Chapter 1

Basic Electrical (DC Circuit) – Part 1

School of Electrical Engineering
Faculty of Engineering
Universiti Teknologi Malaysia

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 Basic Electrical System
■ Electricity is a form of energy
■ Examples of energy source – hydro, coal, wind, nuclear and
solar
■ Electrical systems permits us easily to transmit energy from a
source of supply to a point of application
■ Electrical engineering is the profession concerned with
systems that produce, transmit and measure electrical signals
■ Examples of electrical systems – power system,
communication system, computer system, control system and
signal procesing system

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 …Basic Electrical System
Basic electrical system;
1. The source - to provide energy for the electrical system,
eg. Battery, generator, socket outlet
2. The load - to absorb the electrical energy supplied by
the source, eg. Lamps, aircond
3. The transmission system - conducts energy from the
source to the load, eg. Insulated wire
4. The control apparatus - permits energy to flow or
interrupts the flow, eg. switch

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 …Basic Electrical System
■ Example of Electrical System

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 …Basic Electrical System
■ Example of Electrical System

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 …Basic Electrical System
■ Example Power System

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 …Basic Electrical System
■ Example Communication System

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 Electrical Quantities & Units
■ Electric Charge, Q
– Charge can be in proton (positive) or electron (negative)
– Energy exists at proton and electron
– Unit: Coulomb (C)
– 1 C = electrical quantity when 1 Ampere current flows for 1 second in a
conductor
■ Current, I
– Rate of charge flows
– Two types: DC and AC
– Unit: Ampere (A)
– 1 A = transfer of 1 C charge in 1 s
– I=Q/t

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 … Electrical Quantities & Units
■ Energy, W
– Capacity for doing work
– Unit: Joule (J)
■ Voltage, V or E
– Potential between 2 points in a circuit
– Unit: Volt (V)
– 1 V = Energy needed to transfer 1 C charge through an
element
– V = W/Q

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 … Electrical Quantities & Units
■ Power, P
– Rate for doing work
– Unit: Joule/s or Watt (W)
– 1 W = Power used when 1 A current flowing through a potential of 1 V
– P = VI
■ Resistance, R
– All conductors have their own resistance
– To limit the flow of current in a circuit
– Unit: Ohm (W)
– 1 W = Element with resistance of 1 W will allow 1 A to pass through if 1 V
voltage is applied across the element
– If R = 0 Ω, then short circuit (large current flow)
– If R = µ , then open circuit (no current flow)

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 … Electrical Quantities & Units
No Quantity Symbol Unit Formula
1 Charge Q Coulomb, C Ixt
2 Current I Ampere, A Q/t
3 Voltage V Volt, V W/Q
4 Energy W Joule, J Pxt
5 Power P Watt, W W/t
6 Resistance R Ohm, Ω V/I
7 Capacitance C Farad, F Q/V
8 Inductance L Henry, H Φ/I
9 Frequency F Hertz, Hz 1/t
10 Impedance Z Ohm, Ω V/I
11 Admittance X Ohm, Ω V/I

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Standard prefixes for the SI unit of measure

Factor Prefix Symbol

1012 tera T
109 giga G
106 mega M
103 kilo k
101 deka da
10-1 desi d
10-2 centi c
10-3 mili m
10-6 micro μ
10-9 nano n
10-12 piko p

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 Current, Voltage and Power Relationships
■ Voltage – normal sign
- Vba +
Iab
a b

+ Vab -
Vab = -Vba
• (Same magnitude, different direction)

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Active and Passive Element
Active Element I
- electrical element that can
supply power to other +
I
element
+
- Current direction of active
element are same with
Active V V
element
voltage direction for active
-
element
-eg. Voltage supply, current -
supply, transistor
I

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…Active and Passive Element

Passive Element I

- electrical element that used,

+
absorb or stored I
power/energy.
+
- Current direction of passive Passive V
element are opposite with element V
voltage direction for passive
element -

-eg. resistor, inductor, -

capacitor
I

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 Basic Electrical Laws - Ohm’s Law
V
R
I

+ V -
I

(a)
(b)

■ Current, I which flows through resistance R is directly

proportional to the voltage across the resistor, I µ V, ie, V =
IR.
■ If voltage V is applied across resistor R, current I which flows
through the resistor is inversely proportional to the resistance
value.

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 …Basic Electrical Laws – Ohm’s Law
■ V=IR

I R

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 … Current, Voltage and Power Relationships
■ Voltage, V = IR Volt
■ Power, P = VI
= I2R
= V2/R

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 Example 1.1

■ What is the power output of an electric kettle

connected to the 24 V mains, when a current of 10 A
flows?

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 Example 1.2
A 9V battery supplies power to a cordless curling iron with
a resistance of 18 ohms. How much current is flowing
through the curling iron?

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 Example 1.3

■ Calculate R and VS, if power that been absorbed

by the resistor R is 1.6mW.

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 Example 1.4

An electric bulb uses 0.5 A of current with voltage

generated being 120 V. Determine the value of
resistance.
Example 1.5
If a current of 0.5 A flows through resistor of 15 Ω,
calculate the voltage drop across the resistor.

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 Example 1.6
(i) For the circuit shown,
determine current flowing and
power absorbed by the resistor I
if the resistance is 1 kΩ and +
+
voltage across it is 10 V. V R
(ii) If the current flowing Vs
through the circuit is 3A and -
power absorbed is 72 W, -
determine the resistor value
and voltage across it.

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 Network Topology
Network topology refers placement of elements in the
network (circuit) and the geometric configuration of the
network
Consider the equivalent circuits drawn below

INSPIRING CREATIVE AND INNOVATIVE MINDS

 Node, Branch and Mesh
Node: a joint point between 2 or more circuit element that
been represented by dot

Branch: a section that have one elements on it and ended

with two node

Mesh: a closed loop path in electrical circuit

Loop: a path whose last node is the same as the starting

node

INSPIRING CREATIVE AND INNOVATIVE MINDS

 Example 1.7
How many node, branch and mesh in the circuit ?

INSPIRING CREATIVE AND INNOVATIVE MINDS

 Series
•Network topology is useful in describing series and parallel
elements

•Two or more elements are in series if they exclusively

share a single node and consequently carry the same
current

5Ω and 10V
are in series

INSPIRING CREATIVE AND INNOVATIVE MINDS

 Parallel
•Two or more elements are in parallel if they are
connected to the same two nodes and consequently have
the same voltage across them

2 Ω, 3 Ω, and 2A
are in parallel

INSPIRING CREATIVE AND INNOVATIVE MINDS

 Example 1.8
How many node, branch and mesh does the circuit have?
Identify the elements that are in series and in parallel.

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 Basic Electrical Laws –
Kirchoff’s Current Law
■ The sum of current entering N

ån
a point is equal to the sum
of current leaving the point
(node). i = 0
■ Algebraic sum of current
entering or leaving any n =1
point is equal to zero.
■ In is the current entering Sientering = Sileaving
(+ve) or leaving (-ve) any
point

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 …Basic Electrical Laws –
Kirchoff’s Current Law
I1
■ I1 - I2 - I3 + I4 - I5 + I6 = 0.
I2
I6

■ I1 + I4 + I6 = I2 + I3 + I5
I3

I5
I4

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 …Basic Electrical Laws –
Kirchoff’s Current Law
Examples;
1. For the network junction shown, calculate the I1 I2
current I3 given that I1 = 3A, I2 = -4A and I4 = 2A
I1 – I2 + I3 – I4 = 0
I4
\ I3 = -I1 + I2 + I4 = -5A I3

2. Determine the relationship between the currents I1,

I2, I4 and I5.
Node a: I1 + I4 – I3 = 0
I3 = I1 + I4
Node b: I3 + I5 – I2 = 0
I3 = I2 – I5
\ I1 + I4 = I2 – I5 or I1 – I2 + I4 + I5 = 0

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 Example 1.9
• An R1 of 20W , an R2 of 40W and R3 of 60W are
connected in parallel across the 120V power line.
Using Kirchhoff current law, determine the total
current.

IT

120V R1 R2 R3

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 Basic Electrical Laws –
Kirchoff’s Voltage Law
■ Sum of voltage applied N
is equal to the sum of
voltage loss
■ Algebraic sum of
å n
V =
n =1
0
voltage in a loop is
equal to zero.
■ Vn is the voltage drop SVsupply = SVdrop
(or supply) in a loop

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 …Basic Electrical Laws –
Kirchoff’s Voltage Law
+ E2 -

+ ■ E1 - E2 = V1 + V2 + V3 + V4.
+ V1 - V2
+ -
E1
+ ■ E1 - E2 - V1 - V2 - V3 - V4 = 0.
V3
-
-

- V4 +

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 Example 1.10
• For the circuit below, find voltages v1 and v2

Ans:
V1=8V
V2= -12V

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