The positive test charge in Figure 18.15 is q₀=+3.0×10^–8 C and experiences a force F=6.0×10^–8 N in the direction shown in the drawing. (a) Find the force per coulomb that the test charge experiences. (b) Using the result of part (a), predict the force that a charge of +12×10^–8 C would experience if it replaced q₀.

Reasoning  The charges in the environment apply a force F to the test charge q₀. The force per coulomb experienced by the test charge is F/q₀. If q₀ is replaced by a new charge q, then the force on this new charge is the force per coulomb times q.

Solution

(a)	The force per coulomb of charge is
(b)	The result from part (a) indicates that the surrounding charges can exert 2.0 newtons of force per coulomb of charge. Thus, a charge of +12×10–8 C would experience a force whose magnitude is The direction of this force would be the same as that experienced by the test charge, since both have the same positive sign.