CLASSIFICATION OF MATERIALS
Electrical Engineering materials may be divided into four broad groups according to the purposes they serve.
These four groups are:
- conductors,
- semiconductors,
- magnetic materials, and
- electrical insulating materials or dielectrics.
Conductors possess high conductivity and are used in Electrical Engineering to provide paths for electric current in all types of electrical machine windings, electrical apparatus, devices, instruments, etc.
Conductors are required for the manufacture of cables and wires by means of which electrical energy is transmitted over long distances and then distributed to the consumers.
Semiconductors occupy an intermediate position between conductors and insulators (or dielectrics). The properties of semiconductors make them valuable in many branches of Electrical Engineering such as telecommunication and radio communication, electronics in general (over a wide range of frequencies) and power engineering. Semiconductors serve as rectifiers, amplifiers, special sources of electric current, photocells, etc.
Magnetic materials strengthen the magnetic field in which they are placed and possess high magnetic permeability. These properties make them suitable for use as magnetic cores and circuits, magnetic screens, and permanent magnets enabling setting up a magnetic field in the surrounding space.
The characteristic property of electrical insulating materials is poor conductivity. The great quantitative difference between the conductivity of dielectrics and conductors leads to a distinctive qualitative difference between them.
The behaviour of the dielectrics is not governed by electrodynamic phenomena involving the directional flow of enormous number of electric charges (free electrons or ions) but by electrostatic phenomena associated with the presence of an electric field.
The behaviour of real dielectrics is primarily electrostatic; electrodynamic effects, that also take place due to their non-zero conductivity, are quite insignificant in normal operating conditions.
In all kinds of electrical devices and circuits dielectrics serve to insulate one current carrying part from another whenever they operate with a difference in electrical potential relative to each other.
In capacitors dielectrics also serve to provide some required value of capacitance. The dielectrics used in electrical engineering may be gases, liquids or solids.
Among the gases that serve as dielectrics are air, nitrogen, and others. Liquid dielectrics may be natural (for example mineral insulating oils) artificial. Solid dielectrics are extremely diverse in origin and properties.
0 comments: