Electric field

Beginner

Force per unit positive charge.

Key relationE = F/q

The field E = F/q is the force per unit positive test charge. It exists in space around charges and acts on any charge placed there: F = qE.

Worked example

A 10⁻⁶ C charge in a 5000 N/C field feels 5 mN.

Common mistake

The test charge must be small to avoid disturbing the field.

In more detail

An electric field is the force per unit positive charge at a point, E = F/q. It describes the influence a charge distribution has on space around it.

Electric fields in cathode ray tubes steer electron beams.
Capacitors store energy in electric fields between plates.
Electrostatic precipitators in chimneys use electric fields to push charged dust particles onto collecting plates.

What are the units?

Newtons per coulomb, or equivalently volts per metre.

Does a test charge affect the field?

Only if it is large enough to disturb the source charges, which we assume is not the case.

Is the electric field a force?

The field is the force per unit charge, so a test charge placed in it feels a force equal to the charge times the field.