afesheir said:
thanks lot for your answers ...
but I want further information than steves said about my first question (about the factors affecting the dielectric constant)
Ok, here's some quotations from "The New College Physics" Albert Baez, Freeman and Co, 1967:
"when a substance is placed into an electric field, E, every single electric charge, q, within it experiences the force qE; positive charges are pulled in the direction of E, and negative charges are pulled in the opposite direction. Whether the charges move depends upon the substance, and the motion itself depends on the strength of the field and the time during which it acts. In metals there are many so-called free electrons that can travel over large distances within the sample. The atoms of dielectrics, compared with those of metals, have so few free electrons that they are virtually non-conductors, or insulators. The molecules of a dielectric bear positive and negative charges, of course, and each of these experiences a force when the dielectric is placed in a field, but only a slight separation of the charges within each molecule takes place. The effect is called POLARIZATION. Mobility is limited to molecular dimensions."...
they then present an example...
" When an uncharged slab of glass, G, is placed between the charged plates, A and B, of a capacitor, there is no migration of electrons, but the effect of the charge separation induced by the field is a net accumulation of polarization charges on the upper and lower faces of G."
now put a picture into your mind of a simple circuit containing a battery, a switch and our capacitor with glass between the plates.
"when the slab of glass being absent, the switch is closed, charges flow to the plates of the capacitor until the potential difference across the plates is the same as that of the battery." No current flows when this is completed." When the glass, G, is brought between the plates, polarization charges appear on the upper and lower surface of the glass."
Negative polarization charges appear right next to the positive plate of the capacitor, and positive polarization charges appear next to the negative plate of the capacitor.
"The appearance of positive polarization charges on the lower (negative) plate weakens the resultant repulsive electrostatic force, and more electrons can therefore come from the battery to reside on the plate. The capacitance of the combination has thereby been increased."
there in a nutshell is the reason why a dielectric material can increase capacitance. The relative dielectric constant is the number that states how much more the capacitance has increased compared to the case where there is only a vacuum between the plates.