The distance between charges +q and -q is 2l and between +2q and -2q is 4l. The electrostatic potential at point P at a distance r from centre O is -α[ql/r²] × 10⁹ V, where the value of α i $s ______.$ (Use 1/(4πε₀) = 9 × 10⁹ N m² C⁻²)

Question

G1 Physics · Accepted Answer

Image Dipole moment of (+q, -q) pair: p₁ = 2ql Dipole moment of (+2q, -2q) pair: p₂ = 8ql Their resultant dipole moment is: pₙₑₜ = 6ql For a point at angle 120° from p⃗ₙₑₜ: V = k(p⃗ · r⃗)/r³ = k pₙₑₜ cos120°/r² = (9 × 10⁹)(6ql)(-1/2)/r² = -27(ql/r²) × 10⁹ V Therefore, α = 27

The distance between charges +q and -q is 2l and between +2q and -2q is 4l. The electrostatic potential at point P at a distance r from centre O is -α[ql/r²] × 10⁹ V, where the value of α i s ______. (Use 1/(4πε₀) = 9 × 10⁹ N m² C⁻²)