C1 Basic Concepts

8/3/2019 C1 Basic Concepts

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ELECTRIC CIRCUITSELECTRIC CIRCUITS

(BEL 10103)(BEL 10103)LECTURE #01

By:By:Muhammad Hazli MazlanMuhammad Hazli Mazlan

Department of Electronic EngineeringDepartment of Electronic Engineering

Faculty of Electrical and Electronic EngineeringFaculty of Electrical and Electronic Engineering

University Tun Hussein Onn MalaysiaUniversity Tun Hussein Onn Malaysia


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Chapter 1:Chapter 1:

Introduction toIntroduction toElectrical Circuit TheoryElectrical Circuit Theory


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Lecture ContentsLecture Contents

Definitions and UnitsDefinitions and Units

Charge and CurrentsCharge and Currents

Voltage, Energy, and PowerVoltage, Energy, and Power

Circuit ElementsCircuit Elements


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1.1 Definitions and Units1.1 Definitions and Units

Electric circuit, or electric network:Electric circuit, or electric network:

- An electric circuit is an interconnection

of electrical elements in some way.

FIGURE 1: General two-terminal electrical elements

a b


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Example of electric circuitExample of electric circuit

FIGURE 2: A simple electric circuit


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FIGURE 3: Schematic diagram of a radio receiver


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Quantities and SI UnitsQuantities and SI Units

Quantities and SI Units:Quantities and SI Units:

- The International System of Units (SI)

will be used throughout this course.

- One great advantage of the SI unit is

that it uses prefixes based on the power

of 10 to relate larger and smaller units tothe basic unit (Table 3)


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The six basic SI unitsThe six basic SI units

Table 1: The six basic SI Units

QUANTITY BASIC UNIT SYMBOL

Length Meter m

Mass Kilogram kg

Time Second s

Electric current Ampere A

Thermodynamictemperature

Kelvin K

Luminous intensity Candela cd


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Table2: Electrical quantity, symbol and

unit

Quantity Symbol Unit Formula and Unit Charge Q Coulomb (C) Current x time ; As

Current I Ampere (A) Charge / time ; C/s

Energy W Joule (J) Power x time ; Ws

Power P Watt (W) Energy/ time ; J/sVoltage V Volt (V) Energy/charge ; J/C

Resistance R Ohm () Voltage/Current; V/A

Conductance G Siemens (S) Current/Voltage; A/V

Impedance Z Ohm () Voltage/Current; V/AInductor L Henry (H) Weber / Current; Wb/A

Capacitor C Farad (F) Charge/Voltage; C/V

Frequency F Hertz (Hz) 1/time(sec); 1/s

Reactance X Ohm () Voltage/Current; V/A


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Table 3: Prefixes and symbols

Prefix in the SIPrefix in the SI

MULTIPLIER PREFIXES SYMBOL

10^12 tera T

10^09 giga G

10^06 mega M

10^03 kilo k

10^-03 mili m

10^-06 micro

10^-09 nano n

10^-12 pico p


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1.2 Charge and Current1.2 Charge and Current

Charge (Charge (Q or qQ or q):):

- Is an electrical property of the atomic

particles of which matter consist,

measured in Coulomb (C).

- The charge of an electron (negative

charge) and that of a proton (positive

charge) are equal in magnitude


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- Q (Coulomb) = I( ampere) x t( second)

- 1 ampere- hour = 3600 C

- One Coulomb is the total charge

possessed by 6.25 x 10^18 electrons.

-A single electron has a charge of 1.6 x

10^19 C.


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Cont

FIGURE 4: Electric current due to flow of electronic charge in a

conductor

Battery

I


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Explanation of FIGURE 4Explanation of FIGURE 4

When a conducting wire (consisting of severalatoms) is connected to a battery (a source ofelectromotive force).

The charges are compelled to move; positivecharges move in one direction while negativecharges move in the opposite direction.

This motion of charges creates electric

current. It is conventional to take the currentflow as the movement of positive charges,that is, opposite to the flow of negativecharges.


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Electric currentElectric current

Electric current (Electric current (II):):

- Electric current is the time rate of

change of charge, measured in

amperes (A).

- 1 Ampere = 1 coulomb/second (C/s)


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Mathematically:Mathematically:

The relationship between current i, charge q,

and time t, is

dt

dqi = (1)

where current is measured in amperes (A), and1 ampere = 1 coulomb/second

Cont


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The charge transferred between time toand tis obtained by integrating both sides ofEq.(1). We obtain,

=t

to

idtq (2)

Cont


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FIGURE 5:Two common types of current:(a) direct current (dc), (b) alternating current (ac).

CurrentsCurrents


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ContCont

A direct current (dc) is a current that remainsconstant and does not change with time.

By convention the symbol I is used to represent

such a constant current. An alternating current (ac) is a current that varies

sinusoidally with time.

A time-varying current is represented by the

symbol i. A common form of time-varying currentis the sinusoidal current oralternating current(ac).


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Conventional Current FlowConventional Current Flow


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Cont

Once we define that the current as the

movement of charge the direction of

current flow is conventionally taken as the

direction of positive charge movement.

A negative current of -5A flowing in one

direction is the same as a current of +5A

flowing in the opposite direction.


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ExamplesExamples

Given:-

(a) i(t) = 5 sin6 tA. Calculate Q from t=0to t=10ms.

(b) i(t) = e-2tmA . Calculate Q from t=0to t=2s ?


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Solutions


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ExampleExample

How much charge is represented by 4,600

electrons?


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Solution


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1.3 Voltage, Energy and Power1.3 Voltage, Energy and Power

To move the electron in a conductor in a particular

direction requires some work or energy transfer.

This work is performed by an external electromotive

force (emf), typically represented by the battery This emf is also known as voltage orpotential

difference.

The voltage vab between two points a and b in an

electric circuit is the energy (or work) needed tomove a unit charge from a to b; mathematically,


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dq

dwvab = (3)

where wis energy in joules (J) and qis charge in

coulombs (C). The voltage Vab or simply vis measured

in volts (V), From Eq. (3).

It is evident that,

1 volt = 1 joule/coulomb = 1 newton meter/coulomb

ContCont


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VoltageVoltage

Voltage (Voltage (VV):):

Voltage (or potential difference) is the

energy required to move a unit charge

through an element, measured in volts

(V).


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FIGURE 6:Polarity of voltage Vab

Cont


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FIGURE 7:Two equivalentrepresentations of the same voltage Vab:

(a)point ais 9 V above point b,(b)point bis -9 V above point a.

Cont


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Energy (Energy (WW))

Energy is the capacity to do work,measured injoules (J).

Also defined as the rate at which power is

used in a certain length of time. The electric energy used by consumers is

measured in watt-hour (Wh) or kilowatt-

hour (kWh) where1 Wh = 3600 J


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The relationship between energy

and power is given as:

W = P x t (J)1 (J) = 1 (Ws)


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Power (Power (PP))

Power:Power:

- Power is the rate at which energy is

used, measured in watts (W).

In other words, power (P), is a certain amount

of energy (W) used in a certain length of time

(t), expressed as follows:

P = W/t =(W/Q) x (Q/t) = VI (Watt)


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or fortime-varyingpower,

dt

dwp = (4)


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ivp = (5)

Cont

wherep is power in watts (W), wis energy injoules (J), and tis time in seconds (s).

or


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Cont

Power can be delivered or absorbed as defined

by the polarity of the voltage and the direction of

the current.

- +

V

Power delivered or supplied

by voltage source

I

+ -

V

Power absorbed by resistor


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Homework [1]Homework [1]

1. An electrical element draws the currenti(t)=10cos 4tA at a voltage v(t) = 120cos 4tV. Find the energy absorbed by

the element in 2 s.

2. The current of a device is i(t) = 3e-2tA

and the voltage is v(t) = 5di/dtV . Findthe charge delivered betweent = 0 and t= 2s. Calculate the power absorbed.


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Solution Homework [1]Solution Homework [1]


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Solution

1. P = V I = (12) (10 x 10-3) = 0.12W

2. P = W / t = 30 / (5 x 60) = 0.1 WI = P / V = 0.1W / 4V = 0.025A = 25mA


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1.4 Circuit Elements1.4 Circuit Elements

An element is the basic building block of circuit.

An electric circuit is simply an interconnection of

the elements.

Circuit analysis is the process of determiningvoltages across (or the currents through) the

elements of the circuit.

There are two types of elements found in electriccircuits:passive elements and active elements.


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Cont

An active element is capable of generatingenergy 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 voltageor current sources that generally deliver power to

the circuit connected to them. There are two kinds of sources: independent and

dependent sources.


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Independent SourceIndependent Source

An ideal independent source is an active element that

provides a specified voltage or current that is completely

independent of other circuit variables.

FIGURE 7:Symbols for independent voltage sources:(a)used for constant or time-varying voltage,

(b)used for constant voltage (dc).


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A circle is used to represent an

independent source

IV

I


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Dependent SourceDependent Source

An ideal dependent (or controlled) source is an activeelement in which the source quantity is controlled byanother voltage or current.

Dependent sources are usually designated by diamond-shaped symbols.

Since the control of the dependent source is achieved by avoltage or current of some other element in the circuit, andthe source can be voltage or current, it follows that thereare four possible types of dependent source 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).


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Cont

FIGURE 8: Symbol for independent current source.


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FIGURE 9: Symbols for:(a) dependent voltage source(b) dependent current source.

Dependent Source (cont)

A di d i d t t


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A diamond is used to represent a

dependent source.

Is= I

x Vs

=

V

x


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ExampleExample

Voltage controlled voltage source (VCVS)

The parameter is a ratio of two voltages and

therefore is dimensionless.

vx Vs= vx


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ExampleExample

Voltage controlled current source (VCCS)

vxIs=gm vx


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ExampleExample

Current controlled voltage source (CCVS)

Ix

V = m Ix


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ExampleExample

Current controlled current source (CCCS)

ix

Is = ix


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Homework [2]Homework [2]

Calculate the power absorbed by each

component.

P1

P2

P3 P4

+ 16V

-

I6V

22V

6A

0.4I

10A


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Solution Homework [2]

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C1 Basic Concepts