15-11-2012, 04:01 PM
Fundamentals of Electricity & Electrical Equipment
Fundamentals of Electricity.ppt (Size: 355 KB / Downloads: 37)
Introduction
ELECTRICITY
The invisible energy which constitutes the flow of electrons in a closed circuit to do work is called electricity
It is a form of energy which can be converted to any other form very easily.
Modern Electron Theory
Modern Electron theory says that every matter (solid, liquid or gaseous) consists of very small divisible particles called molecules. Molecules are further made of very minute particles called atoms.
Atom further consists of the following parts –
Nucleus
This is the central part of the Atom which contains protons & neutrons. Proton has positive charge (1.692 X 10 –19 coulombs), where as neutron has no charge
In the nucleus, the protons & neutrons are held together with tremendous forces of attraction. The protons and neutrons carry equal mass and this total mass constitute total mass of an atom.
Extra-nucleus (Space)
The outer part of the atom which contains only electrons is called extra-nucleus. An electron has a negative charge (1.692 X 10 –19 coulombs) equal to that of a proton. The mass of an electron is nearly 1/1840 time to that of a proton and thus neglected.
Atomic weight = No of protons plus No. of neutrons in the nucleus
Electrons are not stationary particles, they move around the nucleus in different paths or orbits. The shape of an orbit is more elliptical than round but in drawing it is shown circular. The number of electrons is same as that of protons in an atom, thus an atom on the whole is NEUTRAL.
Atomic Number = No. of protons Or No. of electrons in an atom
Unit of Charge
The negative charge on the electron is very small and it is not convenient to take it as the unit charge, so Coulomb is used as the Unit of charge.
The practical unit of charge is Coulomb.
1 Coulomb = Charge on 628 x 1016 (6.28 x 1018) electrons.
If a body is said to have a negative charge of one coulomb, it means that the body has an excess of 628 x 1016 electrons, these electrons are supplied to this body from outside.
Free Electrons
The valance electrons which are very loosely attached to the nucleus of an atom and can be easily detached are called Free Electrons.
Conductor Resistance
Resistance Unit:
Resistance is measured in Ohms or Kilo-ohms & is denoted by symbol Ω or (K Ω).
A wire is said to have a resistance of 1 ohm, if one ampere current is passing through it produces heat of 0.24 calories.
Laws of Resistance:
The resistance ® of a wire depends upon the various factors -
1. It is directly proportional to its length, l i.e. R l 2. It is inversely proportional to its area of cross section, a i.e. R l/a 3. It depends upon the atomic structure of the material of which wire …is made. 4. It also depends upon the temperature of the wire.
Induced EMF
INDUCED EMF:
When flux linking with a conductor (coil) changes, an e.m.f. is induced in it. This change in flux linkages can be obtained in the following two ways
By moving the conductor and keeping the magnetic field stationary or by moving the magnetic field and keeping the conductor stationary, as in AC, DC generators. (It is called dynamically induced e.m.f.)
By changing the flux linking with the coil without moving coil or field. The change of flux produced by the field linking with the coil is obtained by changing the current in the field as in Transformers. (It is called statically induced e.m.f.)