Chapter 18: Electric Charge

Coulomb's Law

A law stating that the magnitude of the electrostatic force between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Electrostatic Force (F)

The force of attraction or repulsion between two charged objects.

Point Charge

An idealized model of a charged object where the entire charge is concentrated at a single point.

Coulomb's Law Formula

Formula

$F=\frac{k{q}_1{q}_2}{r^2}$

Variables

• F: Electrostatic force (N).
• k: Coulomb's constant (≈ 8.99 × 10⁹ N⋅m²/C²).
• q₁, q₂: Charges (C).
• r: Distance between charges (m).

Properties of Electrostatic Force

• The force is attractive if charges are opposite and repulsive if charges are the same.
• The force acts along the line joining the two charges.
• The force decreases rapidly with increasing distance (inverse square law).

Example 18-1: Using Coulomb's Law

Two charges, q₁ = 2.0 × 10^-6 C and q₂ = -3.0 × 10^-6 C, are separated by a distance of 0.05 m. Calculate the electrostatic force between them.

• Given: q₁ = 2.0 × 10^-6 C, q₂ = -3.0 × 10^-6 C, r = 0.05 m, k = 8.99 × 10⁹ N⋅m²/C².
• F = k × q₁ × q₂ / r².
• F = (8.99 × 10⁹) × (2.0 × 10^-6) × (3.0 × 10^-6) / (0.05)².
• F = 21.6 N (attractive force).

Questions for Students

1. State Coulomb's Law and explain its significance in electrostatics.
2. What factors affect the magnitude of the electrostatic force between two charges?
3. Describe the relationship between distance and electrostatic force.
4. Calculate the electrostatic force between two charges, q₁ = 5.0 × 10^-6 C and q₂ = 1.0 × 10^-6 C, separated by 0.1 m.
5. Explain how the electrostatic force changes if the distance between two charges is doubled.