BASICs DC ccts
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DC Circuit Theory
Relationship Between Voltage, Current and Resistance
The fundamental relationship between voltage, current and resistance in an electrical circuit
is called Ohm’s Law. All materials are made up from atoms, and all atoms consist of protons,
neutrons and electrons. Protons, have a positive electrical charge. Neutrons have no electrical
charge while Electrons, have a negative electrical charge. Atoms are bound together by
powerful forces of attraction existing between the atoms nucleus and the electrons in its outer
shell.
When these protons, neutrons and electrons are together within the atom they are happy and
stable. But if we separate them from each other they want to reform and start to exert a
potential of attraction called a potential difference.
Conductive materials contain many loosely-bound “free” electrons which conduct the flow of
electricity in response to an applied potential difference. Insulating materials contain very
few loosely-bound “free” electrons, hence they ‘resist’ the flow of electricity in response to
an applied potential difference.
Now if we create a closed circuit (using conductors – e.g copper or aluminium) these loose
“FREE” electrons will start to move from the negative end of the applied potential difference
to the positive end (opposites attract), creating a flow of electrons. This flow of electrons is
called an electrical current. The electrons do not flow freely through the circuit as the real
everyday material they move through creates a restriction to the electron flow. This
restriction is called resistance.
Then all basic electrical or electronic circuits consist of three separate but very much related
electrical quantities called: Voltage, ( v ), Current, ( i ) and Resistance, ( Ω ).
Electrical Voltage
Voltage, ( V ) is the potential energy of an electrical supply stored in the form of an electrical
charge. Voltage can be thought of as the force that pushes electrons through a conductor and
the greater the voltage the greater is its ability to “push” the electrons through a given circuit.
As energy has the ability to do work this potential energy can be described as the work
required in joules to move electrons in the form of an electrical current around a circuit from
one point or node to another.
Then the difference in voltage between any two points, connections or junctions (called
nodes) in a circuit is known as the Potential Difference, ( p.d. ) sometimes called the
Voltage Drop.
The Potential difference between two points is measured in Volts with the circuit symbol V,
or lowercase “v“, although Energy, E lowercase “e” is sometimes used. Then the greater the
voltage, the greater is the pressure (or pushing force) and the greater is the capacity to do
work.
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 to force an electrical current of one ampere through a resistance
of one Ohm. Voltages are generally expressed in Volts with prefixes used to denote submultiples 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 and NZ 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.
Voltage Symbols
A simple relationship can be made between a tank of water and a voltage supply. The higher
the water tank above the outlet the greater the pressure of the water as more energy is
released, the higher the voltage the greater the potential energy as more electrons are
released. The greater the pressure, the more current flows through a particular width of pipe
(resistance).
Voltage is always measured as the difference between any two points in a circuit and the
voltage between these two points is generally referred to as the “Voltage drop“. Any voltage
source whether DC or AC likes an open or semi-open circuit condition but hates any short
circuit condition as this can destroy it, because extreme current will flow.
Electrical Current
Electrical Current, ( I ) is the movement or flow of electrical charge and is measured in
Amperes, symbol I, for intensity). 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, but Traditionally for ease of circuit understanding conventional
current flow assumes that the current flows from the positive to the negative terminal.
Generally in circuit diagrams the flow of current through the circuit usually has an arrow
associated with the symbol, I, or lowercase i to indicate the actual direction of the current
flow. However, this arrow usually indicates the direction of conventional current flow and
not necessarily the direction of the actual flow.
Conventional Current Flow
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.
This was the convention chosen during the discovery of electricity in which the direction of
electric current was thought to flow in a circuit. To continue with this line of thought, in all
circuit diagrams and schematics, the arrows shown on symbols for components such as
diodes and transistors point in the direction of conventional current flow.
Then 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.
Electron Flow
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 milli amps ( 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 as Alternating Current, or A.C..
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 Iaverage . 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
The 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.
Resistor Symbols
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 (‘good’
insulator) made from insulating materials such as glass, porcelain or plastic.
A “semiconductor” on the other hand such as silicon or germanium, is a material whose
resistance is half way between that of a good conductor and a good insulator. Semiconductors
are used to make Diodes and Transistors etc.
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.
Pure Resistance and is not affected by frequency with the AC impedance of a resistance being
equal to its DC resistance and as a result can not 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 milli-ohms, ( mΩ´s ) 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.In other words the ease by which current flows.
High values of conductance implies 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 smaller the diameter of the pipe the larger the resistance to the flow
of water, and therefore the larger the resistance.
The relationship between Voltage, ( v ) and Current, ( i ) in a circuit of constant Resistance,
( R ).
Voltage, Current and Resistance Summary
Hopefully by now you should have some idea of how electrical Voltage, Current and
Resistance are closely related together. The relationship between Voltage, Current and
Resistance forms the basis of Ohm’s law which in a linear circuit states that if we increase
the voltage, the current goes up and if we increase the resistance, the current goes down.
Then we can see that current flow around a circuit is directly proportional ( ∝ ) to voltage,
( V↑ causes I↑ ) but inversely proportional ( 1/∝ ) to resistance as, ( R↑ causes I↓ ).
A basic summary of the three units is given below.






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

Voltage or potential difference is the measure of potential energy between two points in a
circuit and is commonly referred to as its “ volt drop ”.
When a voltage source is connected to a closed loop circuit the voltage will produce a
current flowing around the circuit.
In DC voltage sources the symbols +ve (positive) and -ve (negative) are used to denote the
polarity of the voltage supply.
Voltage is measured in “ Volts ” and has the symbol “ V ” for voltage or “ E ” for energy.
Current flow is a combination of electron flow and hole flow through a circuit.
Current is the continuous and uniform flow of charge around the circuit and is measured in
“ Amperes ” or “ Amps ” and has the symbol “ I ”.
The effective (rms) value of an alternating current has the same average power loss
equivalent to a direct current flowing through a resistive element.
Resistance is the opposition to current flowing around a circuit.
Low values of resistance implies a conductor and high values of resistance implies an
insulator.

Resistance is measured in “ Ohms ” and has the Greek symbol “ Ω ” or the letter “ R ”.
Quantity
Symbol
Unit of
Measure
Abbreviation
Voltage
V or E
Volt
V
Current
I
Ampere
A
Resistance
R
Ohms
Ω
In the next tutorial about DC Theory we will look at Ohms Law which is a mathematical
equation explaining the relationship between Voltage, Current, and Resistance within
electrical circuits and is the foundation of electronics and electrical engineering. Ohm’s Law
is defined as: E = I x R.
Ohms law and Power
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Ohms Law
The relationship between Voltage, Current and Resistance in any DC electrical circuit was
firstly discovered by the German physicist Georg Ohm. Ohm found that, at a constant
temperature, the electrical current flowing through a fixed linear resistance is directly
proportional to the voltage applied across it, and also inversely proportional to the
resistance. This relationship between the Voltage, Current and Resistance forms the basis of
Ohms Law and is shown below.
Ohms Law Relationship
By knowing any two values of the Voltage, Current or Resistance quantities we can use
Ohms Law to find the third missing value. Ohms Law is used extensively in electronics
formulas and calculations so it is “very important to understand and accurately remember
these formulas”.
To find the Voltage, ( V )
[V=IxR]
V (volts) = I (amps) x R (Ω)
To find the Current, ( I )
[I=V÷R]
I (amps) = V (volts) ÷ R (Ω)
To find the Resistance, ( R )
[R=V÷I]
R (Ω) = V (volts) ÷ I (amps)
It is sometimes easier to remember this Ohms law relationship by using pictures. Here the
three quantities of V, I and R have been superimposed into a triangle (affectionately called
the Ohms Law Triangle) giving voltage at the top with current and resistance below. This
arrangement represents the actual position of each quantity within the Ohms law formulas.
Ohms Law Triangle
and transposing the above Ohms Law equation gives us the following combinations of the
same equation:
Then by using Ohms Law we can see that a voltage of 1V applied to a resistor of 1Ω will
cause a current of 1A to flow and the greater the resistance value, the less current that will
flow for a given applied voltage. Any Electrical device or component that obeys “Ohms
Law” that is, the current flowing through it is proportional to the voltage across it ( I α V ),
such as resistors or cables, are said to be “Ohmic” in nature, and devices that do not, such as
transistors or diodes, are said to be “Non-ohmic” devices.
Electrical Power in Circuits
Electrical Power, ( P ) in a circuit is the amount of energy that is absorbed or produced within
the circuit. A source of energy such as a voltage will produce or deliver power while the
connected load absorbs it. Light bulbs and heaters for example, absorb electrical power and
convert it into heat or light. The higher their value or rating in watts the more power they will
consume.
The quantity symbol for power is P and is the product of voltage multiplied by the current
with the unit of measurement being the Watt ( W ). Prefixes are used to denote the various
multiples or sub-multiples of a watt, such as: milliwatts (mW = 10-3W) or kilowatts (kW =
103W).
Then by using Ohm’s law and substituting for the values of V, I and R the formula for
electrical power can be found as:
To find the Power (P)
[P=VxI]
P (watts) = V (volts) x I (amps)
Also,
[ P = V2 ÷ R ]
P (watts) = V2 (volts) ÷ R (Ω)
Also,
[ P = I2 x R ]
P (watts) = I2 (amps) x R (Ω)
Again, the three quantities have been superimposed into a triangle this time called a Power
Triangle with power at the top and current and voltage at the bottom. Again, this
arrangement represents the actual position of each quantity within the Ohms law power
formulas.
The Power Triangle
and again, transposing the basic Ohms Law equation above for power gives us the following
combinations of the same equation to find the various individual quantities:
So we can see that there are three possible formulas for calculating electrical power in a
circuit. If the calculated power is positive, (+P) in value for any formula the component
absorbs the power, that is it is consuming or using power. But if the calculated power is
negative, (-P) in value the component produces or generates power, in other words it is a
source of electrical power such as batteries and generators.
Electrical Power Rating
Electrical components are given a “power rating” in watts that indicates the maximum rate at
which the component converts the electrical power into other forms of energy such as heat,
light or motion. For example, a 1/4W resistor, a 100W light bulb etc.
Electrical devices convert one form of power into another. So for example, an electrical
motor will covert electrical energy into a mechanical force, while an electrical generator
converts mechanical force into electrical energy. A light bulb converts electrical energy into
both light and heat.
Also, we now know that the unit of power is the WATT, but some electrical devices such as
electric motors have a power rating in the old measurement of “Horsepower” or hp. The
relationship between horsepower and watts is given as: 1hp = 746W. So for example, a twohorsepower motor has a rating of 1492W, (2 x 746) or 1.5kW.
Ohms Law Pie Chart
To help us understand the the relationship between the various values a little further, we can
take all of the Ohm’s Law equations from above for finding Voltage, Current, Resistance and
of course Power and condense them into a simple Ohms Law pie chart for use in AC and
DC circuits and calculations as shown.
Ohms Law Pie Chart
As well as using the Ohm’s Law Pie Chart shown above, we can also put the individual
Ohm’s Law equations into a simple matrix table as shown for easy reference when
calculating an unknown value.
Ohms Law Matrix Table
Ohms Law Example No1
For the circuit shown below find the Voltage (V), the Current (I), the Resistance (R) and the
Power (P).
Voltage [ V = I x R ] = 2 x 12Ω = 24V
Current [ I = V ÷ R ] = 24 ÷ 12Ω = 2A
Resistance [ R = V ÷ I ] = 24 ÷ 2 = 12 Ω
Power [ P = V x I ] = 24 x 2 = 48W
Power within an electrical circuit is only present when BOTH voltage and current are
present. For example, in an open-circuit condition, voltage is present but there is no current
flow I = 0 (zero), therefore V x 0 is 0 so the power dissipated within the circuit must also be
0. Likewise, if we have a short-circuit condition, current flow is present but there is no
voltage V = 0, therefore 0 x I = 0 so again the power dissipated within the circuit is 0.
As electrical power is the product of V x I, the power dissipated in a circuit is the same
whether the circuit contains high voltage and low current or low voltage and high current
flow. Generally, electrical power is dissipated in the form of Heat (heaters), Mechanical
Work such as motors, Energy in the form of radiated (Lamps) or as stored energy
(Batteries).
Electrical Energy in Circuits
Electrical Energy is the capacity to do work, and the unit of work or energy is the joule ( J ).
Electrical energy is the product of power multiplied by the length of time it was consumed.
So if we know how much power, in Watts is being consumed and the time, in seconds for
which it is used, we can find the total energy used in watt-seconds. In other words,
Energy = power x time and Power = voltage x current. Therefore electrical power is related to
energy and the unit given for electrical energy is the watt-seconds or joules.
Electrical power can also be defined as the rate of by which energy is transferred. If one joule
of work is either absorbed or delivered at a constant rate of one second, then the
corresponding power will be equivalent to one watt so power can be defined as “1Joule/sec =
1Watt”. Then we can say that one watt is equal to one joule per second and electrical power
can be defined as the rate of doing work or the transferring of energy.
Electrical Power and Energy Triangle
or to find the various individual quantities:
We said previously that electrical energy is define as being watts per second or joules.
Although electrical energy is measured in Joules it can become a very large value when used
to calculate the energy consumed by a component.
For example, if a 100 watt light bulb is left-“ON” for 24 hours, the energy consumed will be
8,640,000 Joules (100W x 86,400 seconds), so prefixes such as kilojoules (kJ = 103J) or
megajoules (MJ = 106J) are used instead and in this simple example, the energy consumed
will be 8.64MJ (mega-joules).
But dealing with joules, kilojoules or megajoules to express electrical energy, the maths
involved can end up with some big numbers and lots of zero’s, so it is much more easier to
express electrical energy consumed in Kilowatt-hours.
If the electrical power consumed (or generated) is measured in watts or kilowatts (thousands
of watts) and the time is measure in hours not seconds, then the unit of electrical energy will
be the kilowatt-hours,(kWhr). Then our 100 watt light bulb above will consume 2,400 watt
hours or 2.4kWhr, which is much easier to understand the 8,640,000 joules.
1 kWhr is the amount of electricity used by a device rated at 1000 watts in one hour and is
commonly called a “Unit of Electricity”. This is what is measured by the utility meter and is
what we as consumers purchase from our electricity suppliers when we receive our bills.
Kilowatt-hours are the standard units of energy used by the electricity meter in our homes to
calculate the amount of electrical energy we use and therefore how much we pay. So if you
switch ON an electric fire with a heating element rated at 1000 watts and left it on for 1 hour
you will have consumed 1 kWhr of electricity. If you switched on two electric fires each with
1000 watt elements for half an hour the total consumption would be exactly the same amount
of electricity – 1kWhr.
So, consuming 1000 watts for one hour uses the same amount of power as 2000 watts (twice
as much) for half an hour (half the time). Then for a 100 watt light bulb to use 1 kWhr or one
unit of electrical energy it would need to be switched on for a total of 10 hours (10 x 100 =
1000 = 1kWhr).
Now that we know what is the relationship between voltage, current and resistance in a
circuit, in the next tutorial about DC Theory we will look at the Standard Electrical Units
used in electrical and electronic engineering to enable us to calculate these values and see that
each value can be represented by either multiples or sub-multiples of the standard unit.
Electrical Units
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Electrical Units of Measure
The standard SI units used for the measurement of voltage, current and resistance are the Volt
[ V ], Ampere [ A ] and Ohm [ Ω ] respectively. Sometimes in electrical or electronic circuits
and systems it is necessary to use multiples or sub-multiples (fractions) of these standard
units when the quantities being measured are very large or very small.
The following table gives a list of some of the standard electrical units of measure used in
electrical formulas and component values.
Standard Electrical Units
Electrical
Parameter
Measuring
Unit
Symbol
Description
Voltage
Volt
V or E
Unit of Electrical Potential
V=I×R
Current
Ampere
I or i
Unit of Electrical Current
I=V÷R
Resistance
Ohm
R or Ω
Unit of DC Resistance
R=V÷I
Conductance
Siemen
G or ℧
Reciprocal of Resistance
G=1÷R
Capacitance
Farad
C
Unit of Capacitance
C=Q÷V
Charge
Coulomb
Q
Unit of Electrical Charge
Q=C×V
Inductance
Henry
L or H
Unit of Inductance
VL = -L(di/dt)
Power
Watts
W
Unit of Power
P = V × I or I2 × R
Impedance
Ohm
Z
Unit of AC Resistance
Z2 = R 2 + X 2
Frequency
Hertz
Hz
Unit of Frequency
ƒ=1÷T
Multiples and Sub-multiples
There is a huge range of values encountered in electrical and electronic engineering between
a maximum value and a minimum value of a standard electrical unit. For example, resistance
can be lower than 0.01Ω’s or higher than 1,000,000Ω’s. By using multiples and submultiple’s
of the standard unit we can avoid having to write too many zero’s to define the position of the
decimal point. The table below gives their names and abbreviations.
Prefix
Symbol
Multiplier
Power of Ten
Terra
T
1,000,000,000,000
1012
Giga
G
1,000,000,000
109
Mega
M
1,000,000
106
kilo
k
1,000
103
none
none
1
100
centi
c
1/100
10-2
milli
m
1/1,000
10-3
micro
µ
1/1,000,000
10-6
nano
n
1/1,000,000,000
10-9
pico
p
1/1,000,000,000,000 10-12
So to display the units or multiples of units for either Resistance, Current or Voltage we
would use as an example:
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
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1kV = 1 kilo-volt – which is equal to 1,000 Volts.
1mA = 1 milli-amp – which is equal to one thousandths (1/1000) of an Ampere.
47kΩ = 47 kilo-ohms – which is equal to 47 thousand Ohms.
100uF = 100 micro-farads – which is equal to 100 millionths (1/1,000,000) of a Farad.
1kW = 1 kilo-watt – which is equal to 1,000 Watts.
1MHz = 1 mega-hertz – which is equal to one million Hertz.
To convert from one prefix to another it is necessary to either multiply or divide by the
difference between the two values. For example, convert 1MHz into kHz.
Well we know from above that 1MHz is equal to one million (1,000,000) hertz and that 1kHz
is equal to one thousand (1,000) hertz, so one 1MHz is one thousand times bigger than 1kHz.
Then to convert Mega-hertz into Kilo-hertz we need to multiply mega-hertz by one thousand,
as 1MHz is equal to 1000 kHz.
Likewise, if we needed to convert kilo-hertz into mega-hertz we would need to divide by one
thousand. A much simpler and quicker method would be to move the decimal point either left
or right depending upon whether you need to multiply or divide.
As well as the “Standard” electrical units of measure shown above, other units are also used
in electrical engineering to denote other values and quantities such as:


• Wh – The Watt-Hour, The amount of electrical energy consumed by a circuit over
a period of time. Eg, a light bulb consumes one hundred watts of electrical power for
one hour. It is commonly used in the form of: Wh (watt-hours), kWh (Kilowatt-hour)
which is 1,000 watt-hours or MWh (Megawatt-hour) which is 1,000,000 watt-hours.
• dB – The Decibel, The decibel is a one tenth unit of the Bel (symbol B) and is used
to represent gain either in voltage, current or power. It is a logarithmic unit expressed
in dB and is commonly used to represent the ratio of input to output in amplifier,
audio circuits or loudspeaker systems.
For example, the dB ratio of an input voltage (Vin) to an output voltage (Vout) is
expressed as 20log10 (Vout/Vin). The value in dB can be either positive (20dB)
representing gain or negative (-20dB) representing loss with unity, ie input = output
expressed as 0dB.

• θ – Phase Angle, The Phase Angle is the difference in degrees between the voltage
waveform and the current waveform having the same periodic time. It is a time


difference or time shift and depending upon the circuit element can have a “leading”
or “lagging” value. The phase angle of a waveform is measured in degrees or radians.
• ω – Angular Frequency, Another unit which is mainly used in a.c. circuits to
represent the Phasor Relationship between two or more waveforms is called Angular
Frequency, symbol ω. This is a rotational unit of angular frequency 2πƒ with units in
radians per second, rads/s. The complete revolution of one cycle is 360 degrees or
2π, therefore, half a revolution is given as 180 degrees or π rad.
• τ – Time Constant, The Time Constant of an impedance circuit or linear first-order
system is the time it takes for the output to reach 63.7% of its maximum or minimum
output value when subjected to a Step Response input. It is a measure of reaction
time.
In the next tutorial about DC Theory we will look at Kirchoff’s Circuit Law which along with
Ohms Law allows us to calculate the different voltages and currents circulating around a
complex circuit.