PHY1121 Electrostatics – Answer Sheet
Model Answer Sheet for Questions 1–32
Q1. Electric charge is a physical property responsible for electric forces. Properties:
conservation and quantization.
Q2. Charge is quantized: q = ne, where n is an integer and e = 1.6×10⁻¹⁹ C.
Q3. Total charge of an isolated system remains constant. Example: rubbing glass and silk
transfers charge, not creates it.
Q4. Conduction: charge transfer by contact. Induction: charge redistribution without direct
contact.
Q5. Electrostatics studies charges at rest. Fundamental law: Coulomb’s law.
Q6. Electrons move from glass to silk. No charge is created; total charge is conserved.
Q7. Moisture makes air conductive, allowing charge leakage.
Q8. Coulomb’s law: F = k|q₁q₂|/r² along the line joining charges.
Q9. Net force equals the vector sum of individual forces.
Q10. Photocopiers and electrostatic precipitators.
Q11. Flame ionizes air, providing a conducting path for charge to leak away.
Q12. Bring charged rod near, charges redistribute, ground object, remove ground, then
remove rod.
Q13. Charge conservation, quantization, Coulomb interaction, and superposition.
Q14. Since charge exists in integral multiples of e, q = ne.
Q15. Doubling distance makes force one-fourth. Tripling one charge triples force.
Q16. Gauss’s law exploits symmetry, avoiding difficult integrations.
Q17. For equilibrium with equal distances: q₁ = q₃ (same magnitude).
Q18. Resolve forces from q₂ and q₃ on q₁ into components and add vectorially.
Q19. F = (1/4πε₀)(q₀q)/(y√(y²+L²/4)); for y≫L it approaches Coulomb’s law.
Q20. F = kqq₀/[a(a+l)]; for l≪a, F ≈ kqq₀/a².
Q21. Fz = (1/4πε₀)(q₀qz)/(z²+R²)^(3/2); F→0 at z=0.
Q22. Net force acts along the cube diagonal toward the missing corner.
Q23. Maximum force occurs at R = a/√2.
Q24. ∮E·dA = qenc/ε₀. Net electric flux equals enclosed charge divided by ε₀.
Q25. Flux entering equals flux leaving; therefore net flux is zero.
Q26. Using a spherical Gaussian surface: E = (1/4πε₀)q/r².
Q27. Using a cylindrical Gaussian surface: E = λ/(2πε₀r).
Q28. For a conducting plane: E = σ/ε₀. No flux through curved side because E is parallel to it.
Q29. Positive enclosed charge gives outward flux; negative gives inward flux. Zero net
enclosed charge gives zero net flux.
Q30. Induced opposite charge appears on the near side; attraction occurs. After contact,
charge redistributes and spheres repel.
Q31. At midpoint forces add in same direction. On perpendicular bisector, use vector
superposition of both charges.
Q32. Coulomb’s law is general but often requires integration; Gauss’s law is simpler for
highly symmetric distributions.