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Topic Notes · Easy summaries with diagrams
Chapter 9 — Chapter 9 — Electrostatics and Current Electricity
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Topic 1 of 8
📘Easy summary
Coulomb's law: the force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them: F = kq₁q₂/r², k = 1/4πε₀ = 9 × 10⁹ N m² C⁻², ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻². The force acts along the line joining the charges; like charges repel, unlike attract. Charge is quantized: q = Ne, e = 1.6 × 10⁻¹⁹ C. In a medium F is reduced by the relative permittivity ε_r.
⭐Key points
- F ∝ q₁q₂/r².
- k = 9 × 10⁹ N m² C⁻².
- q = Ne.
🔬Diagram
🌿Real-life example
0.4 N at 5 cm → 1.6 N at 2.5 cm.
💡Exam tip
- Electric force ≈ 10³⁹ × gravitational force in hydrogen.
✅Quick check
1. Distance reduced from 1 m to 0.5 m: force
2. Possible charge:
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