Current electricity

Generally, the free electrons are responsible for the flow of electric current. In a metallic conductor, on an average there will be one free electron available per atom. At any temperature, these free electrons move freely and collide with fixed atoms or ions inside the conductor. These collisions are inelastic in nature and causes the transfer of energy. The resultant velocity will be zero in any direction. But when potential difference is applied between the ends of a conductor, an electric field E will act on these free electrons. As a result, and electron moves with an average velocity in a direction opposite to the field. This is called drift velocity 'Vd' and is of the order of 10^-3m/s. Like that, all electrons will drift in the same direction. This drift is responsible for the flow of charge through the conductor. By convection, the direction of the current is the direction of positive charge motion.

In electrolytes or in gaseous conductors, the charge carries will be either positive or negative ions or both. But in case of semi conductors, the conduction is due to electrons and holes.

Electrolyte-capacitors

An electrolyte capacitor is obtained by passing a direct current between two sheets of aluminium foils
with a suitable electrolyte like aluminium borate between the foils. Due to electrolysis, a very thin film of thickness of the order of 10^-6cm of aluminium oxide is formed on the anode plate and acts as the dielectric between the plates. This oxide layer is very thin, so that capacity of this capacitor becomes very high. Care must be taken to connect this capacitor with proper polarity in a circuit. Otherwise the oxide film will break down. For this reason, in an electrolyte capacitor the polarity of terminals will be indicated.
These capacitors are widely used when high capacitances are required.

paper capacitor

Above figure shows that paper capacitor. In this, a paper soaked in oil or wax is used as the dielectric in between the tin foils that serve as a parallel plates. In order to increase the capacitance to large extent, several number of tin foils and wax papers to be used by arranging alternatively as shown in figure.
This entire capacitor can be rolled and sealed in a cylinder. Since, these capacitors occupy small space and are cheaper in cost and they are widely used in radio circuits and in laboratories.

multiple capacitor

Above figure shows that multiple capacitor which is a parallel combination of several parallel plate capacitors and is of fixed capacitance. Mica is used as dielectric which has dielectric constant 6 between a number of tin foils arranged in parallel.
If the capacitance between two successive plates is C, then the capacitance of the multiple capacitor is nC, where n is the number of dielectric plates used. This whole arrangement is sealed in a plastic case. These capacitors are used in high frequency oscillating circuits. Since, mica's dielectric constant does not change with temperature, so these capacitors are used as standard capacitors in the laboratory.

The above figure shows the variable capacitor which can be varied gradually. This is achieved by varying effective area between the plates. These plates are usually made up of brass or aluminium and semi circular shape. This capacitor consists of two sets of plates. One set of plates is fixed in position and is called the stator. The other set of plates can be rotated over the stator by rotating the piston. This set called rotor. During the rotation of the rotor, the area common to the plates of stator and rotor is varied.