There are two types of electric charge: positive and negative. Like charges repel; opposite charges attract. Charge is quantised – it comes in multiples of the elementary charge e = 1.6×10⁻¹⁹ C. Charge is also conserved: the total charge in an isolated system never changes.
The force between two point charges is: F = kq₁q₂/r², where k = 9×10⁹ N·m²/C². The force is along the line joining the charges. It is attractive for opposite charges and repulsive for like charges. Doubling the distance reduces the force by a factor of 4.
When multiple charges are present, the net force on any charge is the vector sum of individual Coulomb forces. Break into x and y components, sum separately, then combine.
💡F = kq₁q₂/r². Charge is quantised and conserved. Use vector superposition for multiple charges.
📋 Key Formulas
📝 Worked Example 1
Q: Two charges +3µC and −5µC are 0.2 m apart. Find the force.
A: F = 9×10⁹ × 3×10⁻⁶ × 5×10⁻⁶ / (0.2)² = 3.375 N (attractive).
📝 Worked Example 2
Q: Three charges in a line: +2µC at origin, −4µC at x=0.1 m, +1µC at x=0.3 m. Find net force on the middle charge.
A: F₁₂ = k(2)(4)×10⁻¹²/(0.1)² = 7.2 N left. F₃₂ = k(4)(1)×10⁻¹²/(0.2)² = 0.9 N right. Net = 6.3 N left.
📝 Worked Example 3
Q: How many excess electrons give a charge of −3.2×10⁻¹⁸ C?
A: n = Q/e = 3.2×10⁻¹⁸/1.6×10⁻¹⁹ = 2 electrons.
🧠Always convert µC to C.
🧠Check sign: opposite charges → attractive.
🧠Use components for 2D problems.
⚠️Forgetting to square the distance.
⚠️Not converting micro-coulombs to coulombs.
⚠️Ignoring vector directions.
🎯 Try This Yourself
Open and read all sections to complete this module