Practice question

**Potential due to point charge** V = (1/4π ε₀)Q/r, positive Q gives positive V, negative gives negative. At r=0.3 m from Q=3×10⁻⁸ C, V=9×10⁹×3×10⁻⁸/0.3=900 V. Work done moving charge q from infinity (V=0) to point where V=40 V is W = q V =9×10⁻⁶×40=3.6×10⁻⁴ J. Distance to midpoint = 0.06 m. V = 9 × 10⁹ ( (8 × 10⁻⁶/0.06) + (-4 × 10⁻⁶/0.06) ) = 9 × 10⁹ × (4 × 10⁻⁶/0.06) . V = 9 × 10⁹ × (4 × 10⁻⁶/0.06) = 6 × 10⁵ V . Using V = kQ/r, U = k q₁q₂/r, E = -dV/dr, C = ε₀A/d, 1/C_eq series,