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Lecture 1. Introduction into the physics of dielectrics. ii. Electric dipole - definition. a) Permanent dipole moment, b) Induced dipole moment. iii. Polarization and dielectric constant. iv. Types of polarization a) electron polarization, b) atomic polarization,

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lecture 1
Lecture 1
  • Introduction into the physics of dielectrics.
  • ii. Electric dipole - definition.
  • a) Permanent dipole moment,
  • b) Induced dipole moment.
  • iii. Polarization and dielectric constant.
  • iv. Types of polarization
  • a) electron polarization,
  • b) atomic polarization,
  • c) orientation polarization,
  • d) ionic polarization.
slide2

Ancient times

1745 first condensor constructed by Cunaeus and Musschenbroek

And is known under name of Leyden jar

1837 Faraday studied the insulation material,which he called the dielectric

Middle of 1860s Maxwell’s unified theory of electromagnetic phenomena

 = n2

1847 Mossotti

1887 Hertz

1879 Clausius

1897 Drude

Lorentz-Lorentz

1912 Debye

Internal field

Dipole moment

the dynamic range of dielectric spectroscopy

The dynamic range of Dielectric Spectroscopy

Broadband Dielectric Spectroscopy

Time Domain Dielectric Spectroscopy

10-6

10-4

10-2

100

102

104

106

108

1010

1012

f (Hz)

Macromolecules

  • Glass forming
  • liquids
  • Porous materials
  • and colloids

Clusters

Single droplets

and pores

Water

ice

Dielectric spectroscopy is sensitive to relaxation processes

in an extremely wide range of characteristic times ( 10 5 - 10 -12 s)

slide4

P -

N+

Lipids

Head group

region

Dielectric response in biological systems

Dielectric spectroscopy is sensitive to relaxation processes

in an extremely wide range of characteristic times ( 10 5 - 10 -11 s)

Broadband Dielectric Spectroscopy

Time Domain Dielectric Spectroscopy

0

101

102

103

104

105

106

107

108

109

1010

1011

1012

1013

10-1

1014

ice

H

H3N+ — C — COO-

R

f (Hz)

Proteins

Cells

Water

Ala AspArg Asn

Cys Glu Gln His

Ile Leu Lys Met

Phe Ser Thr Trp

Tyr Val

Amino acids

DNA, RNA

Tissues

-Dispersion

 - Dispersion

 - Dispersion

 - Dispersion

ii electric dipole definition

(1.1)

(1.2)

ii. Electric dipole - definition

The electric moment of a point charge relative to a fixed point is defined as er, where ris the radius vector from the fixed point toe. Consequently, the total dipole moment of a whole system of charges eirelative to a fixed origin is defined as:

A dielectric substance can be considered as consisting of elementary charges ei , and

if it contains no net charge.

If the net charge of the system is zero, the electric moment is independent of the choice of the origin: when the origin is displaced over a distancero, the change inmis according to (1.1), given by:

slide6

Thus mequals zero when the net charge is zero.

Then mis independent of the choice of the origin. In this case equation (1.1) can be written in another way by the introduction of the electric centers of gravity of the positive and the negative charges.

These centers are defined by the equations:

and

in which the radius vectors from the origin to the centers are represented by rp and rn respectively and the total positive charge is called Q.

Thus in case of a zero net charge, equation (1.1) can be written as:

slide7

The differencerp-rn is equal to the vector distance between the centers of gravity, represented by a vectora, pointing from the negative to the positive center ( Fig.1.1).

Thus we have:

(1.3)

Therefore theelectric moment of a system of charges with zero net charge is generally called the electric dipole moment of the system.

Figure 1.1

A simple case is a system consisting of only two point charges + e and -e at a distance a.

Such a system is called a(physical) electric dipole, its moment is equal toea, the vectorapointing from the negative to the positive charge.

slide8

A mathematical abstraction derived from the finite physical dipole is the ideal orpoint dipole. Its definition is as follows: the distance abetween two point charges +eand -e i.e. replaced by a/nand the charge eby en.

The limit approached as the number n tends to infinity is the ideal dipole. The formulae derived for ideal dipoles are much simpler than those obtained for finite dipoles.

Many natural molecules are examples of systems with a finite electric dipole moment (permanent dipole moment), since in most types of molecules the centers of gravity of the positive and negative charge distributions do not coincide.

The molecules that have such kind of permanent dipole molecules called polar molecules.

Apart from these permanent or intrinsic dipole moments, a temporary induced dipole momentarises when a particle is brought into external electric field.

Fig.2 Dipole moment of water molecule.

slide9

Under the influence of this field, the positive and negative charges in the particle are moved apart: the particle is polarized. In general, these induced dipoles can be treated as ideal; permanent dipoles, however, may generally not be treated as ideal when the field at molecular distances is to be calculated.

The values of molecular dipole moments are usually expressed in Debye units. The Debye unit, abbreviated as D, equals 10-18electrostatic units (e.s.u.).

The permanent dipole moments of non-symmetrical molecules generally lie between 0.5 and 5D. It is come from the value of the elementary charge eo that is 4.410-10 e.s.u. and the distance s of the charge centers in the molecules amount to about 10-9-10-8 cm.

In the case of polymers and biopolymers one can meet much higher values of dipole moments ~ hundreds or even thousands of Debye units. To transfer these units to CI system one have to take into account that 1D=3.3310-10 coulombsm.

polarization
Polarization

Some electrostatic theorems.

a) Potentials and fields due to electric charges.

According to Coulomb's experimental inverse square law, the force between two charges e and e'with distance r is given by:

(1.4)

Taking one of the charges, say e', as a test charge to measure the effect of the charge e on its surroundings, we arrive at the concept of an electric field produced bye and with a field strength or intensity defined by:

slide11

(1.5)

The field strength due to an electric charge at a distance r is then given by:

(1.6)

in which ris expressed in cm,ein electrostatic units and Ein dynes per charge unit, i.e. the e.s.u. of field intensity.

A simple vector-analytic calculation shows that Eq. (1.6) leads to:

(1.7)

in which the integration is taken over any closed surface around the charge e, and where dS is a surface element having direction of the outward normal.

slide12

Assuming that the electric field intensity is additively built up of the contributions of all the separate charges (principle of superposition) Eqn. (1.7) can be extended to:

(1.8)

This relation will still hold for the case of the continuous charge distribution, represented by a volume charge density  or a surface charge density . For the case of a volume charge density we write:

(1.9)

or , using Gauss divergence theorem:

(1.10)

This equation is the first Maxwell's well-known equations for the electrostatic field in vacuum; it is generally called thesource equation.

slide13

The second of Maxwell's equations, necessary to derive Euniquely for a given charge distribution, is:

(1.11)

or using Stokes's theorem:

(1.12)

in which the integration is taken along a closed curve of which dsis a line element.

Stokes' theorem:

where C is the contour of the surface of integration S, and where the contour C is followed in the clock-wise sense when looking in the direction of dS.

slide14

From (1.12) it follows that Ecan be written as the gradient of a scalar field , which is called the potential of the field:

(1.13)

The combination of (1.10) and (1.13) leads to the famous Poisson's equation:

(1.14)

In the charge-free parts of the field this reduces to the Laplace's equation:

(1.15)

the vector fields e and d
The vector fieldsEand D.

For measurement inside matter, the definition of Ein vacuum, cannot be used.

There are two different approaches to the solution of the problem how to measure E inside matter. They are:

1. The matter can be considered as a continuum in which, by a sort of thought experiment, virtual cavities were made. (Kelvin, Maxwell). Inside these cavities the vacuum definition of Ecan be used.

2. The molecular structure of matter considered as a collection of point charges in vacuum forming clusters of various types. The application here of the vacuum definition of E leads to a so-called microscopic field(Lorentz, Rosenfeld, Mazur, de Groot). If this microscopic field is averaged, one obtains themacroscopic or Maxwell fieldE .

slide16

The main problem of physics of dielectrics of passing from a microscopic description in terms of electrons, nuclei, atoms, molecules and ions, to a macroscopic or phenomenological description is still unresolved completely.

For the solution of this problem of how to determine the electric field inside matter, it is also possible first to introduce a new vector field Din such a way that for this field the source equation will be valid.

(1.16)

According to Maxwell matter is regarded as a continuum. To use the definition of the field vector E, a cavity has to be made around the point where the field is to be determined.

However, the force acting upon a test point charge in this cavity will generally depend on the shape of the cavity, since this force is at least partly determined by effects due to the walls of the cavity. This is the reason that two vector fields defined in physics of dielectrics:

Theelectric field strengthEsatisfying curlE=0, and the dielectric displacementD, satisfyingdiv D=4.

slide17

The Maxwell continuum can be treated as a dipole density of matter. Difference between the values of the field vectors arises from differences in their sources. Both the external charges and the dipole density of the piece of matter act as sources of these vectors.

The external charges contribute to D and to E in the same manner. Because of the different cavities in which the field vectors are measured, the contribution of dipole density toD and Eare not the same. It can be shown that

(1.17)

where Pcalled thePOLARIZATION.

Generally, the polarizationPdepends on the electric strength E.The electric field polarizes the dielectric.

The dependence ofPon Ecan take several forms:

(1.18)

slide18

The polarization proportional to the field strength. The proportional factor is called thedielectric susceptibility.

(1.19)

in which  is called the dielectric permittivity. It is also called the dielectric constant, because it is independent of the field strength. It is, however, dependent on the frequency of applied field, the temperature, the density (or the pressure) and the chemical composition of the system.

(1.20)

For very high field intensities the proportionality no longer holds.Dielectric saturation and non-linear dielectric effects.

(1.21)

slide19

For non-isotropic dielectrics, like most solids, liquid crystals, the scalar susceptibility must be replaced by a tensor. Hence, the permittivity  must be also be replaced by a tensor:

(1.22)

types of polarization
Types of polarization

For isotropic systems and leaner fields in the case of static electric fields

Theapplied electric fieldgives rise to adipole density

There can be two sources of this induced dipole moment:

Deformation polarization

a. Electron polarization - the displacement of nuclear and electrons in the atom under the influence of external electric field. As electrons are very light they have a rapid response to the field changes; they may even follow the field at optical frequencies.

b. Atomic polarization - the displacement of atoms or atom groups in the molecule under the influence of external electric field.

deformation polarization

+

+

-

Electric Field

-

-

Deformation polarization
orientation polarization
Orientation polarization:

The electric field tends to direct the permanent dipoles.

ionic polarization

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

Ionic Polarization

Inionic lattice, thepositive ionsare displaced in the direction of an applied field while thenegative ionsare displaced in the opposite direction, giving a resultant (apparent)dipole momentto the whole body.

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Electric field

polar and non polar dielectrics
Polar and Non-polar Dielectrics

To investigate the dependence of the polarization on molecular quantities it is convenient to assume the polarization P to be divided into two parts:the induced polarizationPcaused by thetranslation effects, and thedipole polarizationPcaused by theorientationof thepermanent dipoles.

A non-polar dielectricis one whose molecules possess no permanent dipole moment.

A polar dielectricis one in which the individual molecules possess adipole momenteven in the absence of any applied field, i.e. the center of positive charge is displaced from the center of negative charge.