Compute the electric field produced by a point charge, and relate electric potential, potential energy, and charge.
Rather than picturing two charges reaching directly across empty space to push or pull on each other, physicists describe an electric field: a charge creates a field filling the space around it, and any second charge placed in that field feels a force because of the field at its location, F = q*E. For a point charge, the field it creates is E = k*Q/r^2 -- essentially Coulomb's law with the second (test) charge divided back out, describing the field itself as "force per unit charge," independent of whatever charge might eventually be placed there to detect it.
Electric potential energy (PE) is the stored energy of a charge's position within a field, and electric potential (voltage, V) is that potential energy PER UNIT CHARGE: V = PE/q, measured in volts (1 volt = 1 joule per coulomb). For a point charge, V = k*Q/r. The voltage printed on a battery is really a potential DIFFERENCE maintained between its two terminals -- and it is exactly this potential difference that drives current through a circuit, the subject of the next two weeks.
Mastery looks like: They compute the electric field from a point charge using E=kQ/r^2, and use V=PE/q (or PE=qV) to relate potential, potential energy, and charge.
Common stumbles: Confusing electric field (a property of the space around a charge, independent of any test charge) with electric force (which requires an actual charge to be present); confusing potential (per unit charge) with potential energy (not divided by charge).