The electricity units cover electrostatics (charges, fields, potential) and DC circuits (current, resistance, circuit analysis) — completing the AP Physics 1 curriculum.
Charge: positive/negative, quantised (e = 1.6×10⁻¹⁹ C), conserved. Coulomb\'s law: F = kq₁q₂/r² (k = 9×10⁹ N·m²/C²). Like charges repel, unlike attract. Electric field E = F/q = kQ/r² (points away from + charges). Electric potential V = kQ/r (scalar). Potential energy U = kq₁q₂/r. Conductors: charges free to move. Insulators: charges fixed.
Current I = ΔQ/Δt (A). Conventional current: direction + charges would flow. Ohm\'s law: V = IR. Resistance R = ρL/A. Series: same I, V adds, R_total = ΣR. Parallel: same V, I adds, 1/R_total = Σ(1/R). Kirchhoff\'s junction rule: Σ I_in = Σ I_out. Loop rule: Σ ΔV = 0 around any closed loop. Power: P = IV = I²R = V²/R. Internal resistance: V_terminal = ε - Ir.
Strategy: (1) Identify series and parallel combinations. Simplify step by step to find total resistance. (2) Find total current from V = IR. (3) Work backwards to find current and voltage through each component. For circuits that cannot be simplified (e.g., Wheatstone bridge): use Kirchhoff\'s rules — assign current variables to each branch, write junction equations (Σ I = 0) and loop equations (Σ V = 0), then solve the system of equations. On the AP exam, circuits are usually simple enough for the simplify-and-expand method.
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