Dielectrics Flashcards
What is a dielectric?
-A dielectric is a substance that is highly resistant to the flow of an electric current.
-In other words a dielectric is electrically non conducting material that provides electrical insulation between two media (conductors) which are at different potentials. Eg - Glass, Wax paper, Ceramics, Porcelain.
-When a dielectric medium interacts with an applied electric field, charges are redistributed within its atoms or molecules. -This redistribution alters the shape of an applied electrical field both inside the dielectric medium and in the region nearby. -When two electric charges move through a dielectric medium, the interaction energies and forces between them are reduce
What is Dielectric Constant?
Faraday discovered that the capacitance of the condenser increases when the region between the plates is filled with dielectric.
-If C0 is the capacitance of the capacitor without dielectric and C is the capacitance of the capacitor with dielectric then the ratio C / C0 gives εr called relative permittivity or Dielectric constant,
-Also for a given isotropic material the electric flux density is related to the applied field strength by the equation D = ε E, Where E ε is Absolute permittivity. In SI system of units the relative permittivity is given by the ratio of absolute permittivity to permittivity of free space. ε = ε0 εr .ε0 is permittivity of free space. εr is relative permittivity or dielectric constant.
-For an isotropic material, under static field conditions, the relative permittivity is called static dielectric constant. It depends on the structure of the atom of which the material is composed.
What is a Dipole?
A dipole is an entity consisting equal number of positive and negative charges separated by a small distance. A dipole moment is a vector directed from positive field.
What is Polarization?
The displacement of charges in the atoms or molecules of a dielectric under the action of applied field leading to the development of dipole moment is called polarization.
Electrical polarization
The polarization of the dielectric is the process of formation of dipoles or alignment of already existing dipoles by the application of an electric field on the dielectric material. The ratio of induced dipole moment to the effective applied electric field is called polarizability
explain Polar and non-Polar dielectrics
-In dielectrics there are no free electrons, the center of positive charges are centered or concentrated at the center of atom and center of negative charges are concentrated in the electron cloud. With the center of gravity positive charges coincide with center gravity of negative charges, then it neutralizes each other effects; hence their dipole moment is zero. Such dielectrics are called non-polar dielectrics.
-In some other dielectrics like water, center of gravity of positive charges never coincides with center of gravity of negative charges even in the presence of applied field. In such dielectrics each molecule behaves as if it contains a pair of positive and negative charges separated by a distance (10-30m). Hence they have permanent dipole moment. They are known as polar dielectrics.
explain polar and non polar with exp
Let DC potential be applied between the plates; the atomic dipoles in the material align in the electric field. The mean position of electrons will align towards the positive plate of capacitor and mean position of positively charged nucleus will align towards negative plate of capacitor. Inside the material the dipoles formed, align such that positively charged particles are attracted towards the negatively charged particles. In fact, at the surface of dielectric layer, negative charge is formed near positively charged plate of capacitor; a layer of positive charge is formed adjacent to the negatively charged plate of capacitor, these charges on the surface of dielectric material is called polarized charges
what are Different Types of Polarization Mechanisms
The polarization is alignment of permanent or induced atomic or molecular dipoles under action of applied field; hence depending on dielectric material and manner of applied electric field, there are four types of polarization mechanisms
1. Electronic polarization.
2. Ionic polarization.
3. Orientation or molecular polarization.
4. Space change polarization
What is Electronic Polarization
This is the most common type of polarization, which occurs in most of the dielectrics. The electronic polarization is due to displacement of center of gravity of negatively charged particles relative to center of gravity of positive charges. This is called electronic because the dipole moment results due to shift of the electron cloud relative to the nucleus. This type of polarization is due to induced dipole moments. The electrons has very high natural frequencies of order 10^15Hz, hence light of frequency 10^15Hz can cause electronic polarization. The electronic polarization is temperature independent. The electronic polarization occurs in over short interval of time 10-15sec.
WRITE THE EQ ALSO
What is Ionic Polarization
The ionic polarization occurs only in ionic materials like Nacl etc. In this type of materials under equilibrium conditions, the cations and anions remain at their mean equilibrium conditions. When the field is applied the cations and anions get displaced from their mean positions in opposite directions and give rise to a net dipole moment .As the dipole moment occurs only under an applied electric field, ionic polarization is due to induced dipoles; also ions are heavier than electrons. This type of polarization is slow process and ionic polarization is limited to frequencies up to 1013 hertz and hence light frequencies of 1015 cannot cause ionic polarization.
What is Molecular Polarization
The orientation polarization occurs in polar dielectrics in which there are molecules with permanent dipole moment.
-The orientation of these molecules are random due to thermal agitation, because of randomness in orientation, the material has net zero dipole moment in the absence of electric field.
-When electric field is applied each dipole undergo rotation so as to orient along the direction of the field, which exert a torque in them, thus material itself develops the dielectric polarization.
-In the orientation polarization restoring forces do not exists, however dipole alignment is balanced by thermal agitation and this type of polarization is strongly temperature dependent.
The orientation polarizability is given by α0 = µ2/3kT
The orientation polarization Po is given by Po = N µ2 E/3kT
Space Charge Polarizations
The space charge polarization occurs in multiphase dielectric substances in which there is a change of resistivity between different phases when electric field is applied at high temperature. The electric charges get accumulated at the interface due to sudden change in conductivity. This accumulation of charges with opposite polarities at opposite parts in low resistivity phase leads to development of dipole moment The space charge polarization is not an important factor in most common dielectrics. The total polarization α of a material is thus given by the sum of electronic, ionic and orientation polarizations, α = αe+ αi+ αo
What is Internal Field
When the electric field is applied to dielectric material either liquid or solids, each atom in the material develops dipole moment and acts like electric dipole, since atoms either in liquids or solids are surrounded on all sides by polarized atoms, the internal field at given point inside the material is equal to the electric field created by the neighboring atoms and the applied field.
“The internal field is defined as the electric field that acts at the site of any given atoms of a solid or liquid. Dielectric field subjected to an external field and is resultant of the applied field and the field due to all the effects of the surrounding atoms.”
Expression for Internal or local fields in solids and liquids
Do Doo eq
Lorentz Field for a cubic lattice
Do Dooo
