Electrostatics – Stage 1
Chapter 1: Electric Charges & Fields – Page 1
1. Why Coulomb’s Law Is Important
Coulomb’s Law is the Newton’s Law of Electrostatics. It gives the exact mathematical expression for the force between two charges.
JEE Fact: Almost every electrostatics problem starts or ends with Coulomb’s Law.
2. Statement of Coulomb’s Law
The electrostatic force between two stationary point charges is:
- Directly proportional to the product of charges
- Inversely proportional to the square of the distance between them
- Acts along the line joining the charges
3. Mathematical Form of Coulomb’s Law
F = k · |q₁q₂| / r²
Where:
- F = electrostatic force (newton)
- q₁, q₂ = charges (coulomb)
- r = distance between charges (meter)
- k = 1 / (4πε₀)
k = 9 × 10⁹ N·m²/C² (in vacuum or air)
4. Nature of Force (Attractive or Repulsive)
- Like charges → Repulsive force
- Unlike charges → Attractive force
Direction of force is always along the line joining the charges.
5. Vector Form of Coulomb’s Law (JEE IMPORTANT)
Force on charge q₁ due to q₂:
𝐅₁₂ = (1 / 4πε₀) · (q₁q₂ / r²) · r̂
Here r̂ is the unit vector from q₂ to q₁.
JEE Tip: Direction mistakes cause maximum negative marks.
6. Medium Dependence (Dielectric Constant)
In a medium:
F = (1 / 4πε₀εᵣ) · (q₁q₂ / r²)
- εᵣ = dielectric constant of medium
- Force decreases inside a medium
7. Comparison with Gravitational Force
| Electrostatic Force | Gravitational Force |
|---|---|
| Can be attractive or repulsive | Always attractive |
| Very strong | Very weak |
| Depends on medium | Independent of medium |
Stage 1 – Page 1 Takeaway
- Coulomb’s Law gives force between charges
- Force ∝ q₁q₂ and ∝ 1/r²
- Vector direction is crucial
- Medium reduces force
Next → Stage 1, Page 2: Superposition Principle
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