The uniform electric field between two plates

What is a uniform field?

An electric field is said to be uniform when the vector E has the same direction, the same sense and the same magnitude at every point in the region under consideration. Unlike the field created by a point charge (which varies as 1/r²), a uniform field does not depend on the position within the region under consideration.

The parallel-plate capacitor

The standard device for producing a uniform field is the parallel-plate capacitor: two parallel metal plates, separated by a distance d, connected to a power source that applies a voltage U between them.

Between the plates (away from the edges), the field is uniform:

E = U / d

where E is in V/m, U in volts (V) and d in metres (m). The field points from the + plate towards the – plate, perpendicular to the plates.

Example

Two plates separated by a distance d = 0.02 m, subjected to a voltage U = 400 V:

E = 400 / 0.02 = 20,000 V/m

Field lines

Between the plates, the field lines are parallel straight lines, perpendicular to the plates and equidistant from them: this is the characteristic graphical representation of a uniform field.

Field	Field lines	Norm
Point charge	radial	varies as 1/r²
Uniform (capacitor)	parallel	constant

Common pitfall

Be careful with units: d must be expressed in metres (not cm) to use E = U/d directly. Forgetting to convert is the most common mistake.