Further Polarization, electric fields and dielectric

Question 1

What happens to covalent bonds when they are placed in an electric field? Why?

Answer 1

> They become polarised and the electrons in the bond move in the opposite direction to the electric field.
This is because electrons are negatively charged and so move towards the positive point. Electric field is drawn from positive to negative.
The induced dipole moment is still drawn from negative to positive and so follows the electric field

Question 2

What is the symbol for covalent bond polarisation?

Answer 2

αe

Question 3

What is the symbol for ionic bond polarisation?

Answer 3

αi

Question 4

What happens in ionic bond polarisation?

Answer 4

The positive and negative ions move in opposite directions in accordance to the electric field (negative against, positive with)

Question 5

Does ionic bond polarisation produce a net dipole moment per ion pair?

Answer 5

Yes

[SEE NOTES FOR VISUAL REASON]

Question 6

What is orientational polarisation?

Answer 6

> In a non-regular structure like a liquid or gas, molecules with a permanent dipole moment may be aligned perpendicular or not with the direction of the electric field.
This causes each of the charges to have a force in opposite directions to each other. This can cause a rotation of the molecule
Similar to a moment (As in physical physics with forces)

Question 7

What happens with orientation polarisation when the temperature is increased?

Answer 7

This causes more of the molecules to rotate out of alignment and then experience forces because of the electric field

Question 8

What is the symbol for orientation polarisation?

Answer 8

αd

Question 9

Describe some of the values used in the equation:

α = 1/3 × (p0)^2 / kb×T

Answer 9

α = Polarisibilty
p0 = Existing dipole moment between atoms
kb = Constant
T = Temperature

Question 10

What is interfacial polarisation?

Answer 10

> When an electric field is placed over a dielectric, more mobile charges within a material are able to move more easily and so will more easily move.
They are unable to leave the dielectric so pile up on one of the faces making it polarised.

Question 11

When can else interfacial polarisation occur?

Answer 11

> When electrons are trapped within defects of the crystal

Question 12

How can the total polarisation be calculated with the local electric field?

Answer 12

P = αeE + αiE + αdE

Question 13

How can you tell if a crystal has a centre of symmetry?

Answer 13

If you draw a line from one ion to another and then another line in the opposite direction and you see the same ion then it has symmetry

Question 14

What are the properties of a crystal with a centre of symmetry?

Answer 14

> The centre of symmetry is all the centre of mass

> When compressed it produces no dipole moments and the centre of mass is still the same

Question 15

What are the properties of a crystal without a centre of symmetry?

Answer 15

> When compressed the centre of mass is in a different place than when it wasn’t
When compressed it produces a dipole moment and polarises the material

Question 16

Which direction is the polarisation of a crystal without a centre of symmetry after is has been compressed?

Answer 16

Not always in the direction of the compression

Question 17

For a crystal without a centre of symmetry how does the centre of mass relate to the direction of polarisation?

Answer 17

The direction of polarisation is from the centre of mass of the negative charges towards the centre of mass of the positive charges after the compression

Question 18

What is it called when compressing a non-symmetric crystal causes a polarisation?

Answer 18

The piezoelectric effect

Question 19

What is the equation for the piezoelectric effect for when a compressive force produces a polarisation? What are the symbols?

Answer 19

Pi=dij×Tj
Pi = Polarisation in the direction i
dij = Piezoelectric coefficients
Tj = Stress in direction j

Question 20

Describe the piezoelectric effect

Answer 20

> When a compressive force is applied to a crystal this produces a potential difference on the faces of the crystal. This can be measured.
When a voltage is applied to the faces of the crystal, this causes it to expand and contract

Question 21

What is equation for the piezoelectric effect for when a voltage causes the change in the shape of the crystal? What are the symbols?

Answer 21

Sj=dij×Ei
Sj = Strain in the direction j
dij = Piezoelectric coefficients
Ei = Electric field in the direction i

Question 22

What is a ferroelectric crystal?

Answer 22

> This is a crystal with a permanent finite polarisation because of the separation of charges in a crystal.

Question 23

What is the Curie temperature?

Answer 23

This a temperature at which below that temperature the crystal has a permanent polarization

Question 24

What is the pyroelectric effect?

Answer 24

When a Ferroelectric crystal is heated, the temperature increases and it expands so the relative distance between the ions changes and changes the polarization

Question 25

What are the symbols for the pyroelectric effect?

Answer 25

δT = Change in temperature
δP = Change in the pyroelectricity