A-Level Physics: Electric Fields & Capacitors — Uniform Fields, Energy Storage, and Common Graphs
A-Level Physics — Electric Fields & Capacitors: Uniform Fields, Energy Storage, and Common Graphs: quick notes, common traps, and an exam-style example.
Electric field basics
Electric field strength E is force per unit positive charge (E = F / q). In uniform fields (parallel plates), field lines are straight, equally spaced, from positive plate to negative. Potential difference V between plates is related by E = V / d. For point charges, E = kQ/r² and direction is radial.
Capacitance
Capacitance C is defined by Q = CV. Energy stored = ½CV² = ½QV = ½Q²/C. Area under a Q–V graph gives energy in the capacitor — this is a favourite data-handling style question in A-Level Singapore papers.
Dielectrics & exam traps
Inserting dielectric (with battery still connected): C ↑, so Q ↑. If battery disconnected first: Q fixed, so V ↓. Students mix these two cases all the time. State clearly whether the capacitor is isolated or still connected.
Exam habits
- Quote definitions exactly: “Capacitance is the charge stored per unit potential difference”.
- Label field direction arrows from + to − in diagrams; missing direction costs marks.
- When calculating energy, include units (Joules) and round to 2–3 s.f. like the data in the question.