Definition of Electronics
Electronics
is
the branch of
science that deals with the study of flow and control of
electrons (electricity) and the study of their behavior and
effects in vacuums, gases, and semiconductors, and with
devices using such electrons. This control of electrons is
accomplished by devices that resist, carry, select, steer,
switch, store, manipulate, and exploit the electron.
Basic Electrical Units and Definition
Passive
Capable of operating without an external power source. Typical passive
components are resistors, capacitors, inductors and diodes (although
the latter are a special case).
Active
Requiring a source of power to operate. Includes transistors (all
types), integrated circuits (all types), TRIACs, SCRs, LEDs, etc.
DC
Direct Current. The electrons flow in one direction only. Current
flow is from negative to positive, although it is often more
convenient to think of it as from positive to negative. This is
sometimes referred to as "conventional" current as opposed to
electron flow.
AC
Alternating Current. The electrons flow in both directions in a cyclic
manner  first one way, then the other. The rate of change of
direction determines the frequency, measured in Hertz (cycles per
second).




Frequency
Unit is Hertz, Symbol is Hz, old
symbol was cps (cycles per second).
A complete cycle is completed when the
AC signal has gone from zero volts to one extreme, back through zero
volts to the opposite extreme, and returned to zero.
The accepted audio range is from 20Hz
to 20,000Hz. The number of times the signal completes a
complete cycle in one second is the frequency.

Voltage
Unit is Volts, Symbol is V or U, old symbol was E . Voltage is the
"pressure" of electricity, or "electromotive force" (hence the old
term
E). A 9V battery has a voltage of 9V DC, and may be positive
or negative depending on the terminal that is used as the
reference.
The mains has a voltage of 220, 240 or 110V depending
where you live  this is AC, and alternates between positive and
negative values. Voltage is also commonly measured in millivolts
(mV), and 1,000 mV is 1V. Microvolts (uV) and nanovolts (nV) are
also used.
Current
Unit is Amperes (Amps), Symbol is I . Current is the flow of electricity
(electrons). No current flows between the terminals of a battery or
other voltage supply unless a load is connected.
The magnitude of
the current is determined by the available voltage, and the
resistance (or impedance) of the load and the power source.
Current
can be AC or DC, positive or negative, depending upon the
reference. For electronics, current may also be measured in mA
(milliamps)  1,000 mA is 1A. Nanoamps (nA) are also used in some
cases.
Resistance
Unit is Ohms, Symbol is R or Ω . Resistance is a measure of
how easily (or with what difficulty) electrons will flow through the
device.
Copper wire has a very low resistance, so a small voltage
will allow a large current to flow.
Likewise, the plastic
insulation has a very high resistance, and prevents current from
flowing from one wire to those adjacent.
Resistors have a defined
resistance, so the current can be calculated for any voltage.
Resistance in passive devices is always positive (i.e. > 0)



Capacitance
Unit is Farads, Symbol is C. Capacitance is a measure of
stored charge. Unlike a battery, a capacitor stores a charge
electrostatically rather than chemically, and reacts much faster.
A
capacitor passes AC, but will not pass DC (at least for all
practical purposes). The reactance or AC resistance (called
impedance) of a capacitor depends on its value and the frequency of
the AC signal. Capacitance is always a positive value.
Inductance
Unit is Henrys, Symbol is H or L (depending on context).
Inductance occurs in any piece of conducting material, but is wound
into a coil to be useful.
An inductor stores a charge magnetically,
and presents a low impedance to DC (theoretically zero), and a
higher impedance to AC dependent on the value of inductance and the
frequency.
In this respect it is the electrical opposite of a
capacitor. Inductance is always a positive value. The symbol "Hy"
is sometimes used in (guess where :) ... the US.
There is no such symbol.
Impedance
Unit is Ohms, Symbol is Ω or Z. Unlike resistance, impedance
is a frequency dependent value, and is specified for AC signals.
Impedance is made up of a combination of resistance, capacitance,
and/ or inductance.
In many cases, impedance and resistance are the
same (a resistor for example). Impedance is most commonly positive
(like resistance), but can be negative with some components or
circuit arrangements.
Decibels
Unit is Bel, but because this is large, deciBels (1/10th
Bel) are used), Symbol is dB.
Decibels are used in audio because
they are a logarithmic measure of voltage, current or power, and
correspond well to the response of the ear.
A 3dB change is half or
double the power (0.707 or 1.414 times voltage or current
respectively). Decibels will be discussed more thoroughly in a
separate section.
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