(A) Types of Charges:
There are two types of charges:
(i) Positive charge:
When an atom looses one electron it becomes a positive charge
(ii) Negative charge:
Electron has a negative charge
The electric Voltage at any point is known as the potential of that point. The Voltage is measured in volts.
Volt is the unit of electromotive force – a measure of the electrical pressure force of an electric level.
Electromotive force is the force which starts & maintains of electronic current through a conductor.
The difference of the electrical voltage between the two points in a circuit is known of the potential difference. Electrical potential difference is work done in moving a unit of charge.
The quantity of electricity residing on an electro statically charged body. The unit of charge is coulomb.
1 coulomb = 6.29 x 101* electrons
1 electron charge = 1.602 x 1019 coulomb
(D) Current –
The rate of How of electric charge is current. The flow of electron in a conductor is current. The unit of current is ampere
(E) Ampere –
Flow of I coulomb of charge in one second is ampere
Resistors – Having high resistance.
Types of resistors.
(A) Carbon Composition resistor:
Carbon composition consisting 0f finely ground carbon region binder and filler is compressed into shape of resistor and fired in the kiln. The end connection is obtained by metal spraying soldering tinned copper wire.
(B) Metal Film Resistor:
These resistors are manufactured by depositing a film of metal (Ni or Cr) on high grade ceramic rod
(C) Carbon Film Resistor:
Carbon film resistors are manufactured by depositing an even film of resistive material on high grade insulating cores. Helical grooving is done to achieve high degree of accuracy.
(D) Wire Wound Resistor:
These are manufactured by winding resistance wire over insulating base as shown in Fig. 6.8. The whole assembly is covered in an insulating material which provides the resistor excellent protection against moisture and environmental conditions.
(E) Variable Resistor:
These resistors permit variation of resistance from zero to maximum and essentially consist of track of some kind of resistive material to which movable wiper makes contact. These resistors can be classified on the basis of material used as:
(i) Carbon composition
(ii) Wire wound
Insulator – high resistance material –
A Good insulating material should have a;
(1) Low dissipation factor.
(2) High insulation resistance.
(3) Good di-electric strength and high mechanical strength
(4) They should also have high thermal conductivity and high thermal strength.
The insulating materials used in electrical engineering may be (1) Gases including vacuum. Following are gases –
(a) Simple gases:
This group includes Air, Nitrogen, Hydrogen, Helium, Argon etc.
(b) Oxide gases:
Carbon dioxide, Sulphur dioxide.
(c) Electronegative gases:
Sulphur hex Fluoride, CH CI
(d) Hydrocarbon gases:
Methane, Ethane, Propane and Freon gases.
Some of the dielectric gases:
(a) Simple Gases:
Air is the most important of all dielectric gases because gift of nature. This dielectric gas provides insulation between the overhead transmission lines without any cost. It forms a part of an electric device or methane whether we want it or not, acting as an insulation along with the specially provided insulating materials (solids or liquids).
Air is reliable insulating material when voltages are not very hash Current leakage through air is far less than through solid or liquid insulating materials under the same conditions.
Nitrogen is next to air in use as a dielectric in electrical apparatus, where it is used for both chemical and electrical purposes. Nitrogen is used in transformers to replace air (which causes oxidation).
Nitrogen under pressure is used in certain types of electrical capacitors. In gas pressure cables, nitrogen gas in conjunction with oil treated paper used is under pressure.
Hydrogen is of special interest to electrical engineers because it is very light gas and possesses properties favouring its use as a coolant in electrical machines instead of air.
Hydrogen when used as a coolant in electrical machines enables the raising of efficiency of an electrical machine.
Solid Insulating Materials
Solid insulating materials may be classified under the following headings:
(i) Paper and press boards.
(ii) Fibrous Material (unimpregnated and impregnated)
(iii) Impregnating, coating, filling and bonding materials.
(iv) Resins (Polymers).
(v) Natural & Synthetic rubbers.
(vi) Inorganic insulating materials:
(c) Mica & Micanite