The uniform electric field

Overview

Recall questions on electric field strength E = F/q and E = V/d between parallel plates, V m⁻¹ and N C⁻¹, field, force and acceleration calculations for an electron, the field's direction and pattern, changing the plate separation, work done W = qV and kinetic energy gained, parabolic beam paths, and deflection with doubled voltage.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define electric field strength.

   Model answer

   Electric field strength is the force per unit positive charge acting at a point, E = F/q, and it is a vector in the direction of the force on a positive charge.

2. 2

   Write the equation for the field strength between two parallel plates.

   Model answer

   For a uniform field between parallel plates E = V/d, where V is the potential difference and d the separation.

3. 3

   Show that the volt per metre is equivalent to the newton per coulomb.

   Model answer

   One volt is one joule per coulomb, so V m⁻¹ is J C⁻¹ m⁻¹, and since a joule is a newton metre this becomes N C⁻¹.

4. 4

   Two parallel plates 5.0 mm apart have a potential difference of 250 V across them. Calculate the field strength between them.

   Model answer

   E = V/d = 250 ÷ 5.0 × 10⁻³ = 5.0 × 10⁴ V m⁻¹.

5. 5

   Calculate the force on an electron in a uniform field of strength 5.0 × 10⁴ V m⁻¹.

   Model answer

   F = qE = 1.60 × 10⁻¹⁹ × 5.0 × 10⁴ = 8.0 × 10⁻¹⁵ N.

6. 6

   In which direction does the electric field point between two charged parallel plates?

   Model answer

   The field points from the positive plate towards the negative plate, which is the direction of the force on a positive charge.

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