Electrostatic forces obey Coulomb's law. Like gravity, electrostatic forces follow the inverse square law: that is, the strength of the force is inversely proportional to the square of the distance of separation. Coulomb's law says the force is also proportional to the two charges interacting. Unlike gravity which is only an attractive force, electrostatic forces can be attractive it the interacting charges are opposite in sign, or repulsive if they are of the same sign.

With this in mind, we can apply Coulomb's law formula

Fe = kQ1Q2/d^2

k is Coulomb's constant = 9.0x10^9Nm^2/C^2

Q1 and Q2 are the two charges and d is the distance of separation.

Fe =9.0x10^9(10x10^-6)(3x10^-6)/4.0 =0.0675 N

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