Lecture 1 - Electric materials

Question 1

What are electric dipoles and which 2 types exist?

Answer 1

In general, positive and negative charges in a material can be thought of as being arranged into pairs called dipoles. They are generally arranged at random and their electric field cancels out.

1) Permanent dipoles
2) Dipoles that form under the influence of external fields

Question 2

What are dielectric materials?

Answer 2

Materials whose dipoles can be polarised, i.e., an applied electric field causes the dipoles to align under its influence.

Question 3

What are polar dielectrics?

Answer 3

Materials whose dipoles are permanently present but disordered.

Question 4

What are paraelectric materials?

Answer 4

Dielectric materials that exhibit a large response to an external electric field.

Ferroelectric materials are usually paraelectric above their transition temperature.

Question 5

What is the net field occuring from the application of an electric field on a dielectric?

Answer 5

E = E_applied -E_induced

The induced field opposes the applied field

Question 6

What are ferroelectric materials?

Answer 6

In ferroelectric materials, the charges are spontaneously aligned, typically exhibiting an electric field even in the absence of an external field.

Question 7

What is the effect of a dielectric in a capacitor?

Answer 7

A dielectric inside a capacitor increases capacitance. It has the effect of reducing the amount of voltage required to store a certain amount of charge.

This is because energy in the capacitor is now stored in both in the electric field in free space and in the ordering of the dipoles in the material between the plates.

Question 8

What are the two contributions to the total electric field?

Answer 8

1) Charges introduced into a capacitor through its leads

2) Charges associated with the material between the plates (Polarisation/Polarisation Density)

Question 9

What is Polarisation / Polarisation Density?

What is the polarisation equation?

Answer 9

Charges associated with the material between the plates.

Has units of Coulomb per square metre.

𝐏 = ε_0χ𝐄

Only applies for linear, homogenous, isotropic materials.

Where χ the electric susceptibility and 1+χ = ε_r

Question 10

What is the Displacement Field (D)?

What relationships does it satisfy?

Answer 10

The displacement field represents the “free” charges flowing through a capacitor. (as opposed to the charges bound by the dielectric)

∇.𝐃 = ρ_f
𝐃 = ε_0*𝐄 + 𝐏

Question 11

What is the electric susceptibility of a material?

Answer 11

The electric susceptibility χ is a material’s property which describes the material’s electric response.
1+χ = ε_r