Fundamental of Electricity and Magnetism

8/10/2019 Fundamental of Electricity and Magnetism

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INTRODUCTION TO FUNDAMENTALS OF ELECTRICITY AND MAGNETISM

Introduction

This is the first unit of the course on Basic Electricity and Electronics. In this unit we will introduce you about the basicsof electron theory. Here we will discuss how the electricity, electric charge, current, voltage and power originates and

what are their properties. We will also discuss about the various safety measures when operating with electricalapplicances. In addition, this unit also provides a basic idea of magnet and their properties. Finally, some commondevices lie power supply, regulator, stabili!er, "#$, function generator, multimeter and %"# meter will be described inbrief to you

ELECTRON THEORY

To now about electricity and electric charge you must now and understand the Electron Theory, so before you nowabout Electron Theory, you must also now a few other things. These are mainly&' (atter' (olecule' )tom

(atterThings that has some mass and occupies some space, they are nown as (atter. (atter can be categorised as

)fter going through this unit, you will be able to*

E+plain the electron theory.

$utline about the electricity, current, voltage and power.

escribe the different forms of electricity.

-arrate some safety measures.

Illustrate what is magnetism.

Identify some common devices

1.

Learning

Obje

ctives2. Introduction

3. Electron Theory

4. Electricity and Electric Charge

1. Free Electrons and Current

2. Electric otential and !oltage

3. o"er

4. Energy

#. $C and %C

#. &a'ety (easures

1. Earthing

2. &a'ety and recautions

3. Fuse

). &"itches

*. (agnet

+. Fa,iliari-ation "ith &o,e Co,,on $evices. Let /s &u, /0

1. %ns"ers to Chec our rogress

11. Further eadings

12. ossible 5uestion


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following&' olid' %i/uid' 0as

(oleculeIf you go on breaing any matter to smaller and smaller parts, a stage will come when it becomes unbreaable. This the

smallest si!e that a matter can occupy, where all the properties of the matter are preserved. If you try to brea it further,it will lose all i ts properties. This smallest si!e that occupies the complete set of properties of a matter is nown asmolecule.

Every matter 1both living 2 non3living4 is made from the combination of such molecules.

)tomIf molecules are further divided, smaller parts become available which are nown as atom. But the atoms cannot e+istindependently5 the only e+ception to it is that when a molecule is made up of only one atom. The properties of anindividual atom are totally different to that of the matter.

)ccording to the Electron Theory all matters are made of such atoms and these atoms consist of three fundamentalelements, vi!.&' 6roton' -eutron' Electron

The central part of an atom is nown as nucleus. It almost contains all the mass of the whole atom. The nucleousconsists of neutron and proton. 6rotons are positively charged and electrons are negatively charged. -eutrons areneutral i.e. they possess no charge. o in this way the nucleus is positively charged. The electrons move around thenucleus in some orbits. The positive charge of a proton is e/uivalent to the negative charge of an electron. o, in anatom the number of proton and electrons are same. The number of electron and proton depends on the matter and isdifferent for different matter. Fig. 7.7 shows the structure of a Hydrogen atom&

In hydrogen it contains one electron and one proton. Whereas in case of a carbon atom it contains si+ protons in itsnucleus and si+ electrons move around it in two orbits3two electrons in the inner orbit and four in the outer orbit. Thema+imum number of electrons that the orbits can have follows the following mathematical formula3e 8 9 n:9where, n 8 count of the orbit 1orbits are counted from the inner side4

e 8 ma+imum number of electrons that the orbit can haveIt means that for the first orbit, n87 so e89, so i t cannot have more than two electrons. imilarly the second orbit canhave a ma+imum number of eight electrons5 the third one can have 7; and so on. When the number of electrons andprotons are same then they neutralise each other-$W7. )t first it was considered that molecules cannot be further broen in smaller parts and molecule is the smallest part ofany matter, but later scientists found that molecules can be further divided into atoms.9. The electrons found in the outer most orbit of an atom are nown as valance electron. These electrons areresponsible for electric charge and also responsible for some chemical reaction and chemical composition.?. )toms are found in every elements of this universe and scientists have invented 7@A elements till now

ELECTRICITY AND ELECTRIC CHARGE

Whenever some materials lie leather, glass etc are rubbed for a while, then it attracts small pieces of paper and suchlighter things towards it. imilarly, if a comb is rubbed with wool or in dry hair i t also ac/uires similar property. Themeaning of ac/uiring this property is that the material has got charge and this phenomenon of getting charge is calledelectricity. The phenomenon of attraction is due to the reaction between two opposite charges. In this process electronsnever flow freely, it ust changes its position once, so it is nown as static electricity. tatic electricity cannot do anywor, but dynamic electricity does a lot of wor. )ll electrical appliances around us run due to this dynamic electricity.Whenever electricity is produced due to free moving electrons it is called dynamic electrici ty. )s you have found earlier,charges are of two types, vi!.&

'6ositive charge

' -egative charge


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When two similar charges come closer they repel and when two charges of opposite nature come closer, they attracteach other.

The unit of charge is nown as "oulomb and it is e/uivalent to the charge on C9; D 7C electrons

Free Electrons and Current

an atom, the attraction of nucleus to the outermost orbit is poor, the electrons in the outermost orbit are also nown asvalance electron. ometimes they get detached from the atom. These electrons are now free to move from one end toanother. This flow of free electrons is termed as electricity and by the increasing movement of these free electronsproduces electricity flow or current. The number of free electrons in a conductor is more and it is very less in case of aninsulator. If a conductor is connected to the battery terminals, then the free electrons in the conductor flows towards thepositive 14 terminal and current flows from positive 14 terminal towards the negative 134 terminal. Figure 7.9 shows thedirection of electron flow and current flow in a conductor.

"urrent is measured as the rate of flow of free electrons or charge per unit time. (athematically&"8Gtwhere, represents the charge and is measured in coulomb5 t represents time and is measured is second. It meanscoulomb per second is the current and is termed as )mpere. o the unit of current is ampere.

%ET = >-$WEarlier it was not nown that current flow is due to the flow of free electrons. o conventionally it was considered thatcurrent flows from positive terminal to negative terminal where as the electron flow is in the reverse direction. But to

show the current flow direction conventional direction was ept

Electric Potential and Voltage

Whenever a body is to be charged, wor is to be done for charging it and this wor done is stored in the body in theform of 6otential Energy. -ow, the charged body is capable of doing some wor in the form of moving another chargedbody by attraction or repulsion. This capacity to do wor of a charge body is nown as electric potential. The electricpotential can be defined as the wor done for charging a body to a unit charge. (athematically&Electric 6otential, 8WG

Where, W is the amount of wor done, and is measured in oules and is the charge, measured in coulomb. o theunit of electric potential is oulesGcoulomb and it is termed as olt.

-ow, oltage, also termed as 6otential ifference or E.(.F 1Electro (otive Force4. This can be defined as the difference

in electric potentials of two charged bodies.

If two charged bodies have different potentials, then they are said to have a potential difference. For e+ample, let ustae two charged bodies ) and B having different potentials and the potential of B is higher than ). o, as shown in theFigure 7.?, if these two bodies are connected together free electrons from ) will flow to B. This occurs as because, B isat a higher potential and, as the definition of electric potential shows charge is inversely proportional to potential, so thissignifies that B is it at higher potential5 so its charge is less, so naturally charge i.e. the free electrons from ) will rushtowards B

It means that a current flow will occur between these two bodies and whose direction is opposite to that of the electronflow. This current will retain till the potential of these two bodies become e/uivalent. o, whenever there is a differencein potential at two ends of a conductor, the current flow will occur in the conductor. )s the unit of potential is olt, so theunit of potential difference or voltage will also be same i.e. volt

Power

0enerally the rate at which wor is done is nown as power. 6ower can also be defined as the wor done per unit time.)s the unit of wor done is oule so unit of power is ouleGsec and this unit is nown as Watt. In case of electricalsystem whenever voltage is applied across a conductor current flow starts because of the movement of electrons.uring this process of movement the electrons collide with the atoms of the conductor and in this process of collisionheat is produced. This heat produced totally depends on both the applied voltage and the current and it is produced aswor has to be done to overcome the friction due to the electron movement. This wor done per unit time is power and it


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is always e/ual to the product of current and the voltage for a particular circuit. o for electrical system mathematically,we can e+press power as&6ower 164 8 oltage 14 J "urrent 1I4Hence, in a conductor or an electrical circuit power is said to be 7 Watt when for an applied voltage of 7 olt, 7 ) currentflow.

Energy

In the simplest way, energy can be defined as the capacity to do wor. Whenever wor is done by a body, the wor doneis stored in the form of energy. To best understand this, consider the case of electrical conductor. Whenever power isapplied to the conductor heat is produced and this heat is one form of energy. Energy is e/uivalent to the amount ofwor done so the unit of energy is same to that of the wor, i.e. oule

DC and AC

Whenever a circuit having a conductor and a load is connected to a voltage source the current flow starts through theconductor. )nd if the voltage source is a battery having a positive and negative pole then a current wil l flow in a singledirection as shown in the Figure 7.K until and unless a potential difference e+ists between the two battery terminals.

This type of unidirectional current flow is nown as irect "urrent or shortly written as ".)gain if the voltage source is alternating, i.e. the polarity of the voltage source eeps changing in regular interval thanthe current flow through at any instant depends on the polarity of the voltage source at that instant. This type of currentflow whose direction of flow changes at regular interval is commonly nown as )lternating "urrent or shortly written as

)".The main advantages of )" over " is its direction changing property which maes the transformer action possible andthis maes easier for power transmitting to long distances.

SAFETY MEASURES

) certain amount of measures should be followed while woring with electrical appliances for the safe use of suche/uipment. (any a time when electrical appliances are used without following any precautionary measures, it createssafety ha!ards. afety measures, therefore, are a set of rules which need to be observed for trouble free operation ofelectrical appliances and for minimi!ation of ris to users.

Earthing

Earthing is re/uired to ensure safety in all electrical appliances. In most cases, the container or chassis is connected toan earthing pin in the mains outlet. This earthing pin is connected to the earth through a conductor. The earthing wire orconductor is deeply buried in the ground. It may so happen that in any electrical appliances, any live conductor maycome into contact with the chassis and may cause electric shoc. If the chassis is earthed the current will flow to theearthing wire to the earth and this will switch on the circuit breaer. This will not only prevent from electric shoc but also

prevents the appliance from damage.

Safety and Precaution


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While handling any electronic e/uipment, certain precautionary measures are very important. It ensures safety. ) littlecarelessness may cause a severe damage and can be risy. )t times it can be dangerous. "areless handling withelectricity is dangerous and can bring threat to anybody


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surrounding area, but it loses its magnetic property as soon as current flow is stopped. These types of magnets arenown as Electromagnet.

6roperties of a (agnet) magnet has two special properties5 first it can attract iron, nicel, cobalt etc. The materials which are attracted by amagnet are nown as magnetic material. econd, another special property that a magnet has is that if a piece ofmagnet is suspended freely in air its two end always point towards the geographical north3south of the earth. The end

which is always directed towards the geographical north of the freely suspended magnet is nown as the north pole ofthe magnet and the other end which points towards the south is nown as outh 6ole of the magnet.

%ET = >-$W(agnets can never e+ist with a single pole, even if a piece of magnet is broen, both the broen piece will individuallydevelop a -orth 6ole and outh 6ole. 6oles of opposite nature always attract each other and repel the poles of samenature. This is similar to the case of electric charge, lie charges always repel each other and opposite charges attract.

Magnetic FieldThe influence of a magnet can be felt only to a certain area surrounding it. The area or space within which magneticinfluence can be felt is nown as (agnetic Field. The strength of (agnetic Field is not same throughout the field. It isstronger near the poles of the magnets. The magnetic field around a magnet is represented by some imaginary lines5these imaginary lines are nown as magnetic lines of force. The lines of force have a uni/ue direction andconventionally these lines of force starts at the north pole of the magnet and ends at the outh 6ole. The lines of force

of a magnet can be easily traced out with the help of a compass needle. Figure 7.C shows the nature and direction ofthese magnetic lines of force.

(agnetic Flu+(agnetic Flu+ is the measure of the strength of the magnetic field. The amount of magnetic field produced due to amagnetic source is nown as magnetic flu+. In other words it can be said that the magnetic flu+ is the measure of thenumber of lines of force produced by a magnet. (agnetic flu+ is denoted by the 0ree letter LM< 1pronounced as phi4.

(agnetic InductionIt is already mentioned that a current carrying coil or conductor produces magnetic field around it. There is also areverse phenomenon. If a coil is e+posed to a changing magnetic field, in other words, if a coil is placed in such anenvironment or condition where the amount of magnetic lines of force passing through the coil changes at every instantof time, current is generated in the coil. The amount of current generated and the direction depends on the rate ofchange of magnetic flu+ through the coil and the direction of the magnetic lines of force.

Fa#iliari$ation with So#e Co##on De%ices

There are many devices which are needed in laboratory or worshop for the testing, fault finding and repairing ofelectronic goods. ome of the widely used such devices are briefly described bellow&

6ower upply6ower supply is one of the most important devices for all pieces of electronic e/uipment. )ll electronic e/uipments run

on electrical energy, but all electronic devices re/uire different voltage and current. o to supply the specific power tothe electronic e/uipment a power supply device is re/uired for them. 0enerally all electronic devices run on ", so thefirst function of the power supply is to supply " current. )gain, depending on their feathers they are of the following


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types&' Battery 6ower upply' %inear 6ower upply' witched (ode 6ower upply' 6rogrammable 6ower upply' =ninterruptible 6ower upply' Battery 6ower upply * This is one of the simplest, portable and reliable power supplies. The sources of the supply are

electrochemical cells. Here electrical energy is produced due to some chemical reaction.' %inear 6ower upply * In these power supplies the )" available on our mains is converted to " through rectifier andthen to the proper voltage and current level through some filters and regulators. )ll these are discussed in detail in =nit?.' witched (ode 6ower upply * ) switched mode power supply or commonly nown as (6 is slightly different fromthe other power supply types. It does not use any transformer before rectification, the )" coming from mains is directlyconverted to " and this " is sliced in many small pieces using very high speed electronic switch. The si!es of theslices are in accordance with the output re/uirement5 slice si!e goes bigger with the increasing power re/uirement.' 6rogrammable 6ower upply * 6rogrammable power supplies are more advanced and sophisticated. )s the namesuggest the main advantage of the type of power supply is that they can be programmed that means the output voltage,current and fre/uency can be controlled through some programming circuits and digital control through computerinterface.

' =ninterruptible 6ower upply * =ninterruptible 6ower upply or =6 is essential for running e/uipments lie computer.The main advantage of a =6 is that, when there is a power failure in the mains it gives bac up from its battery forsome time so that the device gets ample of time for a proper shut down. When the main power is available, itsimultaneously supplies power to devices that are attached to it and charges the battery. When power supply is off the=6 draws power from the battery. The time up to which it can supply power from battery bacup depends on thecharging capacity of the battery.

#egulator) regulator is a device that helps to maintain the constant characteristics of any process. In case of electricity aregulator is a device that maintains a power supply which is free from fluctuation. ) common 1electrical4 regulator is avoltage regulator which provides regulated voltage supply to electrical e/uipment. It is essential for safety of the device.

)" mains supply always has fluctuations. )s a result the e/uipment may get damaged. ) regulator prevents fluctuationsand supplies constant power. ) voltage regulator is an electronics device which is associated with low powerapplications and ". (ost of the electric devices or electrical circuits need a constant voltage level, below which it willnot wor or if the level is above it the device or the circuit may be damaged or blown. The regulator may be of

electromechanical type5 it may consists of active and passive components. -ow3a3days, they are available in the form ofI". (ore about these I" and most popular regulator I" are discussed in subse/uent units.

tabili!ertabili!ers are mainly voltage stabili!ers. )s the name suggests it is a device meant for stabili!ing voltage. In our )"mains line, the voltage is never stable. It sometimes goes much higher or lower than the rated voltage level for electricale/uipment, and this happens much fre/uently. This condition may cause the device either not to wor or damage. o,here comes the need of voltage stabili!er. oltage stabili!er maintains a constant voltage level at its output, irrespectiveof the voltage level at its input. It uses some electronic circuitry for its operation. It helps not only to eep a constantvoltage level without fluctuation but also helps to filter the unwanted noise coming with the incoming power.

"#$"#$ or "athode #ay $scil loscope is one of the most versatile laboratory instrument, mainly used for studying thewaveform of signal, voltage level, fre/uency. Here the signal to be tested is fed to the input of the "#$ and the waveform of the input signal can be viewed at the isplay =nit. Initially the isplay =nit of a "#$ was a cathode ray tube and

hence the name, now a day modern oscilloscopes have %" or %E screens. The signal that is fed to the "#$ isamplified, filtered and then displayed in the display unit of the "#$. The waveform that can viewed at the display unit isgenerally in the form of two dimensional graph. There are some special controls in the front panel through which thefocus, intensity of the beam, scale of the graph etc can be controlled.


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In this unit we have e+plained to you&(atters have molecule, molecule have atom and atoms have electrons, protons and neutrons3the basic units of electriccharge.Electricity is of two types3tatic and ynamicFree electrons are the main cause of current.The direction of current flow is always opposite to the direction of electron flow."apacity to do wor of charge body is nown as electric potential

For electrical system mathematically we can e+press power as& 6ower 164 8 oltage 14 J "urrent 1I4Energy can be defined as the capacity to do worEarthing is re/uired to ensure safety in all electrical appliancesThe most important point while repairing or replacing any part of any electrical e/uipment, one is to mae sure thatpower is cut off. It prevents one from the shoc.When the current through the fuse goes over the rated current, tremendous heat is produced and conse/uently the fusewire melts. It breas the circuit. )s a result, the circuit is saved from suffering damage.(agnets have the special property of attracting magnetic material.The area or space within which magnetic influence can be felt is nown as (agnetic Field(agnetic Flu+ is the measure of the strength of the magnetic fieldTo supply the specific power to the electronic e/uipment a power supply device is re/uired for them

) regulator in general terms is a device that helps to maintain the constant characteristics of any processoltage stabili!ers are a device meant for stabili!ing voltage"athode #ay $scilloscope is mainly used for studying the waveform of signal, voltage level and fre/uency.With the use of function generator we can generate signals of different waveformWith the use of a multimeter we can measure resistance, current and voltage easily in a single device.%"# meter helps to measure Inductance, capacitance and resistance.

)-WE# T$ "HE"> N$=# 6#$0#E

"HE"> N$=# 6#$0#E377. tate True or False*i. False ii. True iii. False iv. False9. Fill in the Blans*i. olid, li/uid and gasii. 6roton, electron and neutron.

iii. same iv. positive v. e 8 9 n9"HE"> N$=# 6#$0#E397. tate True or False*i. False ii. True iii. False iv. True v. Falsevi. True9. Fill in the Blans*i. alance electron ii. $pposite iii. )mpere.iv. Wor v. "ollusion of electronsvi. oltage 14 J "urrent 1I4vii. "apacity, wor viii. Energy i+. oule +. =nidirect ional +i. "hanges +ii. )""HE"> N$=# 6#$0#E3?tate True or False*i. True ii. False ii i. False iv. True v. False

"HE"> N$=# 6#$0#E3K

Fill in the Blans*i. Three ii. #e/uires iii. "ircuits"HE"> N$=# 6#$0#E3Atate True or False*i. False ii. False iii. True iv. True."HE"> N$=# 6#$0#E3CFill in the Blans*i. upply proper power ii. Wave formiii. ignals of different waveformiv. (ultitester or voltGohm meterv. Inductance, "apacitance, #esistance

F=#THE# #E)I-0

Basic Electronics &B.%.Therea 1 "hand 6ublication4

6rinciples of Electronics &. >. (ehta 2 #ohit (ehta 1 "hand 6ublication4Basic Electronics & 1#a 6ublication4Electricity and (agnetism with Electronics &>.>.Tiwari 1 "hand 6ublication4


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P'SS()&E *UEST('"S

7. escribe briefly the constituents of molecule and atom.9. escribe in short about the origin of electricity and electric charge.?. What are free electrons and how these are related to electric currentOK. efine electric potential and voltage.A. efine power and its unit.

C. efine energy and its unit.P. What is the main difference between )" 2 "O;. escribe the safety measures.Q. iscuss in short about the properties of a magnet.7@. efine magnetic flu+, magnetic lines of force, magnetic field and magnetic induction.77. Briefly describe in short the need of a power supply and its various types.79. Write a short note on #egulator.7?. 0ive a definition of a tabili!er.7K. escribe in brief3 "#$ 2 Function generator.7A. escribe in short 3 (ultimeter and %"# (eter.


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("TR'DUCT('" T' FU"DAME"TA&S 'F E&ECTR(CA& C(RCU(T

+, %earning $bectives-, Introduction., Familiari!ation of "ommon "omponents

1. esistor2. Ca0acitor

3. Inductor

4. elay

#. Trans'or,er

/, Interconnection of 6assive "omponents1. Interconnection o' esistors

2. Interconnection o' Ca0acitors

3. Interconnection o' Inductors

0, Basic %aws of Electrical "ircuits1. Oh,6s La".

2. 7irchho''6s La".

C. #eactance and Impedance

P. oltage and "urrent ivision #ule1, %et =s um =p+2, )nswers to "hec Nour 6rogress++, Further #eadings+-, 6ossible uestion

%E)#-I-0 $BE"TIE

)fter going through this unit, you will be able to*

Identify about the common components used in electrical circuit

"lassify the common components

E+plain the interconnection of basic components in the electrical circuit

efine the relationship between current and voltage

etermine currents and voltages in different circuits by using some basic laws

efine the parameters of common components.

I-T#$="TI$-

In the previous unit you have learnt about the basic concepts of electricity and magnetism. In this unit, you will learn thevarious types of components that can be used in any electrical circuit. This unit describes the interconnection of basiccomponents in electrical circuit. This unit also provides a description of basic laws used in different electrical circuits todetermine currents and voltages. Finally, this unit will focus about the basic rules applicable when components areinterconnected in various ways.

F)(I%I)#IR)TI$- $F "$(($- "$(6$-E-T

)n Electronics %aboratory contains a variety of components, e/uipments and related set3ups. )mong the componentssome are called active while some are called passive.

)ctive "omponents* These components are capable of amplifying or processing an electrical signal on their own.E+amples are vacuum tubes, diodes, transistors, silicon controlled rectifier 1"#4s etc.

6assive "omponents* 6assive components cannot amplify or process signals on their own. These are however havevery important role to play as the active devices can wor only with the aid of passive devices. E+amples of passivedevices are* resistors, capacitors, inductors etc.

Resistor

) resistor is a two3terminal electronic component that resists or opposes the flow of current through it. When currentflows through the resistor, it comes across an opposing force called the resistance. The resistance is defined as theability to resists the flow of electricity through any material. It is measured in ohms 1W4.


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When an electric current of one ampere passes through a component across which a potential difference 1voltage4 ofone volt e+ists, then the resistance of that component is one ohm. In resistors, current is directly proportional to theapplied voltage.

#esistors Types*#esistors are mainly of two types& fi+ed and variable.

They are further classified as&7. Wire3wound resistors.9. carbon3composition type?. carbon3film typeK. (etal film type.

The schematic symbol for fi+ed and variable resistors are shown in Figure 9.7

Fig. 9.7* chematic symbols for fi+ed and variable resistor.

Wire wound resistors*This type of resistors consists of a coil of thin metal wire wound on a ceramic core. These can be either fi+ed or variabletype. "ommonly available wire3wound resistors have resistance values ranging from 7 W to 7@@ W, and power ratingup to about 9@@W.These resistors are e+pensive, not so much affected by temperature and not suitable to use at veryhigh fre/uency.

"arbon3"omposition #esistors*It consists of a solid conducting rod formed of a mi+ture of fine carbon particles and powdered insulating material whichis enclosed in a plastic case. The two ends of the carbon resistance are fi+ed to metal caps with leads used forsoldering in the circuits. These resistors have resistance values ranging from 7W to 9@ (W, and power rating up to 9w.These resistors are smaller in si!e, low in cost, stability is high, and useful in low current circuits. The maindisadvantages are highly sensitive to temperature variations and have a tendency to develop electric noise due topassage of current from one carbon particle to another.


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"arbon Film #esistors*It consists of an insulating rod of ceramic core which is surrounded by a conducting film of carbon. The basic structureof carbon film resistor is shown in Fig. 9.K. They are also nown as the deposited&carbon resistors. )vailable resistancevalues are ranging from 7@W to 7@@ (W and power rating up to 9w. They are ine+pensive, nearly stable, and suitable touse at radio fre/uencies.

(etal Film #esistors*It consists of thin film of carbon coating on to a solid ceramic core. They have greater stability to heat and made in asmall s/uare with leads to fit in the circuits.

ariable resistors*ariable #esistors are those whose resistance can be changed between !ero and a certain ma+imum value. They havethree leads, two fi+ed and one movable. There are two inds of variable resistors3 rheostats and potentiometer. Theycan be wire3wound or carbon type.

When two leads of the resistor 1stationary lead and moving lead4 are oined, the variable resistor may be used as arheostat. #heostats are usually used to control current flowing in the circuit. They are generally used for high energydevice. The symbol for a rheostat is given in Figure 9.A

The potentiometer or Spot is a three terminal variable resistor with an adustable sl iding contact that determines theresistance value in the circuit. It has smaller variable resistances than rheostat and is more efficient. The rheostat worsas potentiometer if all the three terminals are used.Fig. 9.C * ymbol for potentiometer Fig. 9.P * ) 6otentiometer

"olour "oding and tandard #esistance alues*

ome resistors are too small in si!e to print the value of their

Fig* 9.A ymbols for #heostat


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four colour bands are printed at one end around the body. Each colour stands for a digit and it can be translated into anumber by using the standard colour code given in the following table.

Table 9.7* "olour "oding

The colour bands are always read left to right from the end that has the bands closet to i t. The first band represents thefirst figure and the second band the second figure and the third band is decimal multiplier i.e. the number of !eros to

add the first two figures to get the total resistance. The limit of accuracy also called tolerance is indicated by the colourof the fourth band.

The resistance having a colour band se/uence yellow, violet, red and gold. By using the colour coding table you canfind that yellow

and violet colour has the digit of KP. The third colour 1red4 value is 95 hence 7@@ have to be added with KP. The total

resistance value is as follows*

Therefore, the resistance should be within the range K.P>W U @.9?A >W or between K.KCA >W to K.Q?A >W.

Ca3acitor

) capacitor is an electronic component which is capable of storing electric charge when a voltage e+ists across it. Thevoltage applied across a capacitor generates an electric field which stores the electrical energy within it.

It can reduce voltage fluctuations in the circuit and blocs dc signal but easily passes ac signal through it. ) capacitorconsists of two conducting plates separated by an insulating material nown as dielectric. The dielectric may be air,mica, ceramic, paper, polyester, oil etc. When a voltage is applied to these plates, a current flows through it, charging upthe plates with electrons giving one plate a positive charge and the other plate an e/ual and opposite negative charge.This flow of electrons to the plates is nown as the "harging "urrent and continues to flow until the voltage across theplates


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"apacitance*The "apacitors ability to store the electrical charge between its plates is nown as the capacitance ". "apacitance " isalways positive. The unit of capacitance is the farad 1F4. The Farad is a very large unit so micro3farads, nano3farads andpico3farads are generally used.For e+ample*(icrofarad 1VF4 7VF 8 7G7,@@@,@@@ 8 @.@@@@@7 8 7@3C F-anofarad 1nF4 7nF 8 7G7,@@@,@@@,@@@ 8 @.@@@@@@@@7 8 7@3Q F6icofarad 1pF4 7pF 87G7,@@@,@@@,@@@,@@@8@.@@@@@@@@@@@787@379 F

The capacitance depends upon the area of plates, distance between the plates and the nature of insulating material 1ordielectric4.

The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separationbetween the plates. "apacitance also depends on the dielectric constant of the substance separating the plates.When a voltage is applied to a capacitor and if it stores coulomb charge then the capacitance " of the capacitor isthe ratio of the charge to the voltage . It is given as* " 8 G or 8 " + .

"apacitors Types*%ie resistors, all capacitors are also classified into two classes&' Fi+ed capacitor

' ariable capacitor.

Fi+ed capacitors are further classified into&' Electrolytic and' -on3electrolytic capacitors.

The schematic symbols for capacitors are shown in figure below.


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Fig. 9.7@ chematic symbols for capacitors.

-on3Electrolytic "apacitors*mall value capacitors are non3electrolytic or unpolarised and may be connected either way round in the circuit. -on3electrolytic capacitors are divided into following types3

(ica "apacitors*It consists of a mass of alternate parallel layers of mica and metal 1"u or )l4 sheets. )lternate sheets are connectedtogether and leads attached for outside connections. The whole structure is enclosed in a plastic or baelite case.These capacitors have small capacitance values and generally use in radio circuits.Fig. 9.77 (ica "apacitor.

6aper "apacitor*It consists of two metal sheets separated by thin layer of paper, plastic or polyester. The metal foil and paper is then

rolled into a cylindrical shape and enclosed in a plastic case. Two leads are internally attached to the metal foil at eachend of the capacitor. They have large si!e and can be used for both dc and ac circuits.

"eramic "apacitor*It consists of a ceramic layer as dielectric material which is surrounded by two thin metal film that acts as capacitorplates. %eads are connected to each plate and the whole unit is enclosed in a moisture3proof coating. They are verysmall in si!e, have large capacitance value compare to mica and paper capacitor, and can be used in both ac and dccircuits.Electrolytic "apacitor*The electrolytic capacitors are polari!ed and must be connected the correct way round in the circuit. They are notdamaged by heat when soldering. There are two designs of electrolytic capacitors, a+ial and radial. In a+ial, the leadsare attached to each end and in radial, both the leads are at the same end.

The electrolytic capacitors contain an aluminum foil which is coated with a thin insulating film of aluminum o+ide 1)l9$?4on one side. The aluminum plate acts as the positive plate and the o+ide layer is acts as the dielectric of the capacitor.The o+ide is covered with a layer of paper or gau!e saturated in electrolyte of negative plate of the capacitor. )notherlayer of metal is coated over the electrolyte to provide contact between one of the terminals and the electrolyte. Thesecapacitors has a large capacitance value and used in the circuits where only dc voltages are applied across thecapacitors.

(nductor

)n inductor is a component that can opposes a sudden change in current in a circuit. It is shaped as a coil. Whenelectric current flows through the coil, it generates a magnetic field. This magnetic field acts to oppose any change inthe current and trying to eep the current flowing at a steady rate. This opposing force it develops is called induced emf1electromotive force4.

The inductance is the ability of an inductor to produce induced emf when the current varies in it. The unit of inductanceis henry 1H4. )n inductor with an inductance of 7 henry produces an emf of 7 volt when the current through the inductorchanges at the rate of 7 ampere per second.

)n inductor offers high impedance to ac and very low impedance to dc i.e. it blocs ac signal but passes dc signal.


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The schematic symbol of inductor is&

Fig. 9.7A chematic symbol of inductor.

They can also be classified on the basis of their core material. They are*' Iron core' )ir3core inductors.The symbols for air core and iron core are given below.

The inductors can also be classified into&' Filter choes' )udio3fre/uency choes' #adio3fre/uency choes

Filter choe has many turns of wire wound on an iron core and is used in smoothing the pulsating current produced byconverting ac into dc. 0enerally power supplies use filter choes having inductance ranging from 7 H to A@ H, capableof carrying current up to @.A).

)udio3fre/uency choe 1)F"4 is used to provide high impedance to audio fre/uencies 1A@ H! to A >H!4. These aresmaller in si!e and have lower inductance in comparison to filter choe.

#adio3fre/uency choe 1#F"4 is used to tune the radio fre/uencies. 1)bove 7@ >H!4. They are smaller than )F"s evenand have very small value of inductance 1about 9mH4.

ariable InductanceIn some applications variable inductors are used. "ircuits lie tuning circuit, phase shifting circuit and switching of bandsin amplifiers, variable inductors are re/uired. Two variable inductors are shown in the figure. In Fig. 9.7C1c4 inductancecan be varied by switching from one tap on the coil to another. In figure 9.7C1d4 a movable core is shown.

Relay

#elays are one ind of switch, that cannot be made $- or $FF with our hand. Their $- and $FF conditions arecontrolled by an e+tra electronic circuit, so it called an electronic switch. They are mainly used where automaticswitching is re/uired and the switching is mainly controlled by an e+ternal controlling circuit.

#elays are mainly wors on the principle of Electromagnetic Induction. It contains mainly a surrounding of a soft ironcore and a movable conducting arm, which can be moved between two connecting points. The movement is controlledby the coil, when no current flows through the coil, the arm rests on a connecting point, when current flows through the

coil, electromagnetism is induced and the arm gets attracted5 now it moves to the other connecting point. )s the currentthrough the coil is stopped, the arm is also released and the movable arm goes to its original position i.e. it goes bac tothe former connecting point. The whole assembly is enclosed in a plastic enclose which contains five connecting points


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at the outside of the enclosure of which two are connected to the relay driving circuit and among the rest three, onepoint is the movable arm, which is nown as the common point of the relay and the other two are the connecting pointwhere the movable arm rests. )mong the last two connecting point and the common point can be connected to thecircuit for which the switching is re/uired.

The relay can be operated in two modes depending upon to which point the circuit to be switched is connected. The two

modes are3

a. -ormally $pen shortly abbreviated as -G$

b. -ormally "lose shortly abbreviated as -G"

When the relay is operated in -G$ mode the circuit to be switched is connected to the common point 1i.e. the movablearm4 and to the point to which the movable arm moves when current flows through the coil due the relay driving circuitand the circuit to be switched is in $pen condition 1switch is $FF4 when no current flows through the coil. )gain in thecase of -G" mode the circuit to be switched is connected to the common point and to the point where the movable armrests when there is no current through the coil, i.e. it is completely reverse to the -G$ mode, here the circuit to beswitched remains in the "losed condition 1switch is $-4.

#elays available in the maret is rated according to their driving voltage, they are generally available in C, Q and 79

rating and circuit to be switched may be connected up to of the rating of 9?@ and ?).

Transfor#er

Transformer is an electrical device used to adust voltage. They can control the value of current and voltage at theiroutput terminal different from what is fed to them in the input. Their ability to step3up or step3down )" voltage maesthem more popular. Transformer

Documents

Fundamental of Electricity and Magnetism