The uniform field between capacitor plates

Overview

Recall questions on The uniform field between capacitor plates.

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

   State two equivalent units for electric field strength and show that they are the same.

   Model answer

   Field strength may be given in V m⁻¹ or in N C⁻¹. A volt is a joule per coulomb and a joule is a newton metre, so V m⁻¹ becomes N m C⁻¹ m⁻¹ and the metres cancel to leave N C⁻¹.

2. 2

   Why is the field near the edges of a pair of parallel plates not uniform?

   Model answer

   The plates are of finite size, so at the edges the field lines bulge outwards and are no longer straight, parallel and equally spaced.

3. 3

   Calculate the electric field strength between two plates 5.0 mm apart with 250 V across them.

   Model answer

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

4. 4

   State the equation for the force on a charge in a uniform electric field, defining every symbol with its unit.

   Model answer

   The equation is F = EQ, where F is the force in newtons, E is the field strength in V m⁻¹ and Q is the charge in coulombs.

5. 5

   An electron is placed in a uniform field of strength 5.0 × 10⁴ V m⁻¹. Calculate the force on it.

   Model answer

   The force is 5.0 × 10⁴ × 1.60 × 10⁻¹⁹ = 8.0 × 10⁻¹⁵ N.

6. 6

   In which direction does the force act on a negative charge placed between charged parallel plates?

   Model answer

   It acts in the direction opposite to the field, so the negative charge is pushed towards the positive plate.

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