For an electric field, we define electric field strength E as follows:
The electric field strength at a point is the force per unit charge exerted on a stationary positive charge at that point.
So, to define electric field strength, we imagine putting a positive test charge +Q in the field and measuring the electric force F that it feels (Figure 1). (If you have used a charged gold leaf to investigate a field, this illustrates the principle of testing the field with a charge.)
From this definition, we can write an equation for E:
It follows that the unit of electric field strength is the newton per coulomb (N C-1).