Uniform electric field: (a) uniform electric field strength; E = V/d

The idea

A uniform electric field exists between two parallel conducting plates connected to a potential difference V. The field strength E is defined as the force per unit positive charge, and its magnitude is given by E = V/d, where d is the plate separation. The field is directed from the positive plate to the negative plate. For example, if V = 200 V and d = 0.050 m, then E = 200 / 0.050 = 4000 V m⁻¹. The unit V m⁻¹ is equivalent to N C⁻¹. The equation applies only when the field is uniform, which is a good approximation away from the edges of the plates. In this topic you must be able to use E = V/d to calculate field strength, potential difference or separation, and to describe the direction of the field.

What earns the marks

• State that electric field strength is force per unit positive charge.
• Use E = V/d to calculate E, V or d correctly.
• Recognise that the field between parallel plates is uniform except near the edges.
• State that the field direction is from the positive plate to the negative plate.
• Use the unit V m⁻¹ or N C⁻¹ for electric field strength.

Common mistakes and how to put them right

• Confusing electric field strength with electric potential.

  Put it right: Field strength is force per unit charge, while potential is work done per unit charge.

• Using E = V/d when the field is not uniform.

  Put it right: This equation applies only to a uniform field, such as between parallel plates away from the edges.

• Forgetting that d must be in metres.

  Put it right: Convert all lengths to metres before calculation.

• Reversing the field direction.

  Put it right: The field points from the positive plate to the negative plate.

Exam tips

• Always convert distances to metres before using E = V/d.
• Check that the calculated field direction matches the plate polarity.
• Remember that V m⁻¹ and N C⁻¹ are equivalent units for electric field strength.