The Field of a Point Charge
The Field of a Point Charge
- Coulomb’s law gives the force between two charges. Electric field strength describes what one source charge does to the surrounding space.
Field strength outside a point charge or sphere
Symbols
- = field strength at the point (N C⁻¹)
- = charge producing the field (C)
- = distance from the charge or sphere centre (m)
Swipe left or right to see the whole diagram →
- Outside a charged sphere, field strength follows an inverse-square curve. Inside a conductor in electrostatic equilibrium, the field is zero.
- On an experimental graph, the point where the field first becomes non-zero gives the sphere radius.
- The field points away from a positive source and towards a negative source.
Worked example
Find the charge from an electric-field graph
A sphere has radius 0.032 m. Its surface field strength is 2.2 × 10⁵ N C⁻¹. Find its charge.
Answer
This graph method is from (9702/42/O/N/24 Q6(c)).
Your turnquick check
The distance from a point charge is doubled. By what factor does the electric field strength change?
Show worked answer
Field strength follows an inverse-square relation.
Answer