Covers: Conductors, capacitance and dielectrics. 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 ideaInside a conductor in equilibrium E = 0, any excess charge sits on the surface, the whole conductor is at one potential, and the field just outside is perpendicular to the surface.
Common mistakePlacing net charge inside the conductor, or a field in its interior.
Key ideaConductors that are connected end at the same potential. Charge moves until that happens, and the total charge is conserved.
Common mistakeSplitting the charge equally between connected spheres of different sizes.
Key ideaC = Q/V depends only on geometry (and any dielectric), e.g. ε₀A/d for parallel plates. Capacitances add in parallel, and their reciprocals add in series.
Common mistakeThinking C changes when Q or V changes, or using the resistor combination rules.
Key ideaU = ½QV = ½CV² = Q²/2C. Use the form with the quantity that stays fixed: Q if the capacitor is isolated, V if it is connected to a battery.
Common mistakeUsing ½CV² with the old V after the capacitor has been disconnected and its geometry changed.
Key ideaA dielectric multiplies C by κ. If the capacitor is isolated, Q is fixed and V and U drop; if it is connected to a battery, V is fixed and Q and U rise.
Common mistakeNot checking whether the battery is still connected before deciding what changes.
This is one of the 24 questions in the Unit 10 quiz.
A positively charged rod is held near a neutral metal sphere. The sphere is briefly connected to ground, the ground connection is removed, and then the rod is taken away. What is the sphere’s final charge?
Follow the steps in order: which charges does the rod pull or push, which way can charge flow while grounded, and what is trapped when the ground is cut?
Answer: A. Negative
The rod pulls electrons toward the sphere’s near side and pushes positive charge away. Grounding lets electrons flow up from the ground to cancel that pushed-away positive charge. With the ground cut first, those extra electrons are trapped, so the sphere ends up negative.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
