Then
the difference in voltage between any two points, connections or junctions
(called nodes) in a circuit is known as the Potential Difference or
sometimes called the Voltage Drop.A constant voltage source is
called a DC Voltage with a voltage that varies periodically
with time is called an AC voltage. Voltage is measured in volts,
with one volt being defined as the electrical pressure required forcing an
electrical current of one ampere through a resistance of one Ohm. Voltages are
generally expressed in Volts with prefixes used to denote sub-multiples of the
voltage such as microvolts (μV =10-6 V), millivolts (mV=10-3 V)or kilovolts (kV=103 V).
Voltage can be either positive or negative.
Batteries
or power supplies are mostly used to produce a steady D.C. (direct current)
voltage source such as 5v, 12v, 24v etc. in electronic circuits and systems.
While A.C. (alternating current) voltage sources are available for domestic
house and industrial power and lighting as well as power transmission. The
mains voltage supply in the United Kingdom is currently 230 volts a.c.
and 110 volts a.c. in the USA.General electronic circuits operate on low
voltage DC battery supplies of between 1.5V and 24V d.c. The circuit symbol for
a constant voltage source usually given as a battery symbol with a positive, + and
negative, - sign indicating the direction of the polarity. The
circuit symbol for an alternating voltage source is a circle with a sine wave
inside.
Electrical Current: Current, (I) is the movement or flow of electrical charge and is measured
in Amperes. It is the continuous and uniform flow (called a drift)
of electrons (the negative particles of an atom) around a circuit that are
being “pushed” by the voltage source. In reality, electrons flow from the
negative (-ve) terminal to the positive (+ve) terminal of the supply and for
ease of circuit understanding conventional current flow assumes that the
current flows from the positive to the negative terminal.
Conventionally this is the flow of positive
charge around a circuit, being positive to negative. The diagram at the left
shows the movement of the positive charge (holes) around a closed circuit
flowing from the positive terminal of the battery, through the circuit and
returns to the negative terminal of the battery. This flow of current from
positive to negative is generally known as conventional current flow.The Conventional
Current Flow gives the flow of electrical current from positive to
negative and which is the opposite in direction to the actual flow of
electrons.The flow of electrons around the circuit is opposite to the
direction of the conventional current flow being negative to positive.The
actual current flowing in an electrical circuit is composed of electrons that
flow from the negative pole of the battery (the cathode) and return back to the
positive pole (the anode) of the battery.
This
is because the charge on an electron is negative by definition and so is
attracted to the positive terminal. This flow of electrons is called Electron
Current Flow. Therefore, electrons actually flow around a circuit from the
negative terminal to the positive.Both conventional current flow and electron
flow are used by many textbooks. In fact, it makes no difference which
way the current is flowing around the circuit as long as the direction is used
consistently. The direction of current flow does not affect what the current
does within the circuit. Generally it is much easier to understand the
conventional current flow – positive to negative.
In
electronic circuits, a current source is a circuit element that provides a
specified amount of current for example, 1A, 5A 10 Amps etc., with the circuit
symbol for a constant current source given as a circle with an arrow inside
indicating its direction.Current is measured in Amps and an
amp or ampere is defined as the number of electrons or charge (Q in
Coulombs) passing a certain point in the circuit in one second, (t in
Seconds).Electrical current is generally expressed in Amps with prefixes used
to denote micro amps (μA=10-6A) or milliamps (mA=10-3A).
Note that electrical current can be either positive in value or negative in
value depending upon its direction of flow.
Current
that flows in a single direction is called Direct Current, or D.C. and
current that alternates back and forth through the circuit is known asAlternating
Current. Whether AC or DC current only flows through a circuit when a
voltage source is connected to it with its “flow” being limited to both the
resistance of the circuit and the voltage source pushing it.Also, as
alternating currents (and voltages) are periodic and vary with time the
“effective” or “RMS”, (Root Mean Squared) value given as Irms produces
the same average power loss equivalent to a DC current Iavg.
Current sources are the opposite to voltage sources in that they like short or
closed circuit conditions but hate open circuit conditions as no current will
flow.
Using the tank of water
relationship, current is the equivalent of the flow of water through the pipe
with the flow being the same throughout the pipe. The faster the flow of water
the greater the current. Any current source whether DC or AC likes a short or
semi-short circuit condition but hates any open circuit condition as this
prevents it from flowing.
Resistance: Resistance,
(R) of a circuit is its ability to resist or prevent the flow of current
(electron flow) through itself making it necessary to apply a greater voltage
to the electrical circuit to cause the current to flow again. Resistance is
measured in Ohms, Greek symbol (Ω, Omega)with prefixes used
to denote Kilo-ohms (kΩ=103Ω) and Mega-ohms (MΩ=106Ω).
Note that Resistance cannot be negative in value only positive.The amount of
resistance determines whether the circuit is a “good conductor” – low
resistance, or a “bad conductor” – high resistance. Low resistance, for example
1Ω or less implies that the circuit is a good conductor made from materials
such as copper, aluminium or carbon while a high resistance, 1MΩ or more
implies the circuit is a bad conductor made from insulating materials such as
glass, porcelain or plastic.
Resistance
can be linear in nature or non-linear in nature. Linear resistance obeys Ohm’s
Law and controls or limits the amount of current flowing within a circuit
in proportion to the voltage supply connected to it and therefore the transfer
of power to the load. Non-linear resistance does not obey Ohm’s Law but
has a voltage drop across it that is proportional to some power of the
current.Resistance is pure and is not affected by frequency with the AC
impedance of a resistance being equal to its DC resistance and as a result can’t
be negative. Remember that resistance is always positive, and never negative.
Resistance
can also be classed as an attenuator as it has the ability to change the
characteristics of a circuit by the effect of loading the circuit or by
temperature which changes its resistivity. For very low values of resistance,
for example milliohms, (mΩ) it is sometimes much easier to use the reciprocal
of resistance (1/R) rather than resistance (R) itself. The reciprocal of
resistance is called Conductance, symbol (G) and represents
the ability of a conductor or device to conduct electricity.High values of
conductance imply a good conductor such as copper while low values of
conductance implies a bad conductor such as wood. The standard unit of
measurement given for conductance is the Siemen, symbol (S).Again,
using the water relationship, resistance is the diameter or the length of the
pipe the water flows through. The small diameter of the pipe has larger
resistance to the flow of water, and therefore the large resistance.
Ohm's
Law
is a formulation of the relationship of voltage, current, and resistance,
expressed as:
V=I*R;I=V/R;R=V/I.
Hence, Volt= Amps times Resistance
Capacitors: Capacitors
derive their name from their capacity to store charge, and are used in a
circuit to damp out rapid changes in voltage. They consist of two conducting
surfaces separated by an insulator (the 'dielectric'), with a lead connected to
each surface. To create the values of capacitance needed for practical
circuits, one needs conducting surfaces which are both very large and very
close together, and separated by a material with a high 'dielectric constant'.
To pack sufficient surface area into a small volume, capacitors usually have a
rolled or stacked internal structure. If a constant voltage is applied to a
capacitor, only a very small 'leakage current' will flow once the capacitor has
fully charged.
Inductors: Inductorsare
coils of wire with many turns, often wound around a core made of a magnetic
material, like iron or ferrite. Current flowing through the inductor produces a
local magnetic field in which energy is stored. This field creates an induced
current in the inductor in a direction which resists any change in the current
flowing in the circuit. Inductors are thus used in circuits to prevent any
rapid changes in current.
Quantity
|
Symbol
|
Unit of
Measure |
Abbreviation
|
Voltage
|
V or E
|
Volt
|
V
|
Current
|
I
|
Ampere
|
A
|
Resistance
|
R
|
Ohms
|
Ω
|
Inductance
|
L
|
Henry
|
H
|
Capacitance
|
C
|
Farad
|
F
|
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