielectric constant of the solution is? = 80 and?0 = 8.85 10^?12 C²(Jm)^?1. Calculate the number of elementary charges on the surface of the sphere. Use the Debye-Huckel expression for the electrostatic repulsion and explain why. You may simply the problem by a

ssuming two plates, instead of a plate and a sphere.