What is the derivation of Coulomb's constant?

Answer 1

Coulomb's constant 'k' in the equation F=kQQ/r2 is derived from Gauss's law. Gauss's law stated that the charge enclosed by a theoretical surface is equal to the permittivity constant, represented by the Greek letter epsilon (because I can't use an epsilon, I will use an X) times the electric flux through the surface. Flux is equal to the closed integral of electric field vector dot the vector dA (infinitesimal change in surface area) of the surface. Becasue the surface surrounding one point charge is a perfect sphere, the dot product can be ignored (The surface is uniform and every change in area is normal to the electric field), and the Electric field is constant so it can be brought out of the integral leaving integral dA. When the integral is solved, the resulting equation is XEA=Q. A equals the surface area of the sphere so XE(4*pi*r2)=Q and E=Q/(4*pi*X*r2) and because F=EQ, F=QQ/(4*pi*X*r2). This is probably looking pretty familiar. All we have to do is make k=1/(4*pi*X) to make this equation equal to good old Coulomb's law. X, the permittivity constant equals 8.854*10-12 Farads per meter, or coulombs squared seconds squared per kilograms meters cubed. If you substitute this constant into the equation k=1/(4*pi*X), you obtain Coulmb's constant.