* Dielectrics * Polar and Nonpolar Molecules * Polarization P * Polarization Charge Density PPT No. 10
Introduction A medium plays a significant role in determining the response of an electric field. The phenomena related to electrostatic fields in matter are discussed using macroscopic model of matter.
Introduction Depending upon the behavior of different materials in an electrostatic field, they are divided in two main categories Conductors 2. Insulators or dielectrics. They are described in brief as follows
A) Conductors Conductors are materials which contain electrons that are free to move and an electric field produces a steady drift of charge i.e. current. They conduct the current or allow the flow of electric charges like electrons fairly easily.
B) Insulators or Dielectrics Dielectric is basically an electrically insulating material. The electrons are strongly bound to the atoms or molecules. They cannot be separated by the application of an electric field. A steady flow of electrons cannot flow through it.
B) Insulators or Dielectrics Literally, Dielectric is a material placed across the plates of a capacitor like a little nonconducting bridge. (The Greek prefix di or dia means "across").
Insulators or Dielectrics Dielectric materials traditionally used in industrial applications are oil-impregnated paper, mica, Elastomers, press-molded resins etc. Presently materials used are synthetic polymers such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, polytetrafluoroethylene, polymethyl methacrylate, polyamide, polyimide etc. Dielectric materials are classified into two broad groups: 1. Polar 2. Nonpolar
Polar and Nonpolar Molecules A) Polar molecules Polar means having electrical poles (i.e. electrical polarity). The molecules in which the arrangement or geometry of the atoms is such that one end of the molecule has a positive electrical charge and the other side has a negative charge are called as polar molecules. Examples of polar molecules are Water (H2O) Ammonia (NH3), Hydrochloric acid (HCl), Sulfur Dioxide (SO2), Hydrogen Sulfide (H2S), Carbon Monoxide (CO) etc
Polar molecules Fig. (a) Polar Molecule - Water and Chemical Bonds in Water
B) Nonpolar molecules: O = C = O (b)chemical Bonds in CO2
Nonpolar molecules A non-polar molecule is that in which the electrons are distributed more symmetrically It does not have an excess /abundance of charges at the opposite sides. The charges all cancel out each other. e.g. CO2, H2, N2, O2, CH4, CCl4 etc.
A Slab in an Electric Field Due to difference in the atomic structures of materials conductors and insulators exhibit different response when placed in an electric field. The particular response is studied by placing in an external electric field E0. a) a slab of conducting material b) a slab of insulating material
A) A Slab in an Electric Field- Conductor Net charge resides on the surface: Electrons move to the positive side of the field. As they cannot leave the material, they accumulate on the surface i.e. charges are induced on the surface.
A) A Slab in an Electric Field- Conductor *The induced charges produce a field of their own E i which is directed opposite to the applied field E 0 and E i tends to cancel E 0. Charge flow continues till E i completely cancels E 0 Result is Einside = 0 E = 0 inside the conductor
A Slab in an Electric Field- Conductor Conductor is an equipotential surface: As Potential φ is constant everywhere inside the conductor.
A Slab in an Electric Field- Conductor ρ = 0: According to Gauss law If E inside = 0 => ρ = 0. E is normal to the surface: The electric lines of force are normal to the surface.
B) Nonpolar Dielectrics in Electric Field Fig (a1) Nonpolar Molecule-Unpolarized (a2) Polarization in an applied electric Field
Dielectric molecules in an Electric Field In nonpolar dielectric molecules, Permanent electric dipoles are not present. However, after applying an external electric field to an insulator or dielectric, positive and negative electric charges within it undergo slight relative shift In opposite directions Electric field distorts the negative cloud of electrons around positive atomic nuclei in a direction opposite to the field.
Dielectric molecules in an Electric Field The slight separation of charge (stretching or shift of a fraction of an atomic diameter) makes one side of the atom somewhat positive and the opposite side somewhat negative i.e. dipoles are created in nonpolar dielectric molecules. Electric field induces dipoles & Polarization in nonpolar dielectric molecules
Polar Dielectric Molecules in the Electric Field In polar dielectric molecules, permanent dipoles are present however, due to thermal agitation, they are generally in random orientations Dielectric is in unpolarizated condition.
Polar Dielectric Molecules in the Electric Field When an electric field is applied, the dipoles are oriented by rotation and aligned in the direction of the electric field so that one type of bound charges (+ve or -ve) appear on a surface and the opposite type on opposite surface. Thus electric field polarizes the material of polar molecules
Polar Molecule in the Electric Field Fig (b1) Polar Molecule-Unpolarized (b2) Polarization in an applied electric Field
Effect of an Electric Field on Dielectric Materials Thus application of an external electric field creates polarization in both types of molecules- Nonpolar and Polar By Two different mechanisms By inducing dipoles (in Nonpolar Dielectrics) or By reorienting & aligning already existing dipoles (in Polar).
Effect of an Electric Field on Dielectric Materials Polarization reduces the electric field intensity within the dielectric material by a factor known as the dielectric constant (or relative permittivity) of the material.
Polarization P Electric polarization (or simply Polarization) or the Polarization density is the vector field that expresses the density of permanent or induced Electric Dipole moment in a dielectric material.
Polarization P The polarization vector P is defined as the dipole moment per unit volume. The SI unit of measure of Polarization P is Coulombs per square meter.
Polarization Density When a dielectric is placed in a uniform external electric field E, electric Polarization creates additional bound charges on the surface of the material as explained above. The surface Charge density is denoted as the bound charge density σ b. where is the unit normal Vector pointing outwards
Surface Charge Density and Volume Charge Density Fig. Surface charge density of a polarized dielectric
Polarization P Consider a rectangular block of the polarized dielectric of length l & each face having uniform cross-sectional area A. Let surface charge densities of polarization or bound charges appearing at the end faces be +σp and -σp. This surface charge is the only bound charge. Therefore, the total charge induced on each face = σp A
Polarization P Total induced electric dipole moment = -ql = σp Al Volume of block = Area x Length = Al Therefore, Polarization P i.e. Induced dipole moment per unit volume Polarization P = σp
Polarization P The polarization inside the material is equal to the surface charge density appearing at faces perpendicular to the direction of applied electric field. If P is not uniform inside the material, then there will be accumulation of bound charges within the volume of material (in addition to bound charges on the surfaces) having volume charge density ρb.
Polarization P If P is not uniform inside the material, Then it can be proved that Polarization P is a vector field whose Divergence depends on the volume charge density ρb due to bound or polarization charges in the material