Basic Electrical and Electronics Engineering Principles

1. Introduction to Units Associated with basic electrical quantities

The system of units used in engineering and science is
the Système Internationaled’Unités (International sys
tem of units), usually abbreviated to SI units, and is
based on the metric system. This was introduced in1960
and is now adopted by the majority of countries as the
official system of measurement.
The basic units in the SI system are listed below with
their symbols:

Quantity                                                              Unit
length                                                           metre, m
mass                                                             kilogram,kg
time                                                             second,s
electriccurrent                                             ampere,A
thermodynamictemperature                       kelvin,K
luminousintensity                                        candela,cd
amountofsubstance                                     mole,mol
Derived SI units use combinations of  basic units and
there are many of them. Two examples are:
Velocity – metres per second(m/s)
Acceleration–metres per second
squared(m/s2)
SI units may be made larger or smaller by using prefixes
which denote multiplication or division by a particular
amount. The six most common multiples, with their
meaning, are listed below:
Prefix     Name                   Meaning
M          mega                  multiplyby1000000(i.e.×106)
k            kilo                     multiplyby1000(i.e.×103)
m           milli                    divideby1000(i.e.×10−3)
μ            micro                 divideby1000000(i.e.×10−6)
n            nano                  divideby1000000000
                                        (i.e.×10−9)

1.1. Basic units in the SI system

   1.2 Charge
The unit of charge is the coulomb (C)where one
coulomb is one ampere second. (1coulomb=6.24×
1018 electrons).The coulomb is defined as the quantity
of electricity which flows past a given point in an electric circuit when a current of one ampere is maintained
for one second. Thus,
charge, in coulombs Q=It
where I is the current in amperes and t is the time in
seconds.

1.3 Force
The unit of force is the newton(N) where one newton
is one kilogram metre per second squared. The newton
is defined as the force which, when applied to a mass of
one kilogram, gives it an acceleration of one metre per
second squared. Thus,
force, in newtons F=ma
where m is the mass in kilograms and a is the acceleration in metres per second squared. Gravitational force, or weight, is mg, where g=9.81m/s2.

1.4 Work
The unit of work or energy is the joule(J) where one
joule is one newton-metre The joule is defined as the work done or energy transferred when a force of one newton is exerted through a distance of one metre in the direction of the force. Thus work one on a body, in joules, W=Fs
where F is the force in newtons and s is the distance in metres moved by the body in the direction of the force.
Energy is the capacity for doing work.

1.5 Power
The unit of power is the watt(W) where one watt is one
joule per second. Power is defined as the rate of doing work or transferring energy. Thus, power, in watts, P=W/t
where W is the work done or energy transferred, in
joules, and t is the time, in seconds. Thus,
energy, in joules, W=Pt

1.6 Electrical potential and e.m.f.

The unit of electric potential is the volt(V), where one
volt is one joule per coulomb. One volt is defined as
the difference in potential between two points in a conductor which, when carrying a current of one ampere, dissipates a power of one watt, i.e.
volts= watts
amperes = joules/second
amperes
= joules
ampere seconds = joules
coulombs
A change in electric potential between two points in
an electric circuit is called a potential difference. The
electromotive force (e.m.f.) provided by a source of
energy such as a battery or a generator is measured in
volts

1.7 Resistance and conductance
The unit of electric resistance is the ohm, where one ohm is one volt per ampere. It is defined as the resistance between two points in a conductor when a constant electric potential of one volt applied at the
two points produces a current flow of one ampere in the conductor. Thus,
resistance,inohms R=V/I

where V is the potential difference across the two points,
in volts, and I is the current flowing between the two
points, in amperes.
The reciprocal of resistance is called conductance
and is measured in siemens(S).Thus
conductance, in siemens G=1
R
where R is the resistance in ohms.

1.8 Electrical power and energy
When a direct current of I amperes is flowing in an
electric circuit and the voltage across the circuit is
V volts, then
power, in watts P=VI
Electrical energy=Power ×time
=VIt joules
Although the unit of energy is the joule, when dealing with large amounts of energy, the unit used is the
kilowatt hour(kWh)where
1kWh=1000 watt hour
=1000×3600 watt seconds or joules
=3600000J

1.9 Summary of terms,units and their symbols

Quantity     Quantity Symbol  Unit         Unit Symbol
Length            l                            metre           m
Mass              m                          kilogram       kg
Time               t                            second          s
Velocity          v                           metres per     m/s or
                                                     second           ms−1
Acceleration   a                           metres per      m/s2or
                                                      second           ms−2
                                                        squared
Force               F                           newton             N
Electrical         Q                           coulomb           C
chargeor
quantity
Electriccurrent   I                            ampere            A
Resistance          R                            ohm
Conductance      G                            siemen           S
Electromotive      E                           volt                 V
force
Potential              V                           volt                 V
difference
Work                    W                           joule              J
Energy                E(orW)                     joule              J
Power                 P                             watt               W

1.2. Key Relationships To Remember

Key Notes on Units

• Prefixes matter:

  • milli (m) =

    ${10}^{-3}$

    , micro (μ) =

    ${10}^{-6}$

    , nano =

    ${10}^{-9}$

    .

  • Example: 1 mA = 0.001 A, 1 μF =

    ${10}^{-6}$

    F.

• Derived relationships:

  • Ohm’s Law:

    $V=I\cdot R$

    .

  • Power:

    $P=V\cdot I$

    .

  • Energy:

    $E=P\cdot t$

    .

• Practical ranges:

  • Household voltage: 120–240 V.

  • Small electronics: 1–12 V.

  • Typical resistors: 1 Ω – 1 MΩ.

  • Capacitors: picofarads (pF) to millifarads (mF).

⚠️ Important Considerations

• Measurement tools:

  • Current → Ammeter (series).

  • Voltage → Voltmeter (parallel).

  • Resistance → Ohmmeter.

  • Power → Wattmeter.

• Safety: High voltage and current can be dangerous. Even small currents (above ~30 mA) can be harmful to humans.

• Applications:

  • Capacitance: energy storage, filtering, timing circuits.

  • Inductance: transformers, motors, filters.

  • Frequency: critical in AC systems and communication.

✅ In summary: Ampere, Volt, Ohm, and Watt form the foundation of electrical measurement, while Coulomb, Farad, Henry, Siemens, Joule, and Hertz extend the framework to cover charge, energy storage, magnetism, and oscillations. These units are interconnected through fundamental laws like Ohm’s Law and energy-power relationships.

2. An introduction to electric circuits

   At the end of this chapter you should be able to:
• appreciate that engineering systems may be represented by block diagrams
• recognize common electrical circuit diagram symbols
• understand that electric current is the rate of movement of charge and is measured in amperes
• appreciate that the unit of charge is the coulomb
• calculate charge or quantity of electricity Q from Q=It
• understand that a potential difference between two points in a circuit is required for current to flow
• appreciatethattheunitofp.d.isthevolt
• understand that resistance opposes current flow and is measured in ohms
• appreciate what an ammeter, a voltmeter, an ohmmeter, a multimeter and an oscilloscope measure
• distinguish between linear and non-linear devices
• state Ohm’s law as V=I /R or I=V/R or R=V/I
• use Ohm’s law in calculations, including multiples and sub-multiples of units
• describe a conductor and an insulator, giving examples of each
• appreciate that electrical power P is given by P=VI=I2R=V2/R watts
• calculate electrical power
• define electrical energy and state its unit
• calculate electrical energy
• state the three main effects of an electric current, giving practical examples of each
• explain the importance  of uses in electrical circuit

2.1. Electrical/electronic system block diagrams

   An electrical/electronic system is a group of components connected together to perform a desired function.
Figure2.1 shows a simple public address system, where a microphone is used to collect acoustic energy in the form of sound pressure waves and converts this
to electrical energy in the form of small voltages
and currents; the signal from the microphone is then
amplified by means of an electronic circuit containing
transistors/integrated circuits before itis applied to the
loudspeaker

2.2. Standard symbols for electrical components

  Symbols are used for components in electrical circuit
diagrams

2.3. Electric current and quantity of electricity

      All atoms consist of protons, neutrons and electrons.
The protons, which have positive electrical charges, and the neutrons, which have no electrical charge, are contained within the nucleus. Removed from the nucleus
are minute negatively charged particles called electrons

3. Resistance variation

      At the end of this chapter you should be able to:
• appreciate that electrical resistance depends on four factors
• appreciate that resistance R=ρl/a, where ρ is the resistivity
• recognize typical values of resistivity and its unit
• perform calculations using R=ρl/a
• define the temperature coefficient of resistance,α
• recognize typical values for α
• perform calculations using Rθ=R0(1+αθ)
• determine the resistance and tolerance of a  fixed resistor from its colour code
• determine the resistance and tolerance of a fixed resistor from its letter and digit code

3.1. Resistance and resistivity

          The resistance of an electrical conductor depends on
four factors, these being:

(a) the length of the conductor,
(b) the cross-sectional area of the conductor, 

(c) the type of material 

(d) the temperature of the material.
Resistance, R, is directly proportional to length, l, of a
conductor, i.e. R ∝ l. Thus, for example, if the length of a piece of wire is doubled, then the resistance is doubled.

3.2. Temperature coefficient of resistance

The temperature coefficient of resistance (TCR) describes how the electrical resistance of a material changes with temperature. Metals generally have a positive coefficient (resistance increases with heat), while semiconductors and some alloys can have negative or near-zero coefficients.

🔑 Definition

• Temperature Coefficient of Resistance (α): The fractional change in resistance per degree Celsius relative to the resistance at a reference temperature (usually 0 °C or 20 °C).

• Formula:

Where:

• $R_t$

  = resistance at temperature

  $t$
• $R_0$

  = resistance at reference temperature

• $\alpha$

  = temperature coefficient of resistance (/°C)

• $\mathrm{\Delta }T$

  = temperature change

📊 Typical Values of α (at 20 °C)

Metals like copper, aluminum, and iron show positive α, while alloys like manganin and constantan are engineered to have near-zero or negative α, making them ideal for precision instruments.

⚡ Why It Matters

• Circuit Design: Resistance changes affect voltage drops and current flow, especially in sensitive electronics.

• Precision Instruments: Alloys with near-zero α are used to build resistors that remain stable across temperature ranges.

• Semiconductors: Negative α means resistance decreases with heat, which is crucial in devices like thermistors.

🌍 Practical Example

Imagine a copper wire with resistance 30 Ω at 20 °C. If the temperature rises to 35 °C:

This ~6% increase can significantly affect current in precision circuits.

⚠️ Key Considerations

• Positive α (Metals): Resistance increases → can cause overheating in conductors.

• Negative α (Semiconductors): Resistance decreases → useful in temperature sensors.

• Near-zero α (Alloys): Stable resistance → essential for accurate measurement devices.