ASOC Exam Manual | SIARS

8/16/2019 ASOC Exam Manual | SIARS

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Chapter-1

ELEMENTARY ELECTRICITY AND MAGNETISM

Electricity

To explain Electricity we need to look deeply inside of everyday objects to consider their primary

ingredients. They are

Atoms

As you probably know, everything in the world, whether solid, liquid,or gas, is made up of atoms. Each atom contains some number of electrons, protons, neutrons, and other subatomic stu!. Thenucleus "central region# of each atom contains the protons "positivecharge# and neutrons "no charge#. Electrons "negative charge# live ina cloud around the outside. $ince electrons and protons are chargedparticles, each atom prefers to have the same number of electronsas protons.

Molecules

Atoms of one or more types are organi%ed into molecules. There are only a hundred or so typesof atoms, but there are an almost in&nite number of di!erent molecules. 'olecules are thebuilding blocks from which all real objects are made.

Electricity

Electricity is the set of physical phenomena associated with the presence and (ow of electriccharge. Electricity gives a wide variety of wellknown e!ects, such as lightning,staticelectricity,electromagnetic induction and the (ow of electrical current. )n addition, electricity

permits the creation and reception of electromagnetic radiation such as radio waves.

The Diferece !et"ee Electricity a# Electroics

*bviously the words +electron+, +electricity+, and +electronics+ are closely related. or historicalreasons, we divide the science and engineering of electrons into two parts- electricity, andelectronics. Electricity is the domain of motors, light bulbs, generators, and other +large scale+items. Electronics has come to mean the domain of vacuum tubes, transistors, integratedcircuits, and other +small scale+ items. e/re covering electricity theory and practice here,because it is an important subset of electronics that can be dealt with as a standalone topic.

TY$ES %& ELECTRICITY

Direct Curret 'DC(

The current, which (ows in one direction in a circuit. 01 voltage has a &xed polarity "e.g. abattery or an electrical cell# and the magnitude of the current remains constant. )n an electricalcircuit, the (ow of electric current is indicated by an arrow mark originating from the positiveterminal of the battery towards the negative terminal of the battery. This is the conventionalmethod of showing the direction of current (ow. 2ut the real direction of electron (ow is fromthe negative terminal of the battery to the positive terminal.

Alterati) Curret 'AC(

Alternating current (ows &rst in one direction then in the opposite direcion. The samede&nitions apply to alternatingvoltage. A1 voltage switches polarity back and forth. A1voltage3current has a waveform which represent the frequency of the source. The waveform of

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the household ac is known as the 4sine5 wave. The magnitude of the A.1. voltage changes withtime. A1 is obtained from A.1. generators.

A#*ata)e o+ A,C, 6eat is developed in all type of electrical circuits due to the (ow of electriccurrent. The magnitude of the 0.1. being constant produces more heat in a circuit compared tothe heat produced by an A.1. )n long distance transmission lines, large amount of power will bedissipated in the form of heat if 0.1. is used which can be reduced by the use of A.1.

Electricity - Co#uctors a# Isulators hen you study highschool chemistry, you/ll learn that some types of molecules feel morestrongly about their electrons than do other types of molecules. This is a very usefulcharacteristic for electricity and electronics.

Co#uctors

$ome materials are willing to let a few electrons move from molecule to molecule. 'aterials thatlet electrons move through them are called +conductors+.

Although some are better than others, most metals are good conductors of electricity. $ilver,

7old, and 8latinum are good conductors but are expensive, so they are only used when price isnot important compared to function. 1opper and Aluminum are also good conductors and arefairly inexpensive. Thus the wiring in our houses is copper, and the high voltage electric lines thatwe see crossing the country use aluminum cables.

Isulators

*ther substances keep their electrons under very tight control. 'aterials that do not letelectrons move through them are called +insulators+.7lass is an example of a type of material that keeps its electrons tightly controlled. 7lass is madeof silicon molecules, organi%ed very tightly in to crystaline structures. 7lass is an extremely goodinsulator. 'any plastics are good insulators too. 8lastics are cheap, (exible, and durable. That is

why the wiring in our houses is covered with a layer of plastic.

hat is char)e.

1harge is an amount of electrons. )ts unit is coulomb "1# and symbol is 4q5. *ne coulomb isequivalent to 9 x :;: ?olts. =ou also know that atransistor radio battery "with the snaps on the top# creates an electrical force of @ ?olts. "Thereare six little cells of :.> ?olts each inside of the @ ?olt battery.# =ou know that householdelectrical outlets are ; or B; ?olts, and you know that +6igh ?oltage+ is dangerous.

hat else should you know about voltageC Again, lets look at theatomic level.

Every atom has its own complement of electrons. )n a conductor, someof those electrons can jump from atom to atom. 2ut electrons don/tmove from atom to atom without a reason. hen electrons are(owing there is always an electrical force pushing them along. erefer to this force as +?oltage+.

)ts unit is volt "?#. *ne volt is de&ned as the potetial #iferece thatrequires one joule of energy to move one coulomb of charge. ?oltage is always measured



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relative to some other point in a circuit, e.g. the potential across a resistor

Curret

)n very simple terms, current is the (ow rate of the electrons in the circuit. 6ow is that di!erentfrom voltageC Det/s use a water tank and a pipe as an example.

)n some neighborhoods you/ll see a water tank raised high above the ground on strong legs. The

water in this tank has been raised up there to create pressure in the system. A series of pipescarry the water down from the tank, under ground, into your house, and then to each sink,bathtub, and toilet. The water in your pipes is under pressure because the water in the tank ispushing down on it. This pressure is similar to ?oltage. ?oltage is the pressure pushing on theelectrons in a circuit.

)f all of the taps in your house are closed, no water (ows through the pipes. )f you open one tap,some water (ows. )f you open all of the tap, a lot of water (ows. This (ow of water is similar toelectrical 1urrent. 1urrent is the (ow rate of electrons through the circuit.

1urrent is the rate of (ow of charge, i.e., the number of coulombs (owing past a point per

second. )ts unit is ampere "A# or amp. *ne ampere is equal to one coulomb per second.

Curret a# $o"er a# Eer)y

The unit used to measure the (ow of electricity, or current, is the ampere "A#. The unit tomeasure power is the watt "#. Energy is measured in oules "#.

Amperes are really a measure of a rate of electron (ow "electrons per second#, or equivalently,how much charge is passed per second "1oulombs per second#. : A F : 1oulomb per second, and: electron has a charge of :.9; G :;:@ 1oulombs.

$o"er is measure in watts. A watt is an amount of energy per second, or oules per second ":

F : 3s#.

The number of watts is for an electrical circuit given by the number of amperes times the voltage: F : ? G : A.

Eer)y is measured in oules "in $) units#, but power companies use units of kilowatt hours "khr#. $ince a watt is just a joule per second "3s#, a kilowatt hour is a measure of energy. 6ere ishow you convertH

: k hr F :;;; hr F :;;; 3s hr F :;;; 3s G I9;; s F I.9 G :;9

Are /olta)e a# Curret Relate#.

?oltage and current are not the same thing, although they areclosely related. )n simple terms, ?oltage causes 1urrent. 7iven a?oltage and a path for the electrons, current will (ow. 7iven thepath, but no ?oltage, or ?oltage without the path, there will beno current.

This picture illustrates a single cell bed lamp. The :.> ?olt cell ispushing the electrons through the bulb and the wire. ithout thispush, the electrons would be happy to remain stationary. )n thiscase, chemical action within the battery causes the push. hen

the battery gets old, its chemical reaction slows down and its internal push gets weaker andweaker. "That/s why the bulb gets dim.#



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ho Does the or0.

1urrent, not ?oltage, does the work in electrical circuits. The (ow of water through a turbine iswhat makes the turbine spin. The (ow of current through an electrical circuit is what lights thebulb, heats the stove, runs the motor, etc. Jouting and controlling the (ow of current is the goalof every electrical circuit.

hat is a circuit. hat are the parts o+ a circuit.

An electronic circuit is a complete course of conductors through which current can travel.1ircuits provide a path for current to (ow. To be a circuit, this path must start and end at thesame point. )n other words, a circuit must form a loop. An electronic circuit and an electricalcircuit has the same de&nition, but electronic circuits tend to be low voltage circuits.

or example, in the above picture it includes four componentsH a battery , a lamp, a switch andthe wire interconnect them. The circuit allows current to (ow from the battery through theswitch to the lamp, then back to the battery. Thus, the circuit forms a complete loop.

There are four parts to a circuitH

":# There is a source of electricity ExampleH A battery"# There is a path along which charges can move. ExampleH A wire"I# There is a s"itch that opens and closes the circuit. ExampleH A switch"B# There is some loa# that uses the electricity, ExampleH A light bulb

Diferece et"ee a complete circuit a# a icomplete circuit,

hen a switch is closed or turned on, the path of electricity is complete. The charges move. Acircuit whose path is complete is called a complete circuit. hen the switch is open, or turnedo!, the path is broken. The movement of charges stops. The path is incomplete. A circuit whosepath is incomplete is called an incomplete circuit.

Resistace

Jesistance is the friction in an electrical circuit that controls the (ow of current. Aspreviously mentioned, voltage causes current. hen a voltage is present and there is apath "circuit# for electron (ow, then there will be a current. The ohm "symbolH K# is theunit of electrical resistance

2uestio3 4o" much curret "ill 5o" i a circuit .

AnswerH As much as can be produced by the pressure"?oltage# working against the friction "Jesistance# of the

circuit.

1losing a valve in your house water system will reduce the(ow,like wise in electricity a resistor will reduce the (ow of current. )n electrical terms, the valve is an adjustableresistor.

ires, like water pipes, have very low resistance. *thercircuit elements are intentionally designed to have speci&camounts of resistance in order to accomplish the operatinggoals of the circuit. Dight bulb &laments, stove heatingcoils, and motor windings all have resistance that regulates

the (ow of current through them.



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hy #o "e ee# Resistace.

Every realworld circuit must include an appropriate resistance so that there is an appropriatecurrent (ow. Jecall the battery and bulb circuit on the voltage3current page. )n that circuit thebulb is the primary circuit resistance. 6ere are three examples of voltage, resistance, andresulting current in that circuitH

1, Not Eou)h Resistace

)f we use a 9 ?olt battery and a bulb that is intended for :.> ?olts, the pressure is too high andthe &lament resistance allows too much current to (ow. e get a single bright (ash as the bulbburns outL

6, Too Much Resistace

)f we use a :.> ?olt battery and a bulb that is intended for 9 ?olts, the pressure is too low and the&lament resistance allows very little current to (ow. The battery lasts for a very long time butthe bulb is too dim to be of much use.

7, E8actly Eou)h Resistace hen we use a :.> ?olt battery and a bulb that is also intended for :.> ?olts, the current is justright. The bulb is bright, but not doesn/t burn out, and the battery lasts for quite a while.

ACTI/E AND $ASSI/E DE/ICES

hat are Acti*e De*ices.

An active device is any type of circuit component with the ability to

electrically control electron (ow . )n order for a circuit to be properlycalled electronic, it must contain at least one active device. Examples of Active devices are vacuum tubes, transistors, siliconcontrolledrecti&ers "$1Js#, and TJ)A1s etc.

All active devices control the (ow of electrons through them.

hat are $assi*e De*ices.

1omponents incapable of controlling current by means of anotherelectrical signal are called passive devices. Jesistors, capacitors,

inductors, transformers, and even diodes are all considered passivedevices.

8assive devices are the resistors, capacitors, and inductors required tobuild electronic hardware. They always have a gain less than one, thusthey can not oscillate or amplify a signal. A combination of passivecomponents can multiply a signal by values less than one, they canshift the phase of a signal, they can reject a signal because it is notmade up of the correct frequencies, they can control complex circuits,but they can not multiply by more than one because they lack gain.

Dio#es

0iodes are basically a oneway valve for electrical current. They let it (ow in one direction "frompositive to negative# and not in the other direction. 'ost diodes are similar in appearance to a



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resistor and will have a painted line on one end showing the direction or (ow "white side isnegative#. )f the negative side is on the negative end of the circuit, current will (ow. )f thenegative is on the positive side of the circuit no current will (ow. 'ore on diodes in latersections.

Ite)rate# Circuits

)ntegrated 1ircuits, or )1s, are complex circuits inside one simple package. $ilicon and metals are

used to simulate resistors, capacitors, transistors, etc. )t is a space saving miracle. Thesecomponents come in a wide variety of packages and si%es. =ou can tell them by their +monolithicshape+ that has a ton of +pins+ coming out of them. Their applications are as varied as theirpackages. )t can be a simple timer, to a complex logic circuit, or even a microcontroller"microprocessor with a few added functions# with erasable memory built inside.

Trasistors

A transistor is a semiconductor device, commonly used as an ampli&er or an electricallycontrolled switch. The transistor is the fundamental building block of the circuitry in computers,cellular phones, and all other modern electronic devices.

Transistors amplify when built into a proper circuit. A weak signal can be boosted tremendously.

Transistors use the properties of semiconductors, seemingly innocuous materials like geraniumand now mostly silicon.

Capacitors

A capacitor is a passive electrical component that can store energy in the electric &eld between apair of conductors "called +plates+#. The process of storing energy in the capacitor is known as+charging+, and involves electric charges of equal magnitude, but opposite polarity, building up

on each plate. A capacitor/s ability to store charge is measured by its capacitance, in units of farads.

1apacitors are often used by engineers in electric and electronic circuits as energystoragedevices. They can also be used to di!erentiate between highfrequency and lowfrequencysignals. This property makes them useful in electronic &lters.A wide variety of capacitors have been invented, including small electrolytic capacitors used inelectronic circuits, basic parallelplate capacitors, mechanical variable capacitors and ceramiccapacitors are among numerous other types of capacitors.

%hm9s La"

The relationship between ?oltage, 1urrent and Jesistance in any 01 electrical circuit was &rstlydiscovered by the 7erman physicist 7eorg *hm. *hm found that, at a constant temperature, theelectrical current (owing through a &xed linear resistance is directly proportional to the voltageapplied across it, and also inversely proportional to the resistance. This relationship between the?oltage, 1urrent and Jesistance forms the bases of *hms Daw

What is the exact relationship between Voltage, Current, and Resistance?

6ere are two statements that describe the relationshipH

:. 7iven a &xed Jesistance, more ?oltage causes more 1urrent.

. 7iven a &xed ?oltage, more Jesistance causes less 1urrent.

"2efore proceeding remember that we use ?olts for electrical pressure, Amps for electrical



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current, and *hms for resistance.#

*hm/s Daw is a set of three related equations that encapsulate the two statements aboveH/: I ; R I : / < R R : / < I

"M?N to represent ?oltage, +)+ to represent 1urrent, and +J+ to represent Jesistance.#

%hm la" tria)le

)t is sometimes easier to remember *hms law relationship by using pictures. 6ere the threequantities of ?, ) and J have been superimposed into a triangle "called the *hms Daw Triangle#giving voltage at the top with current and resistance at the bottom. This arrangementrepresents the actual position of each quantity in the *hms law formulas.

To calculate voltage, V: put your fnger over V, this leaves you with I R, so the euation is V ! I " RTo calculate current, I: put your fnger over I, this leaves you with V over R, so the euation is I ! V#RTo calculate resistance, R: put your fnger over R, this leaves you with V over I, so the euation is R ! V#I

Some E8ample Calculatios

Osing the *hm/s Daw equations above we can analy%e realworld circuits. =es, it is math, but it isreally fairly simple. 8lease don/t give up until you have tried to understand the examples. 7et outyour calculator and follow along.

There are just three stepsH irst, choose the equation form where the facts you already know areon the right side and the answer you want is on the left side. $econd, plug in your known values.Third, do the arithmetic.

6ere are some examples using the same lamp circuit we have already looked atH

1, Suppose that our ul has a resistace o+ => ohms a# "e 0o" that it shoul# e use#"ith a curret o+ >,>7 Amps, hat attery *olta)e is ee#e#.

e know the current and resistance and want to calculate the voltage, so we should choose the&rst equation " ?F ) G J # then we calculate >; G ;.;I which equals :.> so we need a :.>?

battery.

6, No" suppose that "e ha*e a 7 /olt attery a# that our ul has a resistace o+ 1>>ohms, 4o" much curret is 5o"i) "he the ul is lit.

e know the voltage and resistance and want to calculate the current, so we should choose thesecond equation " ) F ? 3 J #we calculate I 3 :;; which equals ;.;Iso ;.;I Amps of current is (owing.

7, No" suppose that "e ha*e a ?,= / attery a# that there is >,>7 Amps o+ curret 5o"i),hat is the ul resistace.

e know the voltage and the current and want to calculate the resistance, so we should choosethe third equation " J F ? 3 ) #



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and we calculate B.> 3 ;.;I which equals :>;so the bulb resistance is :>; ohms.

@irchhof9s La"s - Curret La" /olta)e La"

)n :, 7erman physicist 7ustav Pirchho! &rst described two laws that became central to electrical

engineering. The laws were generali%ed from *hm law . The following descriptions of Pirchho!/s Dawsassume using a constant current. or timevarying current, or alternating current, the laws must be appliedin a more complex method.

@irchhof9s Curret La" or the &irst La"

Pirchho!/s 1urrent Daw, also known as Pirchho!/s unction Daw and Pirchho!/s irst Daw, de&nes the waythat electrical current is distributed when it crosses through a junction a point where three or moreconductors meet. $peci&cally, the law states thatHThe algebraic sum of current into any junction is zero.

$ince current is the (ow of electrons through a conductor, it cannot build up at a junction, meaning thatcurrent is conservedH what comes in must come out. hen performing calculations, current (owing into andout of the junction typically have opposite signs. This allows Pirchho!/s 1urrentDaw to be restated asH

The sum of current into a junction equals the sum of current out of the junction.

@irchhof9s Brst La" i actio

)n the picture to the right, a junction of four conductors "i.e. wires# is shown.The currents i and iI are (owing into the junction, while i : and iB (ow out of it.

)n this example, Pirchho!/s unction Jule yields the following equationH

i6 i7 : i1 i?

@irchhof9s /olta)e La" or Seco# La"

Pirchho!/s ?oltage Daw describes the distribution of voltage within a loop, or closed conducting path, of anelectrical circuit. $peci&cally, Pirchho!/s ?oltage Daw states thatH

The al)eraic sum o+ the *olta)e 'potetial( #ifereces i ay loop must eual ero,

The voltage di!erences include those associated with electromagnetic &elds "emfs# and resistive elements,such as resistors, power sources "i.e. batteries# or devices "i.e. lamps, televisions, blenders, etc.# pluggedinto the circuit.

As you go around a loop, when you arrive at the starting point has the same potential as it did when youbegan, so any increases and decreases along the loop have to cancel out for a total change of ;. )f it didn/t,then the potential at the start3end point would have two di!erent values.

$ositi*e a# Ne)ati*e Si)s i @irchhof9s /olta)e La"

Osing the ?oltage Jule requires some sign conventions, which aren/t necessarily as clear as those in the1urrent Jule. =ou choose a direction "clockwise or counterclockwise# to go along the loop.

hen travelling from positive to negative "Q to # in an emf "power source# the voltage drops, so the value isnegative. hen going from negative to positive " to Q# the voltage goes up, so the value is positive.



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hen crossing a resistor, the voltage change is determined by the formula )GJ, where ) is the value of thecurrent and J is the resistance of the resistor. 1rossing in the same direction as the current means thevoltage goes down, so its value is negative. hen crossing a resistor in the direction opposite the current,the voltage value is positive "the voltage is increasing#.

@irchhof9s /olta)e La" i actio

)f the picture there is a loop ac# . )f you begin at a and advanceclockwise along the interior loop, the ?oltage Daw yields the equationH

*1 *6 *7 *? : >

)n this case, the current will also be clockwise. 1rossing the resistors willresult in v: , v , and vI all being negative. $ince you/re crossing fromnegative to positive, vB will be positive. )f you consider the dotted linethat has the J> resistor, you get a total of three loops in the circuit. The&rst one has already been described. *ne loop is the largest loop and another is the smallest loop at thebottom, to yield the equationsH

*1 *6 *= *? F ; " abcd taking the new path instead of J= #*7 *=F ; "the small loop cd#

The second equation is of special interest, since it indicates that *7 : -*= . This makes sense, because bothcurrents will be travelling from c to d , so on the small loop you/ll cross one resistor with the current and theother resistor against the current. )f the resistors are of equal value, then the current in both paths will beequal.

)n the voltage loop diagram, we see that at junction c there are three conductors. 1urrent enters from thetop, then goes out in the other two directions and, from the 1urrent Daw, we know the algebraic sum of these must be %ero. )f we knew the resistance values of the resistors, and the current coming into the

junction, we could use the ?oltage Daw equations to determine the currents in each of the lower paths, if the resistors were unequal.

Ma)etism

hat is a magnetC

A piece of iron, nickel, cobalt, steel, alloy "e.g. alloy made from nonmagnetic copper, manganese and aluminum# etc. usually in the formof a bar having properties of attracting or repelling iron is called amagnet. 2ut what gives it its force is not completely known. *ne of the theories to describe magnetism is the+Theory of 0omains+. )tsays that materials that can be made into magnets have many tinycrystal like structures called domains. Each domain is made up of many atoms. Each domain has a small magenetic force of its own.hen the material is not magneti%ed, the domains are hapha%ardlyarrangedpointing in all directionsso that their tiny forces canceleach other. To make the material into a magnet, the domains need tobe lined up so that their individual magnetic forces all help eachother pull the same way. hen most of the domains line up, the magnet becomes strong. hen all of thedomains line up in one direction, the magnet is saturated. )t cannot be made any stronger regardless of howmuch you try to magnetise it. )n a magnetic bar, there are two polesH Rorth and $outh. They are marked as4Rorth5 and 4$outh5 poles because, when the magnetic bar is suspended hori%ontally, one of the ends will

always point towards the Earth5s geographical north and the other pole towards the Earth5s geographicalsouth. This is because of the fact that the Earth itself behaves like a huge magnet. )n a magnet, the likepoles repel and the unlike poles attracta reason for the speci&c alignment of the magnetic bar. The


+

–

a b

d c

v4 v2 R2

R3

v3

R1

v1

R5

v5


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magnetic bar is surrounded by the invisible lines of forces which originate from the 4Rorth5 pole andterminate in the 4$outh5 pole.

&erro-ma)et3

)ron, nickel and cobalt "including the alloy mentioned above# are considered ferromagnetic.erromagnetic materials are diScult to be converted to magnet but once magneti%edunder the in(uence of another magnetic &eld, they cannot be completely demagneti%ed.

erromagnetic materials are used to make permanent magnets. *ne of the strongestpermanent magnets is a combination of iron, aluminum, nickel and cobalt called +Alnico+.

Fse o+ $ermaet ma)ets i Electroics3

8ermanent magnets are used in electronics to make electric meters, headphones,loudspeakers,microphones, radar transmitting tubes etc.Electricity and electronics cannot be discussed byleaving apart 4magnetism5 separately.

Electro-Ma)et

e can create an electromagnet by wrapping many turns of wire on a core, then causing an electricalcurrent to (ow through this coil of wire. ith such a device we can create a magnetic &eld that can be

controlled- we can turn it on and o! by controlling the current. ")n fact, by controlling the current gradually,instead of just on and o! as shown here, we could make the magnetic &eld gradually become stronger orweaker.#

)n the picture above, we are controlling the current by connecting and disconnecting the battery. hen thebattery is not connected, there is no current (ow, and therefore no magnetic &eld. hen the battery isconnected, current (ows through the coil, and a magnetic &eld is created, as shown by the lines. ")n the realworld, magnetic &elds are invisible#

Rote that the magnetic &eld curves around from one end of the coil to the other. The &eld is strongest atthe ends of the coil, and gets weaker as we get farther away. e call the two ends of the coil the +poles+. )n

fact, one is +Rorth+ and the other is +$outh+ just like any other magnet.

)f we use iron or steel as the core of our electromagnet, the &eld at the poles will be stronger. )n some



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sense, the magnetic &eld is +focussed+ by the presence of these materials.

The strength of the &eld is proportional to the amount of current (owing and to the number of turns onthe coil. Each turn of wire on the coil contributes to the strength of the magnetic &eld. )f we double thenumber of turns, we can make an electromagnet twice as strong. 2ut, this exact ratio is only true if thecurrent (ow is the same for both coilsL

A electrical ell 4o" it "or0s.

$ee the &gure,hen the switch is pushed

closed the circuit is completed and current(ows through the electromagnetic coil.

:. The iron striker is attracted to the electromagnet and strikes the bell.

. As the striker moves towards the bell,the contact is broken. Electricity stops (owing through thecoil which loses its magnetism.

I. The spring returns the striker to its original position which makes a new contact and so electricity(ows again.

B. 2ack to number : and the cycle repeats itself. The bell will continue to ring as long as the switch is

held closed.

Sel+ i#uctace a# Mutual i#uctace

Sel+ i#uctace

The phenomenon of selfinduction was &rst recogni%ed by the American scientist oseph 6enry. 6e wasable to generate large and spectacular electric arcs by interrupting the current in a large copper coil withmany turns.

$elf induction is that phenomenon in which a change in electric current in a coil produces an induced emf inthe coil itself. $elf induced emf in a coil is directly proportional to the rate of change of electric current in

the coil.i.e.

*r em+ : -L '#I


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Mutual I#uctace

)n the previous tutorial we saw that an inductor generates aninduced emf within itself as a result of the changing magnetic &eldaround its own turns, and when this emf is induced in the samecircuit in which the current is changing this e!ect is called $elfinduction, " D #.

6owever, when the emf is induced into an adjacent coil situatedwithin the same magnetic &eld, the emf is said to be inducedmagnetically, inductively or by 'utual induction, symbol " ' #. Thenwhen two or more coils are magnetically linked together by a common magnetic (ux they are said to havethe property of 'utual )nductance.

'utual )nductance is the basic operating principal of the transformer, motors, generators and any otherelectrical component that interacts with another magnetic &eld. Then we can de&ne mutual induction asthe current (owing in one coil that induces an voltage in an adjacent coil.

igure above illustrates the con&guration of a typical transformer. 6ere, coils of insulated conducting wireare wound around a ring of iron constructed of thin isolated laminations or sheets. The laminationsminimi%e eddy currents in the iron. Eddy currents are circulatory currents induced in the metal by the

changing magnetic &eld. These currents produce an undesirable byproductheat in the iron. Energy loss ina transformer can be reduced by using thinner laminations, very MsoftN "lowcarbon# iron and wire with alarger cross section, or by winding the primary and secondary circuits with conductors that have very lowresistance. Onfortunately, reducing the heat loss increases the cost of transformers. Transformers used totransmit and distribute power are commonly @< to @@ percent eScient. hile eddy currents are a problemin transformers, they are useful for heating objects in a vacuum. Eddy currents are induced in the object tobe heated by surrounding a relatively nonconducting vacuum enclosure with a coil carrying a highfrequency alternating current.

)n a transformer, the iron ensures that nearly all the lines of magnetic (ux passing through one circuit alsopass through the second circuit. $o that each turn of the conducting coils has the same magnetic (ux- thus,the total (ux for each coil is proportional to the number of turns in the coil. As a result, if a source of

sinusoidally varying electromotive force is connected to one coil, the electromotive force in the second coilis given by

Thus, depending on the ratio of Rs to Rp, the transformer can be either a stepup or a stepdown device foralternating voltages. or many reasons, including safety, generation and consumption of electric poweroccur at relatively low voltages. $tepup transformers are used to obtain high voltages before electricpower is transmitted, since for a given amount of power, the current in the transmission lines is muchsmaller. This minimi%es energy lost by resistive heating of the conductors.



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I)itio coil

An ignition coil "also called a spark coil# is an induction coil in an automobile/s ignition system whichtransforms the battery/s low voltage to the thousands of volts needed to create an electric spark in thespark plugs to ignite the fuel. The wire that goes from the ignition coil to the distributor and the highvoltage wires that go from the distributor to each of the spark plugs are called spark plug wires or hightension leads.

4o" a I)itio coil i a petrol e)ie system "or0s

An ignition coil consists of a laminated iron coresurrounded by two coils of copper wire. Onlike apower transformer, an ignition coil has an openmagnetic circuit the iron core does not form aclosed loop around the windings. The energy that isstored in the magnetic &eld of the core is the energythat is transferred to the spark plug.

The primary winding has relatively few turns of heavy wire. The secondary winding consists of thousands of turns of smaller wire, insulated for thehigh voltage by enamel on the wires and layers of oiled paper insulation. The coil is usually insertedinto a metal can or plastic case with insulatedterminals for the high voltage and low voltageconnections. hen the contact breaker closes, it allows a current from the battery to build up in theprimary winding of the ignition coil. The current does not (ow instantly because of the inductance of thecoil. 1urrent (owing in the coil produces a magnetic &eld in the core and in the air surrounding the core.*nce the current has built up to its full level, the contact breaker opens. $ince it has a capacitor connectedacross it, the primary winding and the capacitor form a tuned circuit, and as the stored energy oscillatesbetween the inductor formed by the coil and the capacitor, the changing magnetic &eld in the core of the

coil induces a much larger voltage in the secondary of the coil. The timing of the opening of the contacts"or switching of the transistor# must be matched to the position of the piston in the cylinder. The sparkmust occur after the air3fuel mixture is compressed. The contacts are driven o! a shaft that is driven by theengine crankshaft, or, if electronic ignition is used, a sensor on the engine shaft controls the timing of thepulses.

Tesla Coil

A Tesla coil is an electrical resonanttransformer circuit invented by RikolaTesla around :


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The Tesla coil is one of Rikola Tesla/s most famousinventions. )t is essentially a highfrequency aircoretransformer. )t takes the output from a :;vA1 to severalkilovolt transformer V driver circuit and steps it up to anextremely high voltage. ?oltages can get to be well above:,;;;,;;; volts and are discharged in the form of electrical

arcs. Tesla coils are unique in the fact that they createextremely powerful electrical &elds. Darge coils have beenknown to wirelessly light up (orescent lights up to >; feetaway, and because of the fact that it is an electric &eld thatgoes directly into the light and doesn/t use the electrodes,even burnedout (orescent lights will glow.

A Tesla coil transformer operates in a signi&cantly di!erentfashion from a conventional "i.e., iron core# transformer. )n aconventional transformer, the windings are very tightlycoupled and voltage gain is determined by the ratio of thenumbers of turns in the windings. This works well at normalvoltages, but, at high voltages, the insulation between thetwo sets of windings is easily broken down and this preventsironcored transformers from running at extremely highvoltages without damage.

Onlike those of a conventional transformer "which may couple @WXQ of the &elds between windings#, aTesla coil/s windings are +loosely+ coupled, with a large air gap, and thus the primary and secondarytypically share only :;Y;X of their respective magnetic &elds. )nstead of a tight coupling, the coiltransfers energy "via loose coupling# from one oscillating resonant circuit "the primary# to the other "thesecondary# over a number of radio frequency cycles.

As the primary energy transfers to the secondary, the secondary/s output voltage increases until all of theavailable primary energy has been transferred to the secondary "less losses#. Even with signi&cant sparkgap losses, a welldesigned Tesla coil can transfer over X of the energy initially stored in the primarycapacitor to the secondary circuit. The voltage achievable from a Tesla coil can be signi&cantly greater thana conventional transformer, because the secondary winding is a long single layer solenoid widely separatedfrom the surroundings and therefore well insulated. Also, the voltage per turn in any coil is higher becausethe rate of change of magnetic (ux is at high frequencies.

ith the loose coupling the voltage gain is instead proportional to the square root of the ratio of secondaryand primary inductances. 2ecause the secondary winding is wound to be resonant at the same frequency asthe primary, this voltage gain is also proportional to the square root of the ratio of the primary capacitor to

the stray capacitance of the secondary.


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