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.
Felect = k • Q1 • Q2 / d2
In the above equation, Q1 might be the source charge Q and Q2 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 / d2
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