Coulomb's constant is 8.99×109 Nm2C-2. It is equal to 1/(4πε0) where ε0 is the permittivity of a vacuum which is 8.85×10-12 Fm-1. If the constant is to be used to calculate the Coulomb force with Coulomb's law and the electric field is not in a vacuum, then the constant will have to be recalculated with the relative permittivity instead.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
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