# Dielectrics Flashcards

1
Q

What is a dielectric?

A

> It is an insulator

> Changes the electric field passing through it by becoming polarised

2
Q

How are dielectric used?

A

> Capacitors are designed to work at different frequencies and the dielectric chosen is important to enable this.
They change the capacitance of a capacitor
Increase the electrical breakdown field between high-voltage conductors:

3
Q

What is the relative permittivity?

A

This is a constant for a material and is dependent on the frequency of the changing electric field

4
Q

What is the equation for the capacitance of a capacitor?

A

Capacitance = Charge stored / Volt

5
Q

How is the relative permittivity of a dielectric calculated?

A

Calculating the capacitance without the dielectric: C0 = Q0/V
Calculating the capacitance with the dielectric: C = Q/V
To calculate the relative permittivity you compare the two capacitances.
εr = Q/Q0 = C/C0

6
Q

What happens when a dielectric is placed in an electric field?

A

It is an insulator so the charges do not flow within the material but they do shift a bit from their equilibrium position causing the material to become polarised.

7
Q

What is polarisation?

A

> Occurs in a medium when the positive and negative charges are displaced relative from their equilibrium position
The opposite surface of a dielectric acquires the opposite surface charge densities that are related to the polarization in the medium

8
Q

What is the electric dipole moment?

A

It is kind of like the magnitude of the polarization

9
Q

What is the equation for the electric dipole moment?

A

P = Qa
Where a is the separation between the positive and negative charges in the dielectric.
The dipole moment and separation are both vectors with a direction: P ⃗ = Qa ⃗

10
Q

How is the distance ‘a’ and dipole moment ‘p’ defined for the polarisation of a material?

A

From NEGATIVE TO POSITIVE

11
Q

Even though the charges are separated by distance ‘a’ the overall net charge is….
but unless….

A

….zero

….the charges are at the same point then there will be a dipole moment

12
Q

How is this movement of charge causing polarisation applied to a dielectric material atomically?

A

For an atom with a positive nucleus and circular cloud of electrons the overall point of positive and negative charge are at the same point.
When placed into a magnetic field the centre point of the negative electron cloud moves from the centre point of the positive nucleus. The distance moved is ‘a’

13
Q

How can an unpolarised material become polarised?

A

When placed into a magnetic field

14
Q

How can the induced polarisation be calculated? What is the equation? What is the constant?

A
```Pinduced = αE
α = Polarizability of the material```
15
Q

For neutral atoms what is the polarizibility constant?

A

αe = Electronic polarization (for electrons)

16
Q

What happens when a dielectric is placed within a capacitor?

A

The dielectric becomes polarized and the capacitance changes

17
Q

What is going on within the dielectric in a capacitor?

A

> Inside the centre, the charges cancel out and so the inside of the dielectric is effectively neutral
The edges of the dielectric becomes charged because it does not cancel (surface polarization)

18
Q

When a battery is connected to the capacitor with the dielectric in it, how does the polarisation change and which direction is the electric field and which direction is ‘a’ ?

A

> One plate on the capacitor is positive
One plate on the capacitor is negative
The electric field is from positive to negative
The polarisation is such that on the surface the negative side is by the positive plate and positive side is by the negative plate
a is in the same direction as the electric field

19
Q

What are the two symbols for dipole moment and polarisation?

A
```p = dipole moment
P = polarisation```
20
Q

How are dipole moment and polarization related?

A

Polarisation = Sum of dipole moments / Unit of volume

21
Q

What are the equations to calculate the polarization from dipole moments?

A

P = (1/Volume)×(p1+p2+p3+…+pN)
P = n×Pavg
Where n = Number of charges per volume

22
Q

What is surface charge density?

A

This is the number of charges per unit of area

23
Q

What is the equation for surface charge density?

A

σ = Q/A

surface charge density = surface charge / area

24
Q

How can the charge of the whole dielectric be modelled? What is the equation for this?

A

> As one large dipole moment
ptotal = Qd
Total dipole moment = surface charge × length of dielectric

25
Q

If we know the overall dipole moment how can we then calculate the polarizibility?

A
```P = ptotal/volume
P = Q×d/Volume```
26
Q

What can be related between surface charge density and polarizability? What is the equation for this?

A

They are the same

> P = Q×d/Volume = Q×d/A×d = Q/A = σ

27
Q

What differentiates polarization and surface charge density?

A

polarization is a vector and is perpendicular to the surface charge

28
Q

What is electric susceptibility? What is its symbol?

A

> It is a proportional constant relating electric field strength to Polarization
Symbol: Xe

29
Q

What is the equation for electric susceptibility?

A

Xe = nα/ε0

30
Q

What are the equations used to calculate the polarizability using the electric susceptibility constant?

A

P = Xe × ε0 × E

31
Q

What is the Clausius-Mossotti equation?

A

This relates macroscopic to microscopic
εr-1 / εr+2 = nα / 3ε0
[YOU DONT NEED TO REMEMBER THIS BUT NEED TO KNOW HOW TO USE IT]