Elctrons and Conductors Atoms consist primarily of lctrons, protons, and nutrons. A modifid Bohr modl of th atom is shown blow. nuclus composd of a clustr of protons and nutrons orbital lctrons lctron proton nutron outr shll Modifid Bohr s modl of th atom. Thr ar two typs of charg, labld positiv (+) and ngativ (-), with th proprty that lik chargs rpl, and unlik chargs attract. Elctrions ar ngativly chargd, protons ar positivly chargd and nutrons carry no charg. In Bohr s modl, th pull of th protons kps th lctrons from flying away. Th laws of quantum physics say that lctrons orbiting a nuclus can only travl in spcific bands or shlls. In addition, ach shll has a limit as to how many lctrons it can hold. Th lctrons orbiting in th shlls closst to th nuclus hav th last nrgy so thos shlls ar normally filld. Most atoms hav outr shlls that ar not fully filld. Th forc kping th lctrons in orbit is invrsly proportional to th squar of th distanc to th protons in th nuclus. Thus lctrons can b addd and takn away most asily from th outrmost shlls. In a conductor, th outr lctrons of th atoms ar loosly bound and fr to mov through th matrial. In fact, th lctrons ar so mobil, that thy ar somtims dscribd as an lctron gas, in which th atomic nucli rst. Th bst naturally occurring conductor is silvr. Coppr is almost as good at conducting as silvr, and it costs a lot lss. Thrfor, coppr is th most popular matrial usd in lctrical wiring. Most mtals ar good lctrical conductors, most nonmtals ar not. Th rsistivity of a mtal is a masur of its opposition to th flow of lctrons. Th lctrical rsistanc of a wir is gratr for a longr wir, lss for a wir of largr cross sctional ara, and dpnds upon th matrial th wir is mad of. Gnrally spaking, th rsistanc of a wir can b xprssd as: R = ρ L A whr ρ is rsistivity L is th lngth of th conductor A is th cross sctional ara of th conductor 1
Th factor in th rsistanc which taks into account th natur of th matrial is th rsistivity. Although it is tmpratur dpndnt, it can b usd at a givn tmpratur to calculat th rsistanc of a wir of givn gomtry. Tabl 1 blow givs th rlativ rsistivity of various mtals as compard to coppr. Rsistivity is masurd in Ohms. W will discuss rsistanc furthr a fw sctions from now. Matrial Rsistivity Compard to Coppr Aluminum 1.6 Brass 3.7-4.9 Gold 1.4 Iron(pur) 5.68 Lad 12.8 NIckl 5.1 Silvr 0.94 Stl 7.6-12.7 Tin 6.7 Zinc 3.4 Insulators In most solid matrials th outrmost lctrons ar so tightly bound that thr ar no fr lctrons that can frly mov throughout th matrial. Ths matrials ar known as insulators. Typically, th lctrons ar tightly shard in th orbits of two adjacnt atoms. Most compounds of carbon and hydrogn ar lik this. Plastics, whos molculs ar mad from long combinations of carbon and hydrogn atoms, hav fw or no fr lctrons. This mans that plastics mak xcllnt lctrical insulators. With nough voltag applid howvr, any insulating matrial will vntually succumb to th lctrical prssur and lctron flow will occur. For voltags blow a crtain thrshold, virtually no lctrons will flow, but if th voltag xcds that thrshold, thr will b a rush of currnt. This occurs at what is calld th matrials brakdown voltag. Thicknss of an insulating matrial plays a rol in dtrmining its brakdown voltag, othrwis known as dilctric strngth. Spcific dilctric strngth is oftn listd in trms of kilovolts pr inch, but th rlationship btwn brakdown voltag and thicknss is not xactly linar. Howvr, for rough stimation us, volt-pr-thicknss ratings ar fin. Blow is a tabl showing th dilctric strngth of svral common matrials. 2
Matrial Dilctric Strngth (kv/inch) Vacuum 20 Air 20-75 Porclain 40-200 Rubbr 450-750 Papr 1250 Tflon 1500 Glass 2000-3000 Mica 5000 In summary, w kp lctrical currnt flowing whr w want it to by us of insulators. Insulation on a lctrical wir is lik th walls of a watr pip. Smiconductors Most simply put, matrials whos conductanc is btwn conductors and insulators ar calld smiconductors. To xamin this to a gratr dtail, w will brifly digrss into th band thory of matrials. Th band thory givs us a clar way to distinguish th diffrncs btwn conductors, smiconductor and insulators. Whn an atom is in isolation, w spak of its lctrons occupying particular discrt nrgy stats. But whn a solid consisting of many atoms is formd, th lctrons occupy nrgy bands. Th outr most band in an atom is th valnc band. But whn atoms form a solid, lctrons may b lvatd into a highr nrgy lvl calld th conduction band. Th dtrmining factor as to whthr a matrial is a conductor, smiconductor or insulator is if thr ar lctrons prsnt in th conduction band. In most matrials, th valnc band and th conduction band ar sparatd by a wid forbiddn zon. Substantial outsid nrgy is rquird to lvat th valnc lctrons into th conduction zon. Thus ths matrials ar insulators. In conductors w find that th conduction and valnc bands ovrlap. In smiconductors, th forbiddn zon is vry small. With such a small gap, a small thrmal or lctrical xcitation can lvat lctrons into th conduction zon. Thus with just a littl coaxing, th matrial may look lik a conductor or an insulator. Th figur blow shows how th bands may diffr btwn insulators, smiconductors and conductors. 3
lctron nrgy lctron nrgy lctron nrgy Suprconductors Whn coold to supr-low tmpraturs (nar absolut zro, or -273 dgrs Clsius) conductors los all of thir lctrical rsistanc. Conductors gradually los rsistanc as tmpratur lowrs. Howvr, suprconductivity occurs with a suddn, quantum lap drop in rsistivity from somthing to nothing. A suprconducting matrial has absolutly zro lctrical rsistanc, not just a small amount. Suprconductors ar spcial matrials which whn coold blow thir critical tmpratur (100K or lss) bcom prfct conductors. Suprconductors ar at on of th last grat frontirs of scintific discovry. Th limits of suprconductivity not yt bn rachd, and th thoris that xplain suprconductor bhavior ar constantly undr rviw. Fully xploitd, suprconductivity could provid lctric circuits that xhibit xtraordinary prformanc. Powr losss in lctric systms du to rsistanc could b liminatd. Elctric motors could b mad narly 100% fficint. Som suprconducting motors, hav bn dvlconduction band valnc band conduction band valnc band conduction band valnc band Insulator Th wid nrgy gap btwn valnc and conduction bands kps lctrons out of th conduction band at ordinary tmpraturs. Conductor In conductors th conduction band ovrlaps th valnc band so that at ordinary tmpraturs conduction occurs. Smiconductor In smiconductors, th band gap is small nough that thrmal nrgy can lvat som lctrons to th conduction band. Applid lctric filds can lvat many lctrons to th conduction band. Smiconductors ar rlativly poor conductors until thy ar dopd with lmnts lik arsnic, phosphorous or boron. Th doping procss insrts a small numbr of doping atoms into th crystal lattic of th original (or bulk) smiconductor. This gratly incrass its conductivity by adding an additional lvl btwn th original valnc and conduction bands. On way doping is applid provids th bulk matrial with a population of fr lctrons which hav bn borrowd or donatd by th dopant. Th xtra availabl lctrons transform th bulk matrial into a conductor with spcial proprtis which allow th fabrication of smiconductor dvics such as transistors and diods. Th first transistors and diods wr mad with grmanium, but making sufficintly pur matrial was difficult. Now th most commonly usd smiconductor is silicon. Othr xampls of smiconductor matrials ar Gallium Arsnid (GaAs), Indium Phosphid (InP) and Silicon Grmanium (SiG). 4
opd, but thir us is limitd du to th problms in maintaining vry low tmpraturs. Critical tmpraturs for suprconductors ar gnrally nar absolut zro, but considrabl progrss has bn mad in dvloping high-tmpratur suprconductors; matrials which suprconduct at warmr tmpraturs. On typ is a cramic mixtur of yttrium, barium, coppr, and oxygn which bgins to suprconduct at 83 dgrs Klvin. Th critical tmpraturs for a fw common substancs ar shown hr in th tabl blow. Matrial Critical Tmpratur (dgrs Klvin) Aluminum 1.2 Mrcury 4.6 Tin 3.72 Niobium/Tin 18.1 YBCO (Yttrium Barium Coppr Oxid) 83 5