<P> The equations of electromagnetism are best described in a continuous description . However, charges are sometimes best described as discrete points; for example, some models may describe electrons as point sources where charge density is infinite on an infinitesimal section of space . </P> <P> A charge q (\ displaystyle q) located at r 0 (\ displaystyle \ mathbf (r_ (0))) can be described mathematically as a charge density ρ (r) = q δ (r − r 0) (\ displaystyle \ rho (\ mathbf (r)) = q \ delta (\ mathbf (r - r_ (0)))), where the Dirac delta function (in three dimensions) is used . Conversely, a charge distribution can be approximated by many small point charges . </P> <P> Electric fields satisfy the superposition principle, because Maxwell's equations are linear . As a result, if E 1 (\ displaystyle \ mathbf (E) _ (1)) and E 2 (\ displaystyle \ mathbf (E) _ (2)) are the electric fields resulting from distribution of charges ρ 1 (\ displaystyle \ rho _ (1)) and ρ 2 (\ displaystyle \ rho _ (2)), a distribution of charges ρ 1 + ρ 2 (\ displaystyle \ rho _ (1) + \ rho _ (2)) will create an electric field E 1 + E 2 (\ displaystyle \ mathbf (E) _ (1) + \ mathbf (E) _ (2)); for instance, Coulomb's law is linear in charge density as well . </P> <P> This principle is useful to calculate the field created by multiple point charges . If charges q 1, q 2,..., q n (\ displaystyle q_ (1), q_ (2),..., q_ (n)) are stationary in space at r 1, r 2,...r n (\ displaystyle \ mathbf (r) _ (1), \ mathbf (r) _ (2),...\ mathbf (r) _ (n)), in the absence of currents, the superposition principle proves that the resulting field is the sum of fields generated by each particle as described by Coulomb's law: </P>

Which one of the following cannot be used as a unit for electric field strength