Covers: Coulomb’s law, continuous charge, flux and Gauss’s law. 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 ideaFind each charge’s field or force separately, then add them as vectors, component by component.
Common mistakeAdding magnitudes without using symmetry or direction, or getting the attraction/repulsion direction wrong.
Key ideaWrite dq (λ dx, σ dA or ρ dV), write dE for that piece, and use symmetry to cancel components before integrating.
Common mistakeIntegrating the magnitude dE when the components partly cancel.
Key ideaΦ = ∫E · dA counts the field through a surface. Only the component perpendicular to the surface contributes.
Common mistakeUsing E·A when the field is not uniform or not perpendicular to the surface.
Key ideaThe net flux through a closed surface equals qenc/ε₀. It is useful when symmetry makes E constant and perpendicular on the surface.
Common mistakeUsing the total charge instead of the charge enclosed by the Gaussian surface.
This is one of the 21 questions in the Unit 8 quiz.
A charge +q is at x = 0 and a charge −4q is at x = d. Where on the x-axis, other than at infinity, is the electric field zero?
Between two opposite charges, which way does each field point? Look for the zero on the side nearer the smaller charge.
Answer: A.
Between opposite charges both fields point the same way, so they can’t cancel there. The zero must be outside, nearer the smaller charge: to the left of . At distance to the left, , so and : .
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
