ECA Lecture 1

8/19/2019 ECA Lecture 1

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Copyrights © Syed Usama Yunas , Faculty Member ElectricalEngineering Department

COMSA S !nstitute o" !n"ormation echnology, Attoc# Campus

Circuit Variables

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Helps to produce, transmit and measure electricalsi nals!Helps to produce s"stems t at meet our real li$eneeds!

T ere are $our ma%or cate ories o$ electricals"stems

Communication Systems (tele&ision e'uipment, radiotelescopes, atellite s"stems, radar s"stems and telep ones"stems)!Computer Systems (calculators, personal computers andsupercomputers)!Control Systems (controllin temperatures, pressuresand o* rates in re+neries mec anisms in doors, li tsand ele&ators automatic s"stems li-e autopilot and auto.landin )!Signal $rocessing Systems (ima e processin s"stems

Introduction to /lectrical /n ineerin

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T e International "stem o$ nits is used b" allma%or en ineerin societies most en ineerst rou out t e *orld!

T e I units are based on se&en de+ned 'uantities,listed *it t eir basic units and s"mbols in t e

$ollo*in table

T e International "stem o$ nits

%uantity &asic Unit Sym'olLen t 3eter m3ass 4ilo ram -

Time econd s/lectric current 5mpere 5

T ermod"namictemp

6e ree 4el&in 4

5mount o$substance

3ole mol

Luminous intensit" Candela cd


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ome standardi7ed pre+8es used b" en ineers areas $ollo*s,

tandardi7ed 9re+8es

Prefix Symbol Power of 10

tera T 10 12

giga G 109

mega M 106

kilo k 103

milli m 10-3

micro µ 10-6

nano n 10-9

pico P 10-12


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/lectric c ar e is t e most $undamental concept in/C5!C ar e is bipolar, eit er can be ne ati&e orpositi&e! ;or e8ample, electrons a&e ne ati&ec ar e and protons a&e positi&e c ar e!

C ar e e8ists in discrete 'uantities * ic areinte ral multiples o$ electronic c ar e, 1! C! 5 sin le electron as ne ati&e c ar e o$.1! C and sin le proton as a positi&e

c ar e o$ 1!

C!/lectrical e?ects are attributed due to t eseparation and motion o$ c ar es!

/lectric C ar e

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T e electrical e?ect caused b" t e c ar es inmotion is called electrical current!/lectrical current depends upon t e rate o$ o* o$c ar e and can be e8pressed as,

i $ d%&dt

* ere i @ current in amperes % @ c ar e in coulombs

t @ time in seconds

Current

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5n ideal basic circuit element as t e $ollo*in attributes:

1! It as onl" t*o terminals * ic are points o$ connectionto ot er circuit components!

2! It is described usin current and &olta e!0! It cannot be subdi&ided into sub elements!

T e polarit" re$erence is used $or t e &olta e is indicated b"plus and minus si n! T e direction re$erence is used $or t ecurrent is indicated b" t e current si n! Dot re$erences $or&olta e and current can be summari7ed as $ollo*s,

Ideal Dasic Circuit /lement

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9assi&e si n con&ention states t at‘Whenever the reference direction for thecurrent in an element is in the direction ofreference voltage drop, use a positive sign

convention to related voltage and currentotherwise use a negative sign’

9assi&e i n Con&ention

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T e use$ul output $rom t e s"stem iscon&entionall" e8pressed in terms o$ po*er andener "!9o*er is t e time rate o$ e8pandin or absorbinener "! ;or e8ample, a *ater pump rated A -B

can deli&er more litres per second t an t e onerated A! -B!3at ematicall", po*er can be e8pressed in termso$ ener " and time as

p = dw/dt where p @ po*er in *atts

( @ ener " in %oules t @ time in seconds

9o*er and /ner "

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In terms o$ &olta e and current, po*er can bee8pressed as

p = dw/dt p = (dw/dq ! (dq/dt

p = viwhere p @ po*er in *atts

' @ &olta e in &olts i @ current in amperes

9o*er and /ner "(continuedE)

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I$ po*er is positi&e, po*er is bein deli&ered to t ecircuit inside t e bo8! I$ po*er is ne ati&e, po*er isbein e8tracted $rom t e circuit inside t e bo8!

sin passi&e si n con&ention, po*er can bee8pressed in t e $ollo*in $our *a"s

9o*er and /ner "(continuedE)

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;i&e basic circuit elements are1" #oltage sources2" $urrent sources%" &esistors'" nductors)" $apacitors* ic can be classi+ed on t e basis o$ ener "

eneration as, Acti'e Elements) Circuit elements capable o$

eneratin electrical ener " are called acti&eelements! ;or e8ample, &olta e and currentsources!*assi'e Elements) Circuit elements t at cannot

enerate electrical ener " are called passi&e

Dasic Circuit /lements

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Dasicall" electrical source is a de&ice t at iscapable o$ con&ertin nonelectrical ener " toelectric ener " or &ice &ersa!

;or e8ample, disc ar in batter" co&erts c emicalener " to electrical ener "! On t e ot er and,

c ar in batter" con&erts electrical ener " toc emical ener "!!deal +oltage Source)

Ideal &olta e source is a circuit element t at

maintains a prescribed &olta e across its terminalre ardless o$ t e current o*in in t ose terminal!!deal Current Source)

Ideal current source is a circuit t at maintains a

prescribed current t rou its terminals re ardlesso$ t e &olta e across t ose terminals!

Volta e and current sources

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Dependent sources) T ese sources depend upon ot er &olta e andcurrent sources to maintain &olta e or current in acircuit! 6ependent sources are also calledcontrolled sources! T e circuit s"mbols are as

$ollo*s,

(a) Dependent +oltage!Controlled +oltage Source (') Dependent+oltage!Controlled Current Source

Volta e and current sources(continuedE)

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Let us test some interconnections o$ dependent andindependent sources

Valid Valid

In&alid In&alid


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In&alid Valid

Valid In&alid


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/8ample 1:

1! ;ind V so t at t e interconnection remains&alid!2! ;ind t e po*er associated *it 5 source!


2=

ECA Lecture 1

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