neub-cse-121-lec-1.pdf Basic-Electric-and-Circuit

NEUB CSE 121 Lecture 1: Introduction to Electrical Engineering
Prepared BY
Shahadat Hussain Parvez
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What is engineering?
Engineering can be simply described as the purposeful use of science.
Oxford dictionary defines engineering as
The branch of science and technology concerned with the design, building, and use of
engines, machines, and structures.
Wikipedia defines engineering as
Engineering is the application of mathematics, science, economics, empirical evidence, and
social and practical knowledge to invent, innovate, design, build, maintain, research, and
improve structures, machines, tools, systems, components, materials, processes, solutions,
and organizations.
No matter how we describe engineering it is clear engineering the application of science in such a
matter, which helps human to lead life easily. Engineering can be also considered as simplification of
scientific knowledge for use in everyday life.
What is Electrical Engineering?
Electrical engineering is the is the branch of engineering concerned with systems that produce,
transmit, and measure electric signals. Electrical engineering combines the physicist's models of
natural phenomena with the mathematician's tools for manipulating those models to produce
systems that meet practical needs. Electrical systems pervade our lives; they are found in homes,
schools, workplaces, and transportation vehicles everywhere. We begin by presenting a few
examples from each of the five major classifications of electrical systems:
• communication systems
• computer systems
• control systems
• power systems
• signal-processing systems
The international system of units
Before diving into details of engineering, we have to recall about the units necessary for
representing any measurements.
Base units:
All units in science are derived from seven base units:
Table 1 Seven base units in SI
Quantity Basic Unit Symbol
Mass kilogram kg
Distance metre m
Time second s
Current ampere A
Amount mole mol
Temperature Kelvin K
Light Intensity candela cd

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Derived units
There are many other units that we use, but all of these are derived by multiplication or division of
some combinations of the base units. You can think of it like letters and words. We have 26 letters
in the alphabet but we have thousands of words in our language. Here are some of the derived
units:
Table 2 Some of derived units
Quantity Unit Symbol Formula
Velocity metre per second ms-1
ms-1
Acceleration metre per second squared ms-2
ms-2
Force Newton N kg ms-2
Work or Energy joule J kg m2
s-2
Pressure Pascal Pa kg m-1
s-2
Frequency hertz Hz s-1
Charge coulomb C A s
Power Watt W J/s
Electric Potential Volt V J/C
Electric Resistance Ohm Ω V/A
Electric Conductance Siemens S A/V
Electric Capacitance Farad F C/V
Magnetic Flux Webber Wb V.S
Inductance Henry H Wb/A
Prefixes
Now you have units, you often need to group these into larger or smaller numbers to make them
more manageable. For example, you don't say that you are going to see someone who lives 100,000
m away from you, you say they live 100 km away from you. Here a quick list of the common
quantities used:
Table 3 Standardized prefixes to signify power of 10
Name Symbol Scaling factor Common example
Peta P 1015
Data center storages are measured in petabytes
Tera T 1012
1,000,000,000,000Large computer hard drives can be terabytes in size.
Giga G 109
1,000,000,000 Computer memories are measured in gigabytes.
mega M 106
1,000,000 A power station may have an output of 600 MW (megawatts).
Kilo k 103
1,000 Mass is often measured in kilogrammes (i.e. 1000 grammes).
Deci d 10-1
0.1 Fluids are sometimes measured in decilitres (i.e. 0.1 litre).
centi c 10-2
0.01 Distances are measured in centimetres (i.e. 100th
of a metre).
Milli m 10-3
0.001 Time is sometimes measured in milliseconds.
Micro µ 10-6
1,000,000th micrometres are often used to measure wavelengths of
electromagnetic waves.
Nano n 10-9
nanometres are used to measure atomic spacing.
Pico p 10-12
picometres used to measure atomic radii.
femto f 10-15
atto a 10-18

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Other system of units
Although now a days all scientific measurements are measured in SI units, before standardization
was common, there were many different system of units used in the past. The usage varied from
regions to regions. Even to this day, there are examples of widespread usage of other systems of
units. Here are some of the common systems of units.
Table 4 Comparison of the English and metric systems of units.
If you have any problem with some mathematical operations used during the course (Conversion
between levels of power of tens, conversion between system of measurements and/or using
calculators for scientific calculations etc), you can refer to chapter 1 of ‘Introductory circuit analysis’
by Boylestad (10th
edition)
Circuit Theory
An electric circuit can be simply defined as interconnection of electrical elements. For and circuit to
allow current to flow there must have a closed path.
Charge is an electrical property of the atomic particles of which matter consists, measured in
coulombs (C).
Here are some of the points that should be noted about charge
1. The coulomb is a large unit for charges. In1Cof charge, there are 1/(1.602 × 10−19
) = 6.24 ×
1018
electrons. Thus realistic or laboratory values of charges are on the order of pC, nC, or
µC.
2. According to experimental observations, the only charges that occur in nature are integral
multiples of the electronic chargee =− 1.602 × 10− 19
C.
3. The law of conservation of charge states that charge can neither be created nor destroyed
only transferred. Thus the algebraic sum of the electric charges in a system does not change.
The rate of flow of charge is known as the electric current.
𝑖 =
𝑑𝑞
𝑑𝑡
(1.1)
Here
𝑖 = The current in amperes
𝑞 = The charge in coulomb
𝑡 = The time in seconds

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Figure 1 Electric current due to flow of electronic
charge in a conductor. Figure 2 Conventional current flow: (a) positive current
flow, (b) negative current flow.
The charge transferred between time t0 and t is obtained by integrating both sides of Eq. (1.1). We
obtain
𝑞 = 𝑖
𝑡
𝑡0
𝑑𝑡 (1.2)
A direct current (dc) is a current that remains constant with time. E.g. Batteries, solar cell, etc
An alternating current (ac) is a current that varies sinusoidally with time. E.g. mains supply,
generator etc.
Figure 3 Two common types of current: (a) direct current (dc), (b) alternating current (ac).
Whenever positive and negative charges are separated, energy is expended. EMF (Voltage) is the
energy per unit charge created by the separation. We express this ratio in differential form as
𝑉 =
𝑑𝑤
𝑑𝑞
(1.3)
Potential Difference (also called voltage) is the energy required to move a unit charge through an
element, measured in volts (V).
Notation plays a very important role in the analysis of electrical and electronic systems. To
distinguish between sources of voltage (batteries and the like) and losses in potential across
dissipative elements, the following notation will be used:
• E for voltage sources (volts)
• V for voltage drops (volts)
V for voltage drops (volts)
An occasional source of confusion is the terminology applied to this subject matter. Terms
commonly encountered include potential, potential difference, voltage, voltage difference (drop or
rise), and electromotive force. As noted in the description above, some are used interchangeably.
The following dentitions are provided as an aid in understanding the meaning of each term:
Here
𝑉 = The voltage in volts
𝑞 = The charge in coulomb
𝑤 = The energy in joules

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Potential: The voltage at a point with respect to another point in the electrical system. Typically the
reference point is ground, which is at zero potential.
Potential difference: The algebraic difference in potential (or voltage) between two points of a
network.
Voltage: When isolated, like potential, the voltage at a point with respect to some reference such as
ground (0 V).
Voltage difference: The algebraic difference in voltage (or potential) between two points of the
system. A voltage drop or rise is as the terminology would suggest.
Electromotive force (emf): The force that establishes the ﬂow of charge (or current) in a system due
to the application of a difference in potential. This term is not applied that often in today’s literature
but is associated primarily with sources of energy.
The ideal Basic circuit Element
An ideal basic circuit element has three attributes: (1) it has only two terminals, which are points of
connection to other circuit components; (2) it is described mathematically in terms of current and/or
voltage; and (3) it cannot be subdivided into other elements. We use the word ideal to imply that a
basic circuit element does not exist as a realizable physical component.
Figure 4 An ideal basic circuit element
Passive sign convention
The passive sign convention uses a positive sign in the expression that relates the voltage and
current at the terminals of an element when the reference direction for the current through the
element is in the direction of the reference voltage drop across the element.
Table 5 Interpretation of Reference Directions in figure 4
Power is the time rate of expending or absorbing energy, measured in watts (W). Mathematically it
can be written as
𝑝 =
𝑑𝑤
𝑑𝑡
(1.4)
So 1 watt is equal to 1 J/S
The power associated with the flow of charge follows directly from the definition of voltage and
current
𝑝 =
𝑑𝑤
𝑑𝑡
=
𝑑𝑤
𝑑𝑞
×
𝑑𝑞
𝑑𝑡
𝑝 = 𝑣𝑖 (1.5)
Here
𝑝 = The Power in watts
𝑡 = The Time in seconds
𝑤 = The energy in joules
Here
𝑣 = The Voltage in volts

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The equation 1.5 is known as the power equation.
Some of the other equations for finding the power are as follows
𝑝 =
𝑣2
𝑅
(1.6)
𝑝 = 𝑖2
𝑅 (1.7)
If the power is positive (that is, if p > 0), power is being delivered to the circuit inside the box. If the
power is negative (that is, if p < 0), power is being extracted from the circuit inside the box.
Passive sign convention is satisfied when the current enters through the positive terminal of an
element and p =+ vi. If the current enters through the negative terminal, p =− vi.
Figure 5 Polarity reference and the expression of power
The law of conservation of energy must be obeyed in any electric circuit. For this reason, the
algebraic sum of power in a circuit, at any instant of time, must be zero:
𝑝 = 0 (1.8)
This confirms the fact that the total power supplied to the circuit must balance the total power
absorbed.
From Eq. (1.5), the energy absorbed or supplied by an element from time t0 to time t is
𝑤 = 𝑝
𝑡
𝑡0
𝑑𝑡 = 𝑣𝑖
𝑡
𝑡0
𝑑𝑡 (1.9)
Energy is the capacity to do work, measured in joules (J).
Some of the other equations for finding the energy are as follows
𝑤 =
𝑣2
𝑅
𝑡 (1.10)
𝑤 = 𝑖2
𝑅 𝑡 (1.11)
The electric power utility companies measure energy in watt-hours (Wh),where
1𝑊ℎ = 3,600 𝐽
Sometimes bills can be measured in Kilo Watt-hour (KWh), where
1𝐾𝑊ℎ = 3,600,000 𝐽
Here
𝑅 = The Resistance in ohm
Here
𝑤 = The Energy in joules

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Circuit Elements
There are two types of elements found in electric circuits: passive elements and active elements. An
active element is capable of generating energy while a passive element is not. Examples of passive
elements are resistors, capacitors, and inductors. Typical active elements include generators,
batteries, and operational amplifiers.
The most important active elements are voltage or current sources that generally deliver power to
the circuit connected to them. There are two kinds of sources: independent and dependent sources.
An ideal independent source is an active element that provides a specified voltage or current that is
completely independent of other circuit variables.
In other words, an ideal independent voltage source delivers to the circuit whatever current is
necessary to maintain its terminal voltage. Physical sources such as batteries and generators may be
regarded as approximations to ideal voltage sources.
Similarly, an ideal independent current source is an active element that provides a specified current
completely independent of the voltage across the source. That is, the current source delivers to the
circuit whatever voltage is necessary to maintain the designated current.
Figure 6 Symbols for independent
voltage sources: (a) used for
constant or time-varying voltage,
(b) used for constant voltage (dc).
Figure 7 Symbol for independent
current source. Figure 8 Symbols for: (a)
dependent voltage source, (b)
dependent current source.
The figures 6 and 7 above shows the symbols for ideal voltage and current sources respectively
An ideal dependent (or controlled) source is an active element in which the source quantity is
controlled by another voltage or current.
Since the control of the dependent source is achieved by a voltage or current of some other element
in the circuit, and the source can be voltage or current, it follows that there are four possible types
of dependent sources, namely:
1. A voltage-controlled voltage source (VCVS).
2. A current-controlled voltage source (CCVS).
3. A voltage-controlled current source (VCCS).
4. A current-controlled current source (CCCS).
Figure 8 above shows the symbols for ideal dependent sources. Dependent sources are useful in
modeling elements such as transistors, operational amplifiers and integrated circuits. An example of
a current-controlled voltage source is shown below
Figure 9 Example of a current controlled voltage source

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Resistance
The ﬂow of charge through any material encounters an opposing force similar in many respects to
mechanical friction. This opposition, due to the collisions between electrons and between electrons
and other atoms in the material, which converts electrical energy into another form of energy such as
heat, is called the resistance of the material. The unit of measurement of resistance is the ohm, for
which the symbol is Ω, the capital Greek letter omega. The circuit symbol for resistance appears in
Fig. 10 with the graphic abbreviation for resistance (R).
Figure 10 Resistance symbol and notation
The resistance of any material with a uniform cross-sectional area is determined by the following
four factors:
1. Material
2. Length
3. Cross-sectional area
4. Temperature
The chosen material, with its unique molecular structure, will react differentially to pressures to
establish current through its core. Conductors that permit a generous ﬂow of charge with little
external pressure will have low resistance levels, while insulators will have high resistance
characteristics.
As one might expect, the longer the path the charge must pass through, the higher the resistance
level, whereas the larger the area (and therefore available room), the lower the resistance.
Resistance is thus directly proportional to length and inversely proportional to area.
As the temperature of most conductors increases, the increased motion of the particles within the
molecular structure makes it increasingly difficult for the “free” carriers to pass through, and the
resistance level increases.
For a fixed temperature the relationship between Resistance and other factor can be summarized by
the equation below
𝑅 =
𝜌𝑙
𝐴
(1.12)
Table 6 Resistivity of common materials
Here
𝜌 = The Resistivity in
𝑙 = The Length in meter
𝐴 = The Cross sectional area in m2

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Ohm’s law states that the voltage v across a resistor is directly proportional to the current i ﬂowing
through the resistor.
i.e. 𝑣 ∝ 𝑖
𝑣 = 𝑖𝑅 (1.13)
The equation 1.13 is one of the simple representation of ohms law.
The resistance R of an element denotes its ability to resist the ﬂow of electric current; it is measured
in ohms (Ω ).
We may deduce the resistance of a material by the formula
𝑅 =
𝑣
𝑖
(1.14)
A short circuit is a circuit element with resistance approaching zero.
An open circuit is a circuit element with resistance approaching infinity.
A useful quantity in circuit analysis is the reciprocal of resistance R, known as conductance and
denoted by G:
𝐺 =
1
𝑅
=
𝑖
𝑣
(1.15)
Conductance is the ability of an element to conduct electric current; it is measured in mhos (℧) or
Siemens (S).
Equations for power and energy can also be rewritten in terms of conductance
𝑝 =
𝑖2
𝐺
(1.16)
𝑝 = 𝑣2
𝐺 (1.17)
𝑤 =
𝑖2
𝐺
𝑡 (1.18)
𝑤 = 𝑣2
𝐺 𝑡 (1.19)
TYPES OF RESISTORS
Fixed Resistors
Resistors whose values are fixed are known as fixed resistors. If only resistor is mentioned, fixed
resistor is considered by default.
Variable Resistors
Variable resistors, as the name implies, have a terminal resistance that can be varied by turning a
dial, knob, screw, or whatever seems appropriate for the application. They can have two or three
terminals, but most have three terminals. If the two- or three-terminal device is used as a variable
resistor, it is usually referred to as a rheostat. If the three-terminal device is used for controlling
potential levels, it is then commonly called a potentiometer. Even though a three-terminal device
can be used as a rheostat or potentiometer (depending on how it is connected).
Figure 11 Potentiometer: (a) symbol; (b) and (c) rheostat connections; (d) rheostat symbol.
Here
Here
𝑝 = The power in watts
𝑤 = The Energy in joules
𝐺 = The Conductance in Siemens

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NODES, BRANCHES, AND LOOPS
A branch represents a single element suchas a voltage source or a resistor.
A node is the point of connection between two or more branches.
A loop is any closed path in a circuit.
Figure 12 Nodes branches and loops Figure 13 Three noded circuit of figure 12 redrawn
A network with b branches, n nodes, and l independent loops will satisfy the fundamental theorem
of network topology:
𝑏 = 𝑙 + 𝑛 – 1 (1.20)
Two or more elements are in series if they are cascaded or connected sequentially and consequently
carry the same current.
Two or more elements are in parallel if they are connected to the same two nodes and consequently
have the same voltage across them.
1. Alexander Example 2.1-2.4
2. Alexander Exercise problems 2.1-2.4 [Based on Ohm’s law]
3. Alexander Exercise problems 2.5-2.7 [Based on Nodes, branches and loops]
4. Boylestad Chapter 1-4 examples
Reference books
1. Introductory Circuit analysis by Robert L. Boylestad
2. Fundamentals of Electric circuits by C. K. Alexander and M. N. O. Sadiku

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