Practice question

**Equipotential surface** is surface where V constant, no work done moving charge along it because W = q ΔV =0 when ΔV=0. Electric field E always perpendicular to equipotential surface, direction from higher to lower potential, magnitude E = -dV/dr, steeper potential gradient means stronger field. U = (1/4 π ε₀) (q₁ q₂/r) = 9 × 10⁹ × (18 × 10⁻⁶ × (-6 × 10⁻⁶)/0.18) . U = 9 × 10⁹ × (-108 × 10⁻¹²/0.18) = -5.4 J . Using V = kQ/r, U = k q₁q₂/r, E = -dV/dr, C = ε₀A/d, 1/C_eq series, C_eq = ΣC parallel, U = ½ C V² and