Covers: Potential from fields and charge distributions. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaU = qV, and ΔU = −W done by the field. A positive charge loses potential energy moving toward lower V; a negative charge gains it.
Common mistakeForgetting the sign of the charge when deciding whether U increases or decreases.
Key ideaV = kq/r is a scalar, so the potentials from several charges add as signed numbers.
Common mistakeAdding potentials as vectors, or using 1/r² (the field formula) for potential.
Key ideaV = ∫k dq/r adds signed scalars, so there are no components to cancel. Pieces with the same sign all add, however they are arranged.
Common mistakeCancelling contributions by symmetry as you would for field components. Only charges of opposite sign reduce V.
Key ideaΔV = −∫E · dl. Moving along the field lowers the potential.
Common mistakeDropping the minus sign, or integrating from the wrong endpoint.
Key ideaE = −dV/dx: the field points toward lower potential, and its size is how steeply V changes.
Common mistakeThinking E = 0 wherever V = 0, or that E is large wherever V is large.
This is one of the 24 questions in the Unit 9 quiz.
Three charges, each , sit at the corners of an equilateral triangle with side . What is the electric potential energy of the arrangement?
Potential energy is a sum over pairs of charges. How many distinct pairs are there?
Answer: A.
Add the energy of each pair once: there are 3 pairs, each , so .
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
