Physical Science Matric Revision: Electrostatics and Coulomb’s Law

Revision Notes for CAPS Grade 12 Physical Science: Electricity and Magnetism – Electrostatics and Coulomb’s Law


Electrostatics is the study of electric charges at rest. This topic is fundamental in understanding how charges interact with each other and the principles governing these interactions. Coulomb’s Law specifically describes the force between two point charges. These concepts are crucial for understanding various phenomena in physics and for solving problems in electric fields and force calculations.

Key Points

  1. Coulomb’s Law:
  2. States that any two point charges exert a force on each other that is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.
  3. Formula: ( F = \dfrac{k Q_1 Q_2}{d^2} ), where:

    • ( F ) is the force between the charges (in Newtons, N).
    • ( Q_1 ) and ( Q_2 ) are the amounts of the charges (in Coulombs, C).
    • ( d ) is the distance between the centers of the two charges (in meters, m).
    • ( k ) is the Coulomb constant, ( 9 \times 10^9 \, \text{N·m}^2/\text{C}^2 ).
  4. Principles of Electrostatics:

  5. Action-Reaction for Electrostatic Forces: If ( Q_1 ) exerts a force on ( Q_2 ), then ( Q_2 ) exerts an equal and opposite force on ( Q_1 ).
  6. Charge Quantization: Charges in nature are discrete and typically measured in microcoulombs (μC, ( 10^{-6} ) C) or nanocoulombs (nC, ( 10^{-9} ) C).
  7. The net charge (( Q )) on an object is given by ( Q = ne ), where ( n ) is an integer and ( e ) is the elementary charge ( (-1.6 \times 10^{-19} \, \text{C}) ).

  8. Electric Fields:

  9. An electric field (( E )) is a region around a charged object where other charged objects experience a force.
  10. Formula: ( E = \dfrac{F}{q} ), where:
    • ( E ) is the electric field (N/C or V/m).
    • ( F ) is the force (N).
    • ( q ) is the test charge (C).

Real-World Applications

  • Charged Spheres: Consider a scenario where two spheres, one with -50 μC and another with +40 μC, are brought into contact and then separated by a specific distance.
  • Determine whether electrons were added or removed:
    • Electrons were added if the sphere is negatively charged.
  • Calculate the number of electrons added or removed: Using ( n = \dfrac{Q}{e} ).
  • Electric Force Calculation:
    • After contact, the charge on each sphere becomes ( Q = \dfrac{Q_1 + Q_2}{2} ).
    • Use Coulomb’s Law to find the force between the spheres.
  • Example: Calculate the force between two spheres with charges -5 μC and 5 μC separated by 15 cm:
    F = k\dfrac{Q^2}{d^2} = 9 \times 10^9 \dfrac{(5 \times 10^{-6})^2}{(0.15)^2} \approx 1.1 \, \text{N}

Common Misconceptions and Errors

  1. Incorrect Signs in Formulas:
  2. Misunderstanding Positive and Negative Charges: Coulomb’s Law formula only requires the magnitudes of the charges; signs are only necessary to determine the direction of the force.
  3. Forgetting to Square the Distance: Always square the distance between the charges when using Coulomb’s Law.

  4. Measurement Units:

  5. Ensure that all units are in the SI system (meters for distance, Coulombs for charge).

  6. Force Direction:

  7. Remember that like charges repel and unlike charges attract.

Practice and Review

Practice Questions

  1. Calculate the force between two point charges, ( 3 \times 10^{-6} \text{C} ) and ( 4 \times 10^{-6} \text{C} ), positioned 10 cm apart.
  2. Determine the electric field ( 20 \text{ cm} ) from a charge of ( 5 \times 10^{-6} \text{C} ).


  1. [
    F = k \dfrac{Q_1 Q_2}{d^2} = 9 \times 10^9 \dfrac{(3 \times 10^{-6})(4 \times 10^{-6})}{(0.1)^2} = 10.8 \, \text{N}

  2. [
    E = k \dfrac{Q}{d^2} = 9 \times 10^9 \dfrac{5 \times 10^{-6}}{(0.2)^2} = 1.125 \times 10^6 \, \text{N/C}

Examination Tips

  • Look for keywords such as “magnitude,” “electric field,” and “force.”
  • Always check your units.
  • Use diagrams to assist in visualizing forces and fields.

Connections and Extensions

  • Link to Other Topics: Explore how electrostatics principles apply to electric circuits and field mapping.
  • Interdisciplinary Links: Relate electrostatics to concepts in chemistry such as ionic bonds and electron transfer.


  • Coulomb’s Law states the relationship between force, charge, and distance.
  • Electric Fields describe the force experienced per unit charge in space around a charged object.
  • Accurate unit conversion and correct usage of formulas are critical in solving problems in electrostatics.

Additional Resources

  • Khan Academy: Online tutorials and videos about Coulomb’s Law and electric fields.
  • PhET Interactive Simulations: Explore the forces and interactions between multiple charges.

These notes should offer a comprehensive but accessible overview of electrostatics and Coulomb’s Law for Grade 12 material, ideal for both introductory study and exam preparation.

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