Describe how objects become electrically charged and apply Coulomb's law to compute the electric force between two point charges.
Electric charge comes in two types, positive and negative, with like charges repelling and opposite charges attracting. Ordinary objects become charged by the movement of electrons (never protons, which stay locked inside atomic nuclei): charging by friction transfers electrons between two rubbed materials (a balloon rubbed on hair); charging by conduction transfers charge through direct contact until both objects share it; charging by induction rearranges charge within a nearby object with no contact or transfer at all. Materials that let charge move freely are conductors (most metals); materials that resist that movement are insulators (rubber, plastic, glass).
Coulomb's law gives the force between two point charges: F = k*q1*q2 / r^2, with k = 8.99x10^9 N*m^2/C^2 and charge measured in coulombs (C). This should look strikingly familiar -- it has the exact same inverse-square structure as Newton's law of gravitation from a few weeks ago. The key difference is that the electric force can be either attractive or repulsive depending on the signs of the two charges, while gravity is only ever attractive.
Mastery looks like: They compute the electric force between two point charges using Coulomb's law, and correctly describe how an everyday object becomes charged.
Common stumbles: Forgetting to square the distance in the denominator; assuming a charged object must physically touch another object to have any effect on it at all (missing induction).