Coulomb’s Law - MCAT
Coulomb’s Law describes what happens when two charges are placed close to one another. The magnitude of the force between the two charges, called the electrostatic force, depends on two variables: the magnitudes of the two charges, and the distance between the two charges. Whether the force is attractive or repulsive depends on the signs of the two charges: like charges will repel, and unlike charges will attract. Mathematically, two charges that repel will have a positive value for the electrostatic force, while two charges that attract will have a negative value for the electrostatic force.
Dependence on the Magnitudes of the Charges:
The electrostatic force is directly dependent on the magnitude of each of the two charges. If one charge’s magnitude is doubled, the electrostatic force between them will be doubled. If the second charge’s magnitude is doubled, again, this will double the force between them. If the magnitudes of both charges are doubled, the force between them will be quadrupled.
F 
q1
F q2
Dependence on the Distance Between the Charges:
The electrostatic force is indirectly dependent on the square of the distance between the two charges. This is why Coulomb’s Law is known as an inverse square law, like Newton’s Law of Universal Gravitation. If the distance between the two charges is doubled, the force is cut by one-fourth. If the distance between the two charges is halved, the force between them is quadrupled.
F 1/(r^2)
Putting all of these variables together with a permittivity constant gives us Coulomb’s Law:
F = k(q1)(q2)/(r^2)
You should understand and memorize Coulomb’s Law, but you do NOT need to memorize the value of k for the MCAT.
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