ELECTROSTATICS (Coulomb's Law)

We begin with Coulomb's Law for the magnitude of the electrostatic force (in newtons) between two point charges q and Q (in coulombs). It is convenient to label one of these charges, q, as a test charge, and call Q
a source charge. As we develop the theory, more source charges will be added. If r is the distance (in meters) between two charges, then the force is:

where ε₀ is the vacuum permittivity, or permittivity of free space:

The SI units of ε₀ are equivalently  A²s⁴ kg⁻¹m⁻³ or C²N⁻¹m⁻² or F m⁻¹. Coulomb's constant is:

The use of ε₀ instead of k₀ in expressing Coulomb's Law is related to the fact that the force is inversely proportional to the surface area of a sphere with radius equal to the separation between the two charges.
A single proton has a charge of e, and the electron has a charge of −e, where,

These physical constants (ε₀, k₀, e) are currently defined so that ε₀ and k₀ are exactly defined, and e is a measured quantity.