Coulomb's Law

Overview

This lesson develops the inverse-square force model for electric charges. You will use Coulomb’s law for point charges, decide force direction from the signs of the charges, and compare the model with Newton’s law of gravitation.

What You Need to Know

• For a point outside a spherical conductor, the charge can be treated as if it were concentrated at the centre.
• Coulomb’s law gives the force between two point charges in free space.
• The force follows an inverse-square relationship with separation.
• Like charges repel; unlike charges attract.
• Electric force can be attractive or repulsive, unlike gravitational force between masses.
• The separation r is measured between the centres of the point charges or spherical conductors.

How to Work Through It

1. Start by recalling Newton’s law of gravitation and identifying the matching electric quantities.
2. Practise Coulomb’s law calculations with charge in coulombs and distance in metres.
3. Add force directions using the signs of the two charges.
4. Compare electric and gravitational fields in a short written explanation.

Check Your Understanding

• What happens to the electric force if the separation is doubled?
• When can a charged sphere be treated as a point charge?
• How do you decide whether the force is attractive or repulsive?
• What is similar and what is different between Coulomb’s law and Newton’s law of gravitation?

Common Mistakes

• Using microcoulombs or nanocoulombs directly without converting to coulombs.
• Measuring r from the surface of a sphere instead of its centre.
• Forgetting that the equation gives the magnitude of force; direction comes from the charge signs.
• Assuming all inverse-square fields behave exactly like gravity.

Next Steps

• Practise one full Coulomb’s law calculation with unit conversions.
• Use Coulomb’s law to build the point-charge field strength equation next.