Dielectrics and relative permittivity

Overview

Recall questions on Dielectrics and relative permittivity.

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

   What is a dielectric?

   Model answer

   A dielectric is an insulating material placed in the space between the plates of a capacitor.

2. 2

   Define the relative permittivity of a dielectric.

   Model answer

   It is the ratio of the capacitance of a capacitor with the material completely filling the space between its plates to the capacitance of the same capacitor with a vacuum between the plates.

3. 3

   State the equation for the capacitance of a parallel plate capacitor filled with a dielectric, and define every symbol with its unit.

   Model answer

   The equation is C = ε₀εᵣA/d, where C is the capacitance in farads, ε₀ is the permittivity of free space in F m⁻¹, εᵣ is the relative permittivity of the dielectric, A is the plate area in m² and d is the plate separation in metres.

4. 4

   Explain in terms of molecules why a dielectric increases the capacitance.

   Model answer

   The field polarises the molecules of the dielectric, so bound charges appear on its surfaces and set up a field opposing the applied one, and the reduced resultant field means a smaller potential difference for the same stored charge, so Q/V is larger.

5. 5

   State the value and unit of the permittivity of free space.

   Model answer

   The permittivity of free space is 8.85 × 10⁻¹² F m⁻¹.

6. 6

   A capacitor has plates of area 0.020 m² separated by 0.10 mm of a dielectric of relative permittivity 4.0. Calculate its capacitance, taking ε₀ as 8.85 × 10⁻¹² F m⁻¹.

   Model answer

   The capacitance is 8.85 × 10⁻¹² × 4.0 × 0.020 ÷ 1.0 × 10⁻⁴ = 7.1 × 10⁻⁹ F. In more usual units that is 7.1 nF.

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