Further Polarization, electric fields and dielectric Flashcards
What happens to covalent bonds when they are placed in an electric field? Why?
> 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
What is the symbol for covalent bond polarisation?
αe
What is the symbol for ionic bond polarisation?
αi
What happens in ionic bond polarisation?
The positive and negative ions move in opposite directions in accordance to the electric field (negative against, positive with)
Does ionic bond polarisation produce a net dipole moment per ion pair?
Yes
[SEE NOTES FOR VISUAL REASON]
What is orientational polarisation?
> 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)
What happens with orientation polarisation when the temperature is increased?
This causes more of the molecules to rotate out of alignment and then experience forces because of the electric field
What is the symbol for orientation polarisation?
αd
Describe some of the values used in the equation:
α = 1/3 × (p0)^2 / kb×T
α = Polarisibilty p0 = Existing dipole moment between atoms kb = Constant T = Temperature
What is interfacial polarisation?
> 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.
When can else interfacial polarisation occur?
> When electrons are trapped within defects of the crystal
How can the total polarisation be calculated with the local electric field?
P = αeE + αiE + αdE
How can you tell if a crystal has a centre of symmetry?
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
What are the properties of a crystal with a centre of symmetry?
> 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
What are the properties of a crystal without a centre of symmetry?
> 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
Which direction is the polarisation of a crystal without a centre of symmetry after is has been compressed?
Not always in the direction of the compression
For a crystal without a centre of symmetry how does the centre of mass relate to the direction of polarisation?
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
What is it called when compressing a non-symmetric crystal causes a polarisation?
The piezoelectric effect
What is the equation for the piezoelectric effect for when a compressive force produces a polarisation? What are the symbols?
Pi=dij×Tj
Pi = Polarisation in the direction i
dij = Piezoelectric coefficients
Tj = Stress in direction j
Describe the piezoelectric effect
> 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
What is equation for the piezoelectric effect for when a voltage causes the change in the shape of the crystal? What are the symbols?
Sj=dij×Ei
Sj = Strain in the direction j
dij = Piezoelectric coefficients
Ei = Electric field in the direction i
What is a ferroelectric crystal?
> This is a crystal with a permanent finite polarisation because of the separation of charges in a crystal.
What is the Curie temperature?
This a temperature at which below that temperature the crystal has a permanent polarization
What is the pyroelectric effect?
When a Ferroelectric crystal is heated, the temperature increases and it expands so the relative distance between the ions changes and changes the polarization
What are the symbols for the pyroelectric effect?
δT = Change in temperature δP = Change in the pyroelectricity
