The electric field strength (E) is defined as the amount of force exerted upon a test charge per unit of charge on the test charge (q). That is, E = F / q. The electric force (F) depends upon a number of variables as described by Coulomb's law.
F_{elect} = k • Q_{1} • Q_{2} / d^{2}
In the above equation, Q_{1} might be the source charge Q and Q_{2} might be the test charge q. If the expression for force as given by the Coulomb's law equation is substituted in for F in the electric field strength equation, then the equation for electric field becomes
E = k • Q / d^{2}
