Question

What formula gives the strength of an electric field, E, at a distance from a known source charge?

E = Startfraction F Subscript e baseline over q times d Endfraction.
E = Startfraction k times q over d Endfraction.
E = Startfraction k times q over d Superscript 2 baseline Endfraction.
E = Startfraction F Subscript e baseline over d Endfraction.

Answer 1

E = (k q) / d^2.

Reason: Coulomb's law gives the force on a test charge q by a source charge Q as F = k Q q / d^2. The electric field is force per unit test charge, E = F / q, so E = k Q / d^2 (directed radially, units N/C). Here k = 1/(4πε0) ≈ 8.99×10^9 N·m^2/C^2.