Coulomb's Law
Coulomb's Law
- Coulomb’s law gives the force between two charged objects when they can be treated as point charges.
The two-mark statement
Definition
2 marksWhat is meant by Coulomb's law?
Model answer: The electric force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation.
The product of the charges and the square of the separation are the two mark points (9702/41/M/J/25 Q2(a)) (9702/42/O/N/24 Q6(a)).
Symbols
- = magnitude of the force on each charge (N)
- = the two point charges (C)
- = permittivity of free space (F m⁻¹)
- = separation between the charges (m)
- Use the magnitudes of the charges to calculate the force magnitude. Use the signs to decide whether the force is attraction or repulsion.
- Like charges repel. Unlike charges attract.
- A uniformly charged sphere acts like a point charge at its centre for every point outside the sphere. Measure centre to centre, not surface to surface.
Common mistake
Doubling the separation makes the force one quarter as large, not one half as large.
Your turnquick check
The two charges stay fixed while their separation is tripled. By what factor does the force magnitude change?
Show worked answer
The inverse-square factor is the square of three.
Answer