04-05-2012, 05:24 PM
ELEMENTARY THEORY OF ELECTRICITY AND MAGNETISM
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What is electronics?
Electronics is the field of manipulating electrical currents and voltages using passive and
active components that are connected together to create circuits. Electronic circuits range
from a simple load resistor that converts a current to a voltage, to computer central
processing units (CPUs) that can contain millions of transistors. Electronic devices
operate by the movement of electrons through conductors, e.g. wires, and electronic
components.
What are passive components?
Resistors, inductors, transformers and capacitors are called passive devices. They
don’t alter their resistance, impedance or reactance when alternating currents (ac) are
applied to them.
What are active components?
Vacuum tubes, diodes, transistors etc. are called active devices. They change their
resistance or impedance when varying voltages are applied to them and as a result can
amplify, rectify, modify or distort ac waveforms. Passive devices normally don’t distort
waveforms.
Matter and electricity
Before going to discuss the different theories related to electricity and magnetism,
we would like to give a brief idea about matter. All matter consist of molecules. A
molecule can be defined as the smallest particle, which shows all the characteristics of a
particular matter. For example, molecule of water is obtained by dividing a drop of water
again and again until it can be divided and still be water. Further division of this water
molecule will yield three particles which are not water. Molecule of water contains two
atoms of hydrogen (H) and one atom of oxygen (O). Chemical combination of different
atoms makes a molecule. An atom can be further divided into three particles known as
protons and electrons and neutrons. Protons and electrons are the particles possessing
electrical properties whereas neutron is electrically neutral. These particles can't be
divided further.
What is charge?
Charge is an amount of electrons. Its unit is coulomb © and symbol is ‘q’. One coulomb
is equivalent to 6 x 1018 electrons.
What is current?
Atoms of a metal form a crystal lattice, and in the spaces between the lattice
points free electrons move chaotically, wandering aimlessly here and there. But it is
enough to connect a metal plate to the two poles of a voltage source for the electrons
immediately to acquire an aim. They will move towards the positive pole of the battery,
and an electric current will begin to flow in the metal. An electric current can also flow in
a gas. A voltage applied across a gas-filled tube causes ionization of the gas: free
electrons stream towards the plate with the positive potential, colliding with the atoms in
their way and detaching electrons from their orbits. The positive ions move toward the
opposite end of the tube. Current is the rate of flow of charge, i.e., the number of
coulombs flowing past a point per second. Its unit is ampere (A) or amp. One amp is
equal to one coulomb per second.
What is voltage?
Voltage is also called potential (Potential is defined as the work required from
some energy source in moving a unit positive charge between two points in an electric
field), potential difference, potential drop, or electromotive force-EMF. It is the
electronic potential energy between two points, and is the driving force that causes charge
to flow. Its unit is volt (V). One volt is defined as the potential difference that requires
one joule of energy to move one coulomb of charge. Voltage is always measured relative
to some other point in a circuit, e.g. the potential across a resistor. Voltage measurement
made at a single point in a circuit are made relative to the earth (ground), which is
assigned an “absolute” voltage of zero.