Electrostatics
This domain covers electric charge, field, potential, Gauss’s law and capacitance. PSEB SS Physics questions often combine a vector-direction judgement with a formula substitution, so write the direction explicitly.
Core rules and concepts
- Coulomb’s law: F = kq1q2/r², with k = 9×10⁹ N m² C⁻² in vacuum; force is along the line joining charges and obeys superposition.
- Electric field E = F/q0; field lines start on positive charge and end on negative charge. Potential V = W/q0; E = −dV/dr.
- Gauss’s law: Φ = q_enclosed/ε0. For a charged conductor in electrostatic equilibrium, E inside is zero and charge resides on the outer surface.
- Capacitance C = Q/V. Parallel plate: C = ε0A/d, and with dielectric C = Kε0A/d. Series: 1/C = Σ1/Ci; parallel: C = ΣCi.
- Energy stored U = ½CV² = Q²/2C.
Worked example
Point charges +4 μC and −4 μC are 20 cm apart. Find E at the midpoint. Each charge is 0.10 m away. Magnitude from either charge: E = (9×10⁹)(4×10⁻⁶)/(0.10)² = 3.6×10⁶ N C⁻¹. The field from +4 μC points toward the negative charge; the field from −4 μC also points toward the negative charge. They add: E_total = 7.2×10⁶ N C⁻¹ toward the −4 μC charge.
Traps
- Do not add fields as scalars unless they are collinear; resolve perpendicular components.
- A dielectric reduces E inside the capacitor but increases C; the battery may keep V constant or be disconnected, changing which quantity is fixed.
- In series capacitors the charge is the same; in parallel the voltage is the same.