By the end of this chapter, you will have covered the following key knowledge and skills:

Transcription

1 Chaper 2 Elecriciy Key knowledge and skills By he end of his chaper, you will have covered he following key knowledge and skills: \\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\ \\\\\\\\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\\\\\\\\\\\ \\\\\\\\\\ Apply he conceps of charge (Q), elecric curren (I ), poenial difference (V), energy (E) and power (P) in elecric circuis. Analyse elecrical circuis using he relaionships I Q/, V E/Q, P E/ VI, E VI. Conver energy values o kilowa-hour (kwh).

2 Nelson Physicss 38 VCE Unis 1&2 Charge he amoun of posiive or negaive elecriciy Quaniy of charge exen o which an objec is elecrically charged; uni of charge is he coulomb, C Elemenary charge he firs subaomic quaniy o be shown o come in discree packages: e C 1 1 C elemenary charges Benjamin FRANKLIN ( ) Benjamin Franklin was he firs American o be universally acceped for his scienific achievemens. He is also remembered as a saesman, a priner and an invenor. Franklin was responsible for he naming of he wo ypes of elecric charge as posiive and negaive, and inroduced he erms plus and minus and also charge and baery ino scienific language. Franklin was he 15h of 17 children and was forced o leave school a he age of 10 o help his faher in he family candle and soap shop. Two years laer he became an apprenice priner. He became a skilled priner and also wroe several newspaper aricles. A he age of 17, he ran away o Philadelphia. For he nex 7 years, he worked for various priners in Philadelphia and hen in London. In 1737, he became posmaser of Philadelphia, and his was he firs of many public offices ha he held during his lifeime. He is remembered as a saesman and a diploma for his work. Franklin was one of he firs people o become ineresed in he sudy of elecriciy. His mos famous experimen in 1752 was o prove ha lighning is a form of elecriciy. This led him o inven he lighning rod, which is sill used oday o save buildings from lighning srikes. Oher of Franklin s invenions include arranging flues in soves o improve heaing in rooms, bifocal glasses and improving acid soil by using lime. Inroducion Elecriciy is a very convenien form of energy. I is available from many sources, such as baeries, alernaors and solar panels. I can be generaed by power saions ha use coal, waer (hydro-elecriciy), naural gas, wind or nuclear fuel. Elecric power is ransmied over large disances for domesic, commercial and indusrial use. Elecric energy is easily ransformed ino oher ypes of energy, such as hea energy in oasers, ovens and heaers; sound energy in elevisions, radios and CD players; ligh energy in elecric globes; and mechanical energy in refrigeraor moors, elecric drills, hair dryers and elecric shavers. I can also be used in oher appliances o creae a magneic effec o be sored or used (in porable media players, magneic RAM), or i can be used o operae logic circuis in alarms, compuers, robos and conrol devices. Elecrical quaniies Charge You are probably familiar wih some common examples of he effecs of elecrosaic charge: a plasic comb run hrough your hair, and a plasic ruler or pen rubbed on woollen maerial arac small pieces of issue paper. If you ake off a polyeser or nylon shir or blouse in he dark, you someimes hear a crackling noise and observe flashes of ligh as he saic elecriciy discharges. The ancien Greeks were he firs o realise ha some subsances could be charged. Amber, he hardened sap from sofwood rees, would arac ligh objecs when i was rubbed vigorously wih a cloh. There are wo ypes of charge: posiive and negaive. Like charges always repel each oher and unlike charges arac each oher. Charge Posiive ( ) Negaive ( ) Posiive ( ) Repel Arac Negaive ( ) Arac Repel Q or q is used o represen a quaniy of charge. Charge is measured in coulomb, C. The charge on an elecron is known as he elemenary charge, e. 1 elemenary charge C charge on an elecron charge on a proon 1 1 C elemenary charges Lighning rods Lighning discharges are dramaic illusraions of elecrosaic effecs. The enormous elecric discharge beween hunderclouds and he surface of he Earh (or beween neighbouring clouds) can produce lighning flashes ha are specacular, bu are ofen frighening and dangerous (Figures 2.1 and 2.2).

3 Uni 1 Chaper 2 \\ Elecriciy 39 charge on hundercloud sream of ions lighning conducor induced charge elecrons ino ground meal plae in ground Figure 2.1 Lighning discharges ono he Cenral Plaza, Hong Kong. Elecrosaic precipiaors An effecive means of reducing smoke and ash paricles escaping from coal-fired power saions is elecrosaic precipiaors in he smoke sack. Posiively charged smoke paricles are repelled from a posiive wire mesh in he cenre of he chimney and araced o meal plaes on he sides, where hey sick (Figure 2.4). Figure 2.2 A charged cloud drives elecrons down he lighning rod on a building, which provides a conrolled pah o Earh for he discharge of he cloud. Elecrosaic precipiaors devices ha ake charged ash paricles ou of smoke before discharge o he amosphere Charging a maerial by fricion When a srip of cellulose aceae is rubbed wih a coon cloh, elecrons are ransferred from he aceae o he coon. The cellulose aceae becomes posiively charged and he coon becomes negaively charged. The charge is no desroyed, simply ransferred. An objec becomes negaively charged when i gains an excess of elecrons, or posiively charged when i loses elecrons and so has unbalanced posiive charge. Only elecrons can move in his process of charging a maerial (Figure 2.3). Before rubbing: polyhene srip wool posiively charged paricles are deposied on he meal plaes posiively charged fine wire grid meal plaes neural or uncharged maerials Afer rubbing: excess of elecrons negaive charge elecrons ransferred by rubbing ogeher posiively charged wool deficiency of elecrons posiive charge Figure 2.3 Elecrons are ransferred o and from maerials when hey are rubbed ogeher. smoke and dus paricles rising wih earh he wase gases connecion o meal plaes Figure 2.4 In an elecrosaic precipiaor, he posiively charged wire mesh in he cenre induces negaive charge on he meal plaes on he sides, causing deposiion of ash.

4 Nelson Physicss 40 VCE Unis 1&2 Elecrosaic inducion local movemen of charge on an objec as a resul of a nearby charged objec Conducors aracion neural (equal number of posiive and negaive charges) Figure 2.5 Charges on a neural objec may move around he surface in he presence of a nearby charged objec. The wo objecs will hen be araced. maerials ha allow charge o flow in hem Insulaors maerials ha do no allow charges o flow freely in hem Semiconducors parial conducors ha can enhance or inhibi charge flow The charged maerials do no remain charged for very long. They discharge by ransferring elecrons o or from he surroundings. Charging a maerial by inducion A neural objec can be araced by a charged objec due o elecrosaic inducion (see Figure 2.5). This occurs because elecrons are free o move around he surface of a conducor. If, for example, a highly posiively charged objec is placed near a neural conducor, here is a localised rearrangemen of he posiions of he charges. Negaive charges are drawn o he side neares he posiive objec. The neural objec is hen araced owards he posiive objec. If hey ouch, he ne charge on boh objecs becomes posiive and hey will hen fly apar. Conducors, insulaors and semiconducors Conducors are maerials ha conain charged paricles ha are no very ighly bound o heir nuclei, so charge is able o move in hem relaively easily. Insulaors are maerials ha do no conain free charged paricles, so hey will no allow charge o flow hrough hem. Meals are good conducors because hey have many ouer-shell elecrons ha are no oo ighly bound o he aom or o he srucure in which hey reside. Plasics are insulaors because he elecrons are much more ighly bound ino he compound (Table 2.1). A very imporan group of maerials are he semiconducors, which, given he appropriae condiions, can eiher enhance or inhibi he flow of elecrons. Table 2.1 Examples of conducors, insulaors and semiconducors Good conducors Copper Aluminium Graphie Poor conducors Waer Human body Sugar Insulaors Glass Rubber Dry air Semiconducors Silicon Germanium Curren rae of charge flow: I Q Curren Curren is a flow of posiive charges. This was decided well before he elecron was discovered. Physiciss have kep he convenion because here are significan implicaions for many of he laws and rules of elecriciy, magneism and elecromagneism if hey were now o change he direcion of curren o be he flow of elecrons. Consider he following cases, in which charges are moving pas a line XY, as shown in Figures 2.6, 2.7 and 2.8. In all hree cases, he righ side becomes more posiive by hree unis. X Y conducor Figure 2.6 Three posiive charges moving o he righ makes a curren of 3 unis. X conducor Y Figure 2.7 Two posiive charges moving o he righ and one negaive charge leaving he righ, making he righ more posiive, makes a curren of 3 unis. X conducor Y Figure 2.8 Three negaive charges moving o he lef, ha is leaving he righ, makes a curren of 3 unis.

5 Uni 1 Chaper 2 \\ Elecriciy 41 The quaniy of curren (I ) is defined as he rae of charge flow. If Q charge flows in seconds, he curren I is given by he equaion: I Q The uni of curren is 1 coulomb per second 1 C s 1 1 ampere 1 A. The amoun of charge ha passes a poin in a given ime is: Q I There are wo ypes of curren, namely direc curren (DC) and alernaing curren (AC). In DC, he ne charge flows in one direcion; in AC, he charge flow alernaes. DC is he mehod used in orches, porable radios, lighs in cars, and in oys. The mos common source of DC is a baery. DC is generally safer han AC because he rapid swiching of AC curren has serious effecs on nerve responses. AC is used in many applicaions such as car alernaors, moors and clocks because i is simpler o produce and ransmi, and, during ransmission, power losses in he wires can be reduced. The sandard frequency used hroughou Ausralia is 50 Hz. This is accuraely conrolled, normally wihin a variaion of 1 Hz. Energy In an elecrical sysem, energy is ransferred from place o place. The source of energy, for example, a power saion, a baery or an alernaor, provides energy o each charge. Each charge is hen ready o do work. Energy poenially available o do work is called poenial energy. So, each charge has poenial energy each charge is ready o do work. When he charges are permied o release energy, hey do work, for example, by heaing a oaser, lighing a lamp or driving a moor. The poenial energy is convered o energy in he form of hea, ligh, movemen or any number of possible forms in which energy ransfers are experienced. Poenial difference The energy (E) given o each charge (Q), he poenial energy per charge, is called poenial difference and measured in vol (V ): V E Q The uni of poenial difference is: 1 vol 1 V 1 joule / coulomb 1 J C 1 If he poenial energy of each charge is given up in some way, such as in a circui, he amoun of energy provided, or used, is: E QV Elecromoive force ( ) A one ime i was hough ha he charges were subjeced o some elecrical force o make hem move an elecromoive force (emf ). We sill use his erm, bu now undersand i in erms of energy available o do work. A baery has an emf, which is hen reduced by he inernal resisance of he baery. Thus, he poenial difference across he erminals of he baery is he emf less he poenial difference across he inernal Direc curren (DC) curren ha is always posiive Alernaing curren (AC) curren ha changes from posiive o negaive and back again Charles COULOMB ( ) The uni of elecric charge in he SI sysem is named afer Coulomb in recogniion of his work on elecric charge, especially he force by one charge on anoher charge he elecrosaic force. Charles Coulomb was born in France and, afer obaining an engineering degree, spen 9 years in he Wes Indies, where he firs became ineresed in scienific experimenaion. His work on mechanics gained him elecion o he French Academy of Sciences. This enabled him o devoe mos of his ime o he sudy of mechanics, including fricion, srucure of maerials and soil mechanics. Coulomb invened a orsion balance, which he used o discover he inverse-square law of force ha exiss beween charged bodies. He measured he roaional deflecion of charged spheres when spheres of opposie charge were brough close o hem. This resul was known o Cavendish, bu Cavendish s work was only published by Maxwell 50 years afer his deah. So, Coulomb ges he credi! Coulomb also made imporan discoveries abou he disribuion of charges on conducors, and developed a heory of magneism. Poenial energy energy associaed wih he posiion of an objec; energy ready o do work; sored energy; he energy given o each single charge ha can be delivered up in a circui Poenial difference (V) poenial energy per charge: V E, Q someimes referred o as volage Elecromoive force (emf) energy per charge provided by a source of poenial difference (volage source)

6 Nelson Physicss 42 VCE Unis 1&2 Energy is energy! The name we give o differen forms of energy hides an imporan poin. All energy is he same. I came from he origin of he universe. In he beginning, here was energy, and he energy wen: BOOM! Afer ha he energy condensed ino phoons, elecrons, proons and all he maer of he universe. When we speak of ligh energy we speak of energy ha we can measure wih a ligh-measuring insrumen such as our eyes. Hea energy is measured by a hermomeer, and sound energy by our ears or a sound level meer. The names we give o differen forms of energy depend upon he way we measure he energy, no because he energy is any differen. Energy can be experienced in differen ways. Fundamenally, all energy is he same. Poenial energy per charge a a poin Power (P) rae of energy ransfer: P E resisance. The inernal resisance is reaed as a series resisance ha in mos, bu no all, cases is very small (negligible) compared o he resisance in he res of he circui. V erminals V inernal o baery Poenial and poenial difference The energy is given o each charge by doing work on he charge, as i is brough from infiniy o a poin. The energy per charge a a poin is called he poenial of ha poin. The poenial a a poin is he volage a ha poin. Generally, we are concerned wih energy ransfers leading o energy ransformaions, so he imporan feaure is he difference in poenial beween wo poins. Charge will ransfer energy from a poin a higher poenial o a poin a lower poenial. No ransfer of energy occurs beween poins a he same poenial. The poenial difference is jus he volage drop beween wo poins. I is a measure of he energy per charge being used, convered or ransferred beween he wo poins. Work done by moving a uni posiive charge from one poin o anoher is defined as he poenial difference or volage difference beween he poins. If he work done by moving a quaniy of charge, Q, is W, hen he poenial difference V, in vol, is given by he equaion: Power V W Q A 100 W globe delivers 100 J of energy in 1 second. A 50 W globe delivers he same amoun of energy (100 J), bu in 2 seconds. The 100 W globe is herefore more powerful because i delivers more energy per second han he 50 W globe. From our perspecive, we see beer wih a 100 W globe han a 50 W globe, so he rae a which he energy is delivered, he power, is our judge of he relevan quaniy. Power (P ) is defined as he rae of energy ransfer, eiher delivery or dissipaion. I is he energy ransfer per uni of ime. Therefore: P E When V is measured in vol (V), I is measured in ampere (A) and is measured in second (s), he energy is measured in joule (J). The power is hen measured in wa (W). 1 wa 1 W 1 joule per second 1 J s 1 Power delivery can be deduced from volage and curren values: P E 1 P VQ V Q 1 P VI The amoun of energy ransferred can be deduced from: P E 1 E P 1 E VI

7 Uni 1 Chaper 2 \\ Elecriciy 43 Convering energy values o kilowa-hours For domesic energy usage, he joule is oo small. A larger uni, he kilowa-hour (kwh), is used: One kilowa-hour (1 kwh) is he energy used by a 1 kwh appliance in 1 hour: Thus, E (kwh) P (kw) (h) 10 3 W (60 min 60 s) 1 kilowa-hour 1 kwh J \\ WORKED EXAMPLE A curren of 2.0 A fl owing in a heaer for an hour convers 1.7 MJ of elecric energy ino hea energy. Quesion 1 How much charge is ransferred ino he heaer? Answer I Q, Q I 1 Q 2.0 (60 60) 1 Q C Quesion 2 Wha is he poenial difference across he heaer? Answer V E Q 6 1 V V 240 V Quesion 3 Find he power raing of he heaer in kw. Answer P VI 1 P W 1 P 0.48 kw Quesion 4 How much energy is used if he heaer runs for 2 hours (in kwh)? Answer E P 1 E kwh

8 Nelson Physicss 44 VCE Unis 1&2 Summary of elecriciy Charge Posiive and negaive Elemenary charge (e) is a discree uni of charge: e C Charge Posiive ( ) Negaive ( ) Posiive ( ) Repel Arac Negaive ( ) Arac Repel Uni of charge is coulomb: 1 C elemenary charges. Elecrosaic charging All maerials are neural, bu elecrons can be aken from hem o make hem overall posiive or added o hem o become overall negaive. Elecrosaic inducion: charges can be redisribued o make areas locally charged by he presence of a nearby ne charged body. Conducors have charge carriers ha are free o move. Insulaors have very ighly bound charge carriers ha are no free o move. Semiconducors have opporuniies for charge carriers o move and can be used o amplify or inhibi charge carrier movemen. Curren Number of charges passing a poin in 1 second: I Q Uni of curren is coulomb per second: 1 C s 1 1 ampere 1 A. The amoun of charge ha passes a poin in a given ime is: Q I Energy Poenial energy is energy ready o do work. Poenial difference (volage): poenial energy per charge (poenial): V E Q Uni of poenial difference is vol: 1 V 1 joule per coulomb 1 J C 1. If he poenial energy of each charge is given up in some way, such as in a circui, he amoun of energy provided, or used, is: E QV Elecromoive force, emf, is he poenial difference provided by a baery, no aking ino accoun any energy losses inernal o he baery. Power Power is energy delivered or used every second. P E 1 E P P VI 1 E VI Uni of power is wa: 1 W 1 joule per second 1 J s 1.

9 Uni 1 Chaper 2 \\ Elecriciy 45 Kilowa-hour is a uni of energy. 1 kilowa-hour (1 kwh) is he energy used by a 1 kwh appliance in 1 hour: E (kwh) P (kw) (h) 10 3 W (60 min 60 s) 1 kilowa-hour 1 kwh J Review quesions The elemenary charge e C Quesion 1 If a curren of 0.50 A flows for 10 minues in an elecrical conducor, calculae he number of coulombs of charge ha pass a given poin. Quesion 2 If C of charge flows in a conducor in 1.0 hour, wha is he elecric curren (assuming ha his is a seady curren)? Tes Yourself Word Check Theory Summary Podcas Revision Quesion 3 If elecrons per second pass hrough a curren-measuring device, wha is he curren in amperes? Quesion 4 How long will i ake for a oal charge of 720 C o pass hrough a conducor if a seady curren of 0.60 A is flowing? Quesion 5 A suden claims ha he charge on an ion in a soluion is C. Why mus he suden be incorrec? Quesion 6 A seady curren of 0.50 A flows in a wire. How many elecrons flow pas any poin in he wire in 1.0 s? Quesion 7 Calculae he emf of a dry cell ha supplies 4.5 J of energy o every 3.0 C of charge ha passes hrough he cell. Quesion 8 Calculae he emf of a baery ha supplies 960 J of energy o every 80.0 C of charge ha passes hrough he baery. Quesion 9 Calculae he emf of a baery ha supplies J of energy o every elecron ha passes hrough he baery. Quesion 10 How much energy does a 12.0 V car baery supply o every coulomb of charge? Quesion 11 How much energy does a 12.0 V car baery supply o every elecron? Quesion 12 A 6.0 V orch baery operaes for 0.50 hour o ligh a globe ha draws a curren of 2.0 A. a Wha is he quaniy of elecric charge ha flows hrough he baery in half an hour? b How much energy is supplied o he charge ha flows hrough he baery? c Wha is he power raing of he globe?

10 Nelson Physicss 46 VCE Unis 1&2 Quesion 13 A curren of 2.0 A flows in a dry cell when a ligh globe is conneced across he erminals of he cell. The poenial drop across he erminals is 6.0 V. a Wha is he quaniy of elecrical charge ha flows in he globe each second? b How much energy is given o each coulomb of charge ha passes hrough he baery? c How long does i ake he baery o supply 480 J of energy? d How many coulombs of charge will pass hrough he baery in his ime? Quesion 14 In 30 s a oal of 100 C of charge passes hrough a baery and 600 J of energy is supplied o he elecric charge. a Wha is he emf of he baery? b Wha is he curren flowing in he baery? Quesion 15 Wha is he emf of a baery ha supplies 12 J of energy o each coulomb of charge? Quesion 16 A baery of emf 18.0 V provides a curren of 2.5 A for 3.0 minues. a How many coulombs of charge pass hrough he baery? b How many elemenary charges pass hrough he baery in he 3.0 minues? c How much energy is supplied o each: i coulomb of charge? ii elecron? d Wha was he power expended by he baery? e How much work was done by he baery? Quesion 17 A 100 W ligh globe is lef on for 1.0 hour. How much energy is used by he globe? Give your answer in joule and kilowa-hour. Quesion 18 A 1.5 kw oaser cooks wo pieces of oas in 80 s. If all he energy is used in cooking he oas, how many kilowa-hours of energy are used per piece of oas?