Coulomb's Law

1. Definition

• Coulomb's Law states that the electrostatic force between two charged objects is:
  • Directly proportional to the product of their charges.
  • Inversely proportional to the square of the distance between their centres.
• The force can be attractive (opposite charges) or repulsive (like charges).

2. Formula for Electrostatic Force

$F = k \frac{Q_1 Q_2}{r^2}$

where:

• $F =$ Electrostatic force $(N)$
• $k =$ Coulomb's constant $(:highlight[9.0 \times 10^9] N·m²/C²)$
• $Q_1, Q_2 =$ Charges of the two objects $(C)$
• $r =$ Distance between the charges $(m)$

3. Understanding Coulomb's Law

✔ If charge increases, force increases (doubling charge doubles the force).

✔ If distance increases, force decreases (doubling distance makes force four times weaker).

✔ Force follows an inverse square law, like Newton's Law of Gravitation.

4. Using Coulomb's Law

5. Key Takeaways

✔ Like charges repel, opposite charges attract.

✔ Force increases with charge, decreases with distance.

✔ Electrostatic force follows an inverse square law.

✔ Always convert distances to metres when using the formula.

Exam Tip

💡 When solving problems, remember to square the distance in the denominator!