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 What is earthing type of earthing different type of earthing system

Introduction '

 In power system , * grounding or earthing means connecting frame of electrical equipment ( non - current carrying part ) or some electrical part of the system ( e.g , neutral point in a star - connected system , one conductor of the secondary of a transformer etc. ) to earth i.e. soil . This connection to earth may be through a conductor or some other circuit element ( e.g. a resistor , a circuit breaker etc. ) depending upon the situation . Regardless of the method of connection to earth , grounding or earthing offers two principal advantages . First , it provides protection to the power system . For example , if the neutral point of a star - connected system is grounded through a circuit breaker and phase to earth fault occurs on any one line , a large fault current will flow through the circuit breaker . Thecircuit breaker will open to isolate the faulty line . This protects the power system from the harmful effects of the fault . Secondly , earthing of electrical equipment ( e.g. domestic appliances , hand - held tools , industrial motors etc. ) ensures the safety of the persons handling the equipment . For example , if insulation fails , there will be a direct contact of the live conductor with the metallic part ( i.e. frame ) of the equipment . Any person in contact with the metallic part of this equipment will be subjected to a dangerous electrical shock which can be fatal . In this chapter , we shall discuss the importance of grounding or earthing in the line of power system with special emphasis on neutral grounding .

 

 

Grounding or Earthing

 The process of connecting the metallic frame ( i.e. non - current carrying part ) of electrical equip ment or some electrical part of the system ( e.g. neutral point in a star - connected system , one con ductor of the secondary of a transformer etc. ) to earth ( i.e. soil ) is called grounding or earthing . It is strange but true that grounding of electrical systems is less understood aspect of power system . Nevertheless , it is a very important subject . If grounding is done systematically in the line of the power system , we can effectively prevent accidents and damage to the equipment of the power system and at the same time continuity of supply can be maintained . Grounding or earthing may be classified as : ( i ) Equipment grounding ( ii ) System grounding . Equipment grounding deals with earthing the non - current - carrying metal parts of the electrical equipment . On the other hand , system grounding means earthing some part of the electrical system e.g. earthing of neutral point of star - connected system in generating stations and sub - stations .

 

Type of Grounding

 

Equipment Grounding

 

The process of connecting non - current - carrying metal parts ( i.e. metallic enclosure ) of the electri cal equipment to earth ( i.e. soil ) in such a way that in case of insulation failure , the enclosure effectively remains at earth potential is called equipment grounding . 

 

 

System Grounding

The process of connecting some electrical part of the power system (e.g. neutral point of a star connected system, one conductor of the secondary of a transformer etc. ) to earth ( i.e. soil ) is called system grounding . The system grounding has assumed considerable importance in the fast expanding power system. By adopting proper schemes of system grounding, we can achieve many advantages including protection, reliability and safety to the power system network. But before discussing the various aspects of neutral grounding, it is desirable to give two examples to appreciate the need of system grounding.

 

 

Neutral Grounding

 

The process of connecting neutral point of 3 - phase system to earth ( i.e. soil ) either directly or through some circuit element ( e.g. resistance , reactance etc. ) is called neutral grounding . Neutral grounding provides protection to personal and equipment . It is because during earth fault , the current path is completed through the earthed neutral and the protective devices ( e.g. a fuse etc. ) operate to isolate the faulty conductor from the rest of the system . This point is illustrated in

 

 

Solid Grounding

 When the neutral point of a 3 - phase system ( e.g. 3 phase generator , 3 - phase transformer etc. ) is directly * connected to earth ( i.e. soil ) through a wire of neg ligible resistance and reactance , it is called solid grounding or effective grounding .

 

Resistance Grounding

 

In order to limit the magnitude of earth fault current , it is a common practice to connect the neutral point of a 3 - phase system to earth through a resistor . This is called resistance grounding . When the neutral point of a 3 - phase system ( e.g. 3 - phase generator , 3 - phase transformer etc. ) is connected to earth ( i.e. soil ) through a resistor , it is called resistance grounding .

 

what is electromagnetic induction|| do you mean by electromagnetic induction full details

 

 Electromagnetic Induction

What is Electromagnetic Induction?

 When the magnetic flux linking a conductor ( or coil ) changes , an e.m.f. is induced in the conductor . If the conductor ( or coil ) forms a complete loop , a current will flow in it . This phenomenon is called electromagnetic induction . 

 

What is Electromagnetic Induction?

Laws of Electromagnetic Induction

 Faraday's  First Law :-

When the magnetic flux linking a conductor or coil changes , an e.m.f. is induced in it . It does not matter how the change in magnetic flux is brought about . The essence of the first law is that the induced e.m.f. appears in a circuit subjected to changing magnetic field . Second law . 

 Faraday's second  Law :-

The magnitude of the e.m.f. induced in a conductor or coil is directly proportional to the rate of change of flux linkages .,

 

 

 

 Direction of Induced E.M.F. and Current 

The direction of induced e.m.f. and hence the current ( if the circuit is closed ) can be determined by one of the following two methods :

 ( i ) Lenz's law

 ( ii ) Fleming's right - hand rule 

( i ) Lenz's law . 

The induced current will flow in such a direction so as to oppose the cause that produces it .  Let us apply Lenz's law to . Here the N - pole of the magnet is approaching a coil of several turns . As the N - pole of the magnet moves towards the coil , the magnetic flux linking the coil increases . Therefore , an e.m.f. and hence current is induced in the coil according to Faraday's laws of electromagnetic induction . According to Lenz's law , the direction of the induced current 

 

 

will be such so as to oppose the cause that produces it . In the present case , the cause of the induced current is the increasing magnetic flux linking the coil . Therefore , the induced current will set up magnetic flux that opposes the increase in flux through the coil . This is possible only if the left hand face of the coil becomes N - pole . Once we know the magnetic polarity of the coil face , the direction of the induced current can be easily determined by applying right - hand rule for the coil

 

 

 It may be noted here that Lenz's law directly follows from the law of conservation of energy Le in order to set up induced current , some energy must be expended . In the above case , for example , when the N - pole of the magnet is approaching the coil , the induced current will flow in the coil in such a direction that the left - hand face of coil becomes N - pole . The result is that the motion of the magnet is opposed . The mechanical energy spent in overcoming this opposition is converted into electrical energy which appears in the coil . Thus Lenz's law is consistent with the law of conservation of energy . 

( ii ) Fleming's right - hand rule . 

This law is particularly suitable to find the direction of the induced e.m.f. and hence current when the conductor moves at right angles to a stationary magnetic field . It may be stated as under : Stretch out the forefinger , middle finger and thumb of your right hand so that they are at right angles to one another . If the forefinger points in the direction of magnetic field , thumb in the direction of motion of the conductor , then the middle finger will point in the direction of induced current .

 

Consider a conductor AB moving upwards at right angles to a uniform magnetic field as shown in Fig . 9.3 . Applying Fleming's right - hand rule , it is clear that the direction of induced current is from B to A. If the motion of the conductor is downward , keeping the direction of magnetic field unchanged , then the direction of induced current will be from A to B.

 

Fleming's Left Hand Rule : Fleming's left hand rule is applicable to d.c. motor for the direction of the mechanical force experienced . The rule states that if the fore finger , middle finger and the thumb of the left hand are mutually perpendicular to each other and the fore finger points towards the magnetic field . Middle finger points towards electric current and then the thumb gives the direction of force acting on the current carrying conductor .

 

 

Induced E.M.F. 

When the magnetic flux linking a conductor ( or coil ) changes , an e.m.f. is induced in it . This change in flux linkages can be brought about in the following two ways : ( i ) The conductor is moved in a stationary magnetic field in such a way that the flux linking it changes in magnitude . The e.m.f. induced in this way is called dynamically induced e.m.f. ( as in a d.c. generator ) . It is so called because e.m.f. is induced in the conductor which is in motion . ( ii ) The conductor is stationary and the magnetic field is moving or changing . The e.m.f. induced in this way is called statically induced e.m.f. ( as in a transformer ) . It is so called because the e.m.f. is induced in a conductor which is stationary . It may be noted that in either case , the magnitude of induced e.m.f. is given by Ndo / dt or derivable from this relation .

 

 

-Self Inductance and Mutual Inductance :

 Let us take a closed coil and current is supplied to the coil by a source , then it will produce a magnetic flux . Now with the variation of current , the flux will also change . This change of flux will produce an emf in the coil . This generated or induced emf is called the self - induced emf . Now the self inductance ( L ) of a coil is the characteristic or property of the coil , by which an emf is generated when the current is varied through the coil . It is denoted by ' L ' and the unit is Henry . It depends upon the shape of the coil used , square of the number of turns of the coil and neighbouring any magnetic material .

 

 

Mutual Inductance :

 Let us take a coil ' x ' which is placed close to other coil ' y ' . Now if the current of the coil ' x ' can be varied or changed , then an emf will be generated in coil ' y ' . The generated or induced emf in coil ' y ' exist so long as the current in the coil ' x ' changes , but not found when the current through coil ' x ' is steady . This is known as mutual induced emf . The mutual inductance of a coil is the characteristics or property of a coil by which an emf is generated in a coil when there is a variation or change of current through its nearby coil . It is denoted by ' M ' . The unit is Henry . It depends on the closeness of coil and the magnitude of the variation of current ,

 

 

Co - efficient of Self Induction : When the current in a coil changes , the magnetic field also changes and producing an induced emf in the particular coil . This process is known as self induction . The e.m.f. induced in the coil is directly proportional to the rate of change of current , the constant of proportionality is known as the co - efficient of self induction of the coil .

 
Concept of Eddy Current and Eddy Current Loss : 

Eddy Current :

We know that any rate of change of flux produces induced e.m.f. in the core , As Iron core is a conductor , an e.m.f. is also induced in the same manner in the core called eddy current . As the core is closed in itself a current will flow through the core is known as eddy current orcirculating current or faucault current . The magnetude of current depends on the value eddy current and the resistance of the eddy current path . Eddy Current Loss : The loss of electrical energy in the form of heat energy which may be produced by the flow of eddy current induced in the armature core , magnetic core material and the pole by changing e.m.fs is called eddycurrent loss or circulating current loss . We n.Bmax².f.2t2 x Volume of lamination = ( Where , n = Steinmetz constant a f = frequency of t = thickness of P ac fle Far = C

 

Eddy Current Loss : 

The loss of electrical energy in the form of heat energy which may be produced by the flow of eddy current induced in the armature core , magnetic core material and the pole by changing e.m.fs is called eddycurrent loss or circulating current loss . We = n.Bmax².f.²t² x Volume of lamination ( Where , n = Steinmetz constant f = frequency of reverasl t = thickness of lammation ) The eddy current loss may be reduced ( a ) laminations of the core ( b ) Lower the flux density , decrease the loss ( c ) choosing alloy for magnetic core eddycurrent loss is reduced . 

 

What is underground cable system? types, advantages & disadvantages

Underground Cables 

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What is underground cables?

An underground cable essentially consists of one or more conductors covered with suitable insula tion and surrounded by a protecting cover . 

Although several types of cables are available , the type of cable to be used will depend upon the working voltage and service requirements . In general , a cable must fulfill the following necessary requirements : 

( i ) The conductor used in cables should be tinned stranded copper or aluminium of high con ductivity . Stranding is done so that conductor may become flexible and carry more current .

 ( ii ) The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits . 

( iii ) The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is designed . 

( iv ) The cable must be provided with suitable mechanical protection so that it may withstand the rough use in laying it .

 ( v ) The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout .

 

 Construction of Cables 

shows the general construction of a 3 - conductor cable . The various parts are : 

( i ) Cores or Conductors . A cable may have one or more than one core ( conductor ) depending upon the type of service for which it is intended . For instance , the 3 - conductor cable shown in Fig . 11.1 is used for 3 - phase service . The conductors are made of tinned copper or alu minium and are usually stranded in order to provide flexibility to the cable .

 ( ii ) Insulatian . Each core or conductor is provided with a suitable thickness of insulation , the thickness of layer depending upon the voltage to be withstood by the cable . The commonly used materials for insulation are impregnated paper , varnished cambric or rubber mineral compound .

 ( iii ) Metallic sheath . In order to pro tect the cable from moisture , gases or other damaging liquids ( acids or alkalies ) in the soil and atmosphere , a metallic sheath of lead or aluminium is provided over the insulation

 

 ( iv ) Bedding . Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape . The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring .

 ( v ) Armouring . Over the bedding , armouring is provided which consists of one or two layers of galvanised steel wire or steel tape . Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling . Armouring may not be done in the case of some cables . 

( vi ) Serving . In order to protect armouring from atmospheric conditions , a layer of fibrous material ( like jute ) similar to bedding is provided over the armouring . This is known as serving . It may not be out of place to mention here that bedding , armouring and serving are only applied to the cables for the protection of conductor insulation and to protect the metallic sheath from mechanical injury .

 

Insulating Materials for Cables

 The satisfactory operation of a cable depends to a great extent upon the charac teristics of insulation used . Therefore , the proper choice of insulating material for cables is of considerable importance . In general , the insulating materials used in cables should have the following properties : 

( i ) High insulation resistance to avoid leakage current .

 ( ii ) High dielectric strength to avoid electrical breakdown of the cable .

 ( iii ) High mechanical strength to withstand the mechanical handling of cables .

 ( iv ) Non - hygroscopic i.e. , it should not absorb moisture from air or soil . The moisture tends to decrease the insulation resistance and hastens the breakdown of the cable . In case the insulating material is hygroscopic it must be enclosed in a waterproof covering like lead sheath .

  ( v ) Non - inflammable .

( vi ) Low cost so as to make the underground system a viable proposition . 

( vii ) Unaffected by acids and alkalies to avoid any chemical action .

 

 Classification of Cables

 Cables for underground service may be classified in two ways according to ( i ) the type of insulating material used in their manufacture ( ii ) the voltage for which they are manufactured . However , the latter method of classification is generally preferred , according to which cables can be divided into the following groups : 

( i ) Low - tension ( L.T. ) cables - upto 1000 V 

( ii ) High - tension ( H.T. ) cables -

 ( iii ) Super - tension ( S.T. ) cables

 ( iv ) Extra high - tension ( E.H.T. ) cables

 ( v ) Extra super voltage cables -beyond 132 kV

 A cable may have one or more than one core depending upon the type of service for which it is intended . It may be ( 1 ) single - core ( ii ) two - core ( iii ) three - core ( iv ) four - core etc. For a3 - phase service , either 3 - single - core cables or three - core cable can be used depending upon the operating voltage and load demand .

 

Laying of Underground Cables

The reliability of underground cable network depends to a considerable extent upon the proper lay and attachment of fittings i.e. , cable end boxes , joints , branch con nectors etc. There are three main methods of laying underground cables viz . , direct laying , draw - in system and the solid system . 

1. Direct laying . 

This method of laying underground cables is simple and cheap and is much favoured in modern practice . In this method , a trench of about 1-5 metres deep and 45 cm wide is dug . The trench is covered with a layer of fine sand ( of about 10 cm thickness ) and the cable is laid over this sand bed . The sand prevents the entry of moisture from the ground and thus protects the cable from decay . After the cable has been laid in the trench , is covered with another layer of sand of about 10 cm thickness . The trench is then covered with bricks and other materials in order to protect the cable from mechani cal injury . When more than one cable is to be laid in the same trench , a horizontal or vertical inter axial spacing of atleast 30 cm is provided in order to reduce the effect of mutual heating and also to ensure that a fault occurring on one cable does not damage the adjacent cable . Cables to be laid in this way must have serving of bituminised paper and hessian tape so as to provide protection against corrosion and electorlysis .

 Advantages

 ( i ) It is a simple and less costly method . 

( ii ) It gives the best conditions for dissipating the heat generated in the cables .

 ( iii ) It is a clean and safe method as the cable is invisible and free from external disturbances . 

Disadvantages 

( i ) The extension of load is possible only by a completely new excavation which may cost as much as the original work . 

( ii ) The alterations in the cable netwok cannot be made easily . 

( iii ) The maintenance cost is very high . 

( iv ) Localisation of fault is difficult .

 ( v ) It cannot be used in congested areas where excavation is expensive and inconvenient .

 

2. Draw - in system .

In this method , conduit or duct of glazed stone or cast iron or concrete are laid in the ground with manholes at suitable positions along the cable route . The cables are then pulled into position from manholes . Fig . 11.11 shows section through four - way underground duct line . Three of the ducts carry transmis sion cables and the fourth duct carries relay protection con nection , pilot wires . Care must be taken that where the duct line changes direction ; depths , dips and offsets be made with a very long radius or it will be difficult to pull a large cable between the manholes . The distance between the manholes should not be too long so as to simplify the pull ing in of the cables . The cables to be laid in this way need not be armored but must be provided with serving of hessian and jute in order to protect them when being pulled into the ducts . Advantages

Advantages 

(i)Repairs , alterations or additions to the cable network can be made without opening the ground . ( ii ) As the cables are not armoured , therefore , joints become simpler and maintenance cost is reduced considerably . ( iii ) There are very less chances of fault occurrence due to strong mechanical protection pro vided by the system . 

Disadvantages

 ( i ) The initial cost is very high . 

( ii ) The current carrying capacity of the cables is reduced due to the close grouping of cables and unfavourable conditions for dissipation of heat . This method of cable laying is suitable for congested areas where excavation is expensive and inconvenient , for once the conduits have been laid , repairs or alterations can be made without opening the ground . This method is generally used for short length cable routes such as in workshops road crossings where frequent digging is costlier or impossible . 

3. Solid system .

 In this method of laying , the cable is laid in open pipes or troughs dug out in earth along the cable route . The troughing is of cast iron , stoneware , asphalt or treated wood . After the cable is laid in position , the troughing is filled with a bituminous or asphaltic compound and covered over . Cables laid in this manner are usually plain lead covered because troughing affords good mechanical protection . Disadvantages ( i ) It is more expensive than direct laid system . ( ii ) It requires skilled labour and favourable weather conditions . iii ) Due to poor heat dissipation facilities , the current carrying capacity of the cable is reduced . In view of these disadvantages , this method of laying underground cables is rarely used now - a days .

 

 

what is fuses || Characteristics of Fuse Element|| Current rating||Types of Fuses

What is Fuses?

 Fuses A fuse is a short piece of metal , inserted in the circuit , which melts when excessive current flows through it and thus breaks the circuit . The fuse element is generally made of mate rials having low melting point , high conductivity and least deterioration due to oxidation e.g. , silver , copper etc.

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Advantages

 ( i ) It is the cheapest form of protection available . 

( ii ) It requires no maintenance .

 ( iii ) Its operation is inherently completely automatic unlike a circuit breaker which requires an elaborate equipment for automatic action . 

( iv ) It can break heavy short - circuit currents without noise or smoke . 

( v ) The smaller sizes of fuse element impose a current limiting effect under short - circuit condi tions . 

( vi ) The inverse time - current characteristic of a fuse makes it suitable for overcurrent protection . 

( vii ) The minimum time of operation can be made much shorter than with the circuit breakers 

 Disadvantages

 ( i ) Considerable time is lost in rewiring or replacing a fuse after operation . 

( ii ) On heavy short - circuits , * discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned . 

( iii ) The current - time characteristic of a fuse cannot always be co - related with that of the pro tected apparatus . 

Characteristics of Fuse Element 

The function of a fuse is to carry the normal current without overheating but when the current excee its normal value , it rapidly heats up to melting point and disconnects the circuit protected by it . order that it may perform this function satisfactorily , the fuse element should have the followin desirable characteristics : 48 ( i ) low melting point e.g. , tin , lead . ( ii ) high conductivity e.g. , silver , copper . ( iii ) free from deterioration due to oxidation e.g. , silver . ( iv ) low cost e.g. , lead , tin , copper . The above discussion reveals that no material possesses all the characteristics . For instance , lead has low melting point but it has high specific resistance and is liable to oxidation . Similarly , copper has high conductivity and low cost but oxidises rapidly . Therefore , a compromise is made in the selection of material for a fuse .

Types of Fuses 

Fuse is the simplest current interrupting device for protection against excessive currents . Since the invention of first fuse by Edison , several improvements have been made and now - a - days , a variety of fuses are available . Some fuses also incorporate means for extinguishing the arc that appears when the fuse element melts . In general , fuses may be classified into : ( i ) Low voltages fuses ( ii ) High voltage fuses It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety . If such means of isolation are not available , the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed .

 Important Terms The following terms are much used in the analysis of fuses :

 Current rating of fuse element: It is the current which the fuse element can normally carry without overheating or melting . It depends upon the temperature rise of the contacts of the fuse holder , fuse material and the surroundings of the fuse . ) Fusing current . It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it . Obviously , its value will be more than the current rating of the fuse element . For a round wire , the approximate relationship between fusing current I and diameter d of the wire is

Prospective Current : shows how a.c. current is cut off by a fuse . The fault current would normally have a very large first loop , but it actually generates sufficient en energy to melt the fuse able element well before the peak of this first loop is reached . The r.m.s. value of the first loop of fault current is known as prospective current . Therefore , prospective current can be defined as under : It is the r.m.s. value of the first loop of the fault current obtained if the fuse is replaced by an ordinary conductor of negligible resistance .

 Cut - off current . It is the maximum value of fault current actually reached before the fuse melts . On the occurrence of a fault , the fault current has a very large first loop due to a fair degree of asymmetry . The heat generated is sufficient to melt the fuse element well before the peak of first loop is reached The current corresponding to point ' a ' is the cut off current . The cut off value depends upon : ( a ) current rating of fuse ( b ) value of prospective current ( c ) asymmetry of short - circuit current It may be mentioned here that outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak . This gives the fuse a great advantage over a circuit breaker since the most severe thermal and elector - magnetic effects of short - circuit currents ( which occur at the peak value of prospective current ) are not experienced with fuses . Therefore , the circuits pro tected by fuses can be designed to withstand maximum current equal to the cut - off value . This consideration together with the relative cheapness of fuses allows much saving in cost .

 Pre - arcing time :. It is the time between the commencement of fault and the instant when cut off occurs . When a fault occurs , the fault current rises rapidly and generates heat in the fuse element . As the fault current reaches the cut off value , the fuse element melts and an arc in initiated . The time from the start of the fault to the instant the arc is initiated is known as pre - arcing time . The pre - arcing time is generally small : a typical value being 0.001second ( vii ) Arcing time . This is the time between the end of pre - arcing time and the instant when the arc is extinguished . ( viii ) Total operating time . It is the sum of pre - arcing and arcing times . It may be noted that operating time of a fuse is generally quite low ( say 0-002 sec . ) as compared to a circuit breaker ( say 0-2 sec or so ) . This is an added advantage of a fuse over a circuit breaker . A fuse in series with a circuit breaker of low - breaking capacity is a useful and economical arrangement to provide adequate short - circuit protection . It is because the fuse will blow under fault conditions before the circuit breaker has the time to operate . ( ix ) Breaking capacity . It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage .

What is Electric Potential , Resistance, Conductance?

 What is Electric Potential , Restence, Conductance?

1.3. The Idea of Electric Potential

In Fig. 1.1, a simple voltaic cell is shown. It consists of copper plate (known as anode) and a zinc rod (i.e. cathode) immersed in dilute sulphuric acid (H2SO4) contained in a suitable vessel. The chemical action taking place within the cell causes the electrons to be removed from copper plate and to be deposited on the zinc rod at the same time. This transfer of electrons is accomplished through the agency of the diluted H2SO4 which is known as the electrolyte. The result is that zinc rod becomes negative due to the deposition of electrons on it and the copper plate becomes positive due to the removal of electrons from it. The large number of electrons collected on the zinc rod is being attracted by anode but is prevented from returning to it by the force set up by the chemical action within the cell.

But if the two electrodes are joined by a wire externally, then electrons rush to the anode therebyequalizing the charges of the two electrodes. However, due to the continuity of chemical action, a continuous difference in the number of electrons on the two electrodes is maintained which keeps up a continuous flow of current through the external circuit. The action of an electric cell is similar to that of a water pump which, while working, maintains a continuous flow of water i.e., water current through the pipe (Fig. 1.2).It should be particularly noted that the direction of electronic current is from zinc to copper in the external circuit. However, the direction of conventional current (which is given by the direction of flow of positive charge) is from copper to zinc. In the present case, there is no flow of positive charge as such from one electrode to another. But we can look upon the arrival of electrons on copper plate (with subsequent decrease in its positive charge) as equivalent to an actual departure of positive charge from it. When zinc is negatively charged, it is said to be at negative potential with respect to the electrolyte, whereas anode is said to be at positive potential relative to the electrolyte. Between themselves, copper plate is assumed to be at a higher potential than the zinc rod. The difference in potential is continuously maintained by the chemical action going on in the cell which supplies energy to establish this potential difference.

Resistance

It may be defined as the property of a substance due to which it opposes (or restricts) the flow of electricity (i.e., electrons) through it. Metals (as a class), acids and salts solutions are good conductors of electricity. Amongst pure metals, silver, copper and aluminium are very good conductors in the given order.* This, as discussed earlier, is due to the presence of a large number of free or loosely-attached electrons in their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide and other atoms and electrons, thereby producing heat. Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like bakelite, mica, glass, rubber, p.v.c. (polyvinyl chloride) and dry wood etc. Amongst good insulatorscan be included fibrous substances such as paper and cotton when dry, mineral oils free from acids and water, ceramics like hard porcelain and asbestos and many other plastics besides p.v.c. It is helpful to remember that electric friction is similar to friction in Mechanics.

1.5. The Unit of Resistance

The practical unit of resistance is ohm.** A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals. For insulators whose resistances are very high, a much bigger unit is used i.e., mega-ohm = 106 ohm (the prefix ‘mega’ or mego meaning a million) or kilo-ohm = 103 ohm (kilo means thousand). In the case of very small resistances, smaller units like milli-ohm = 10−3 ohm or micro- ohm = 10−6 ohm are used. The symbol for ohm is Ù.

 

Ohm’s Law

This law applies to electric to electric conduction through good conductors and may be stated as follows :

The ratio of potential difference (V) between any two points on a conductor to the current (I) flowing between them, is constant, provided the temperature of the conductor does not change.

In other words, V/I= constant or V/I= R

where R is the resistance of the conductor between the two points considered. Put in another way, it simply means that provided R is kept constant, current is directly proportional to the potential difference across the ends of a conductor. However, this linear relationship between V and I does not apply to all non-metallic conductors. For example, for silicon carbide, the relationship is given by V = KIm where K and m are constants and m is less than unity. It also does not apply to non-linear devices such as Zener diodes and voltage-regulator (VR) tubes.

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