1/24/2017. Electrical resistance

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1 1/24/2017 Photocopirs and th National Grid Photoconductors so far.. On xampl of a smiconducting matrial Elctrical insulator in th dark, conductor in th light mportant componnt in a photocopir butt Slctiv discharg Documnt Light rflctd off documnt Charg imag Day 4: Finish Photoconductors Start Powr transmission Rmindrs/Updats: Attnd HW sssions today/fri/mon Lab 2 nxt wk Rading quiz Tus: 11.1, 11.2, Elctrical rsistanc Primarily dtrmind by th numbr of chargd particls (lctrons) that can b acclratd by th battry So what ar lctrons doing insid a solid how ar thy arrangd? Microscopic look at diffrnt matrials Conduction rul: For lctrons to mov (whn a voltag is applid) thr must b mpty nrgy lvls immdiatly abov thm Conductor: mpty lvls vry clos nsulator: Big jump to mptis. mpty Smiconductor: Half way in btwn. Small jump to mptis Quantum mchanics: Elctrons must occupy allowd nrgy lvls. n a solid thr ar bands of billions of vry closly spacd lvls. nbtwn bands thr ar rgions of forbiddn nrgy, calld band gaps. Up to 2 lctrons pr nrgy lvl Conduction rul: For lctrons to mov and carry currnt (whn a voltag is applid) thr must b mpty nrgy lvls immdiatly abov thm mpty lctron lik ball rolling on almost flat ground movs asily gap- no lvls Elctron lik a ball in pit. Can t mov without big boost. mpty Small gap - Elctron lik ball in shallow pit. - Small boost rquird to mov. 4 mpty Littl gap Bhavs as insulator Rviw of smiconductors. Hat nrgy or photons mpty Littl gap Small gap btwn filld and mpty nrgy lvls. Gap is much biggr than th nrgy givn to an lctron by th battry. Assuming no lctrons ar in th conduction band thn th gap prvnts currnt flowing. Gap is small nough that lctrons can b xcitd into th conduction band by absorbing hat nrgy or photons (with nrgy qual to or gratr than band gap nrgy) Thus w can us hat and light to control th rsistanc of a smi-conductor. Concpt bhind all modrn lctronics! Us smiconductors to control lctric currnts. 5 Bhavs as conductor Back to Photocopying Corona wir Slctiv discharg Now w undrstand photoconductors and how this stp works Documnt Photoconductor Groundd mtal Charg photoconductor vlocity Light rflctd off documnt Charg imag 6 1

2 1/24/2017 Photocopir summary Sprad charg on photoconductor Us light to slctivly rmov som charg and mak a charg imag of original Elctrostatic attraction sticks tonr to rgions whr charg rmains Us hat to bond tonr to nw pic of papr Copying is complt! Elctric powr distribution Th powr distribution systm and th physics that maks it th way it is. Qustion to think about, and b rady to answr in a coupl of classs: What is th rlationship btwn th fact that lights somtims dim with you turn on a hair dryr and that w hav giant powr transmission towrs with cabls hanging from thm all ovr th country? W will rturn to smi-conductors and thir us in lctronics in a coupl of wks 8 Powr distribution and gnration - Powr loss to wirs - Dlivring Powr - transformrs - Crating lctrical currnt - gnrators. Powr loss to wirs whn distributing powr across th country Long wirs from powr gnrating plants to homs (100 s of mils) W can t ignor th powr loss in th wirs How much powr will w los in wirs? How can w minimiz powr lost in th wirs? Big physics idas Changing magntic filds produc voltags and currnts Currnts produc magntic filds. 9 First, lts xplor what affcts powr loss in wirs Us old hous wiring as an xampl 10 What happns whn w can t ignor wir rsistanc? Wir has high rsistanc (long, thin, poor quality) Currnt is larg What happns whn w can t ignor wir rsistanc? Vry long wir around walls of hous Vry long wir around walls of hous in Wall outlt Hatr Low R, High Light High R, Low in Wall outlt Exampl: Plugging multipl dvics into an outlt in an old hous What happns to th light bulb whn w plug th hatr in? HEATER 1500 Watts - Low R - High LGHT 60 Watts - High R - Low Visualiz hous wirs as prfct wir with small R w in it Turn on Hatr (clos switch), obsrv light bulb. Light bulb dims. Why? Discuss with your group and com up with possibl rasons. Shar with class Not: Powr company supplis limitd voltag to hous (120V) but ffctivly UNLMTED currnt. 2

3 1/24/2017 THNK lik an lctron. What is going on? Hr is th circuit with th hatr off: Circuit with hatr off Long wirs, ralistically som R Visualiz as prfct wir with small R w in it lots of nrgy at start. 120V xhaustd! All - nrgy (or V) usd up gtting through cours. glid down hous wirs rlativly asily, just a fw bumps. Small amount of - nrgy (or V) lost!?#%, bridg out, stuck. Almost all - nrgy (or V) lost in bulb Whr is voltag and powr going? Work through togthr. Voltag supplid to hous = olts Total rsistanc of hous wirs, R w = 0.1 Ohms Rsistanc of 60W lightbulb, R B = 240 Ohms Total rsistanc = R w + R B = Ohms Currnt = Hous Supply Voltag / Total R = olts / Ohms = 0.5 Amps Voltag drop in hous wirs (V w) = Currnt x rsistanc = 0.5 Amps x 0.1 Ohms = 0.05 Volts Voltag drop in bulb = 120 V w = V Powr into hous wirs = Currnt x Voltag drop in wirs = 0.5 Amps x 0.05 Volts = Watts Powr into bulb = currnt x voltag drop in bulb = 0.5 Amps x Volts = Watts Hr is th circuit with th hatr on (bridg fixd) What changs occur in th hous wirs whn hatr turnd on? R W Low R High A) Currnt through hous wirs is now largr B) Currnt through th hous wirs is th sam C) Currnt through th hous wirs is smallr D) t dpnds on th kind of wir High R Low Hr is th circuit with th hatr on (bridg fixd) What othr changs occur in th circuit whn th hatr turns on? HNT: this is tricky REMEMBER th wirs hav a small (but non-zro) R!. Currnt through hous wirs is now largr. Voltag drop across hous wirs (V w) is now largr. Voltag drop around whol circuit is now largr V. Voltag drop across th bulb () is smallr V. Currnt through th bulb is smallr R W a. and b. and c.,, V d.,, V, V.,,, V, V Hr is th circuit with th hatr on (bridg fixd) Powr transmission - Powr loss to wirs R w R w Clos hatr switch incrass V w incrass (V w = *R w) drops ( = 120 V w) Light bulb dims, wir gts warm Not: Why do lctrons los mor nrgy (or voltag) in wirs whn incrass? A: Thy hav to travl fastr, hit obstacls hardr, los mor nrgy in collision Voltag droppd in wir V w = R w Powr wastd in wir = 2 R w Significant whn: is larg (supplying lots of appliancs) R W is larg (long, thin, poor quality wirs) Exampls whn wir rsistanc can t b ignord: - Lights dim whn connct hatr in old hous - Hadlights dim whn start car - Transporting lctricity ovr long distanc from powr station to towns 3

4 1/24/2017 What will mak bulb vn dimmr?. Shortr wirs. Longr wirs. Adding anothr hatr in paralll V. Thinnr wirs V. Fattr wirs a. and V b.,, and c.,, and V d.,, V.. only. Back to powr distribution. To minimiz powr loss in transmission wirs ( 2 R): 1. Want low R: - thick wirs - no longr than ncssary, - mad of mtal with vry low lctrical rsistanc R = ρ L A powr plant But ovr many mils R will still b significant.. 2. So how can w achiv low? powr plant Back to powr distribution. Say w dlivr 100 W to hous, P = V V = 10 V, has to b 10 A. V= 100 V, has to b 1 A V= 1000 V, has to b? What is powr loss in wirs for ach voltag if R w = 1 ohm? P = 2 R w = 2 (1ohm) V = 10V P = 100 Watt = sam as powr to hous V = 100V P = V = 1000V P = 21 powr plant Powr distribution qustions Voltag supplid by powr company Voltag supplid to hom. (som voltag drop in wirs) Powr plant dlivrs 10,000 W to powr a hous nitially it dlivrs th powr to th hom at a voltag of 100V Thn ralizs that this is vry infficint and changs to dlivr powr at 10,000V Q: What changs occur whn th powr company incrass its supply voltag? Currnt through transmission wir ndd to supply powr will b Voltag drop across transmission wirs will b Powr going into hating th transmission wirs will b a) sam, sam, sam b) lss, sam, lss c) mor, sam, mor d) lss, lss, lss ) mor, mor, mor. 22 Qustion: f high voltag/low currnt powr transmission is so fficint, dos powr arriv at my hous at a masly 120V? So how do w chang from high to low voltag transmission? TRANSFORMERS! Powr distribution is a trad off btwn fficincy and safty: High voltag: Efficint transmission, dadly..g. 300,000V usd for rmot cross-country powr lins Low Voltag: Lss dangrous but lss fficint. Usd for short distancs in populatd aras.g. 1000s V round town in hous Transformrs only work with AC (altrnating currnt) - On of th rasons that th national grid runs on AC Transformr physics involvs 2 idas: 1. Currnts produc magntic filds 2. Changing magntic filds produc voltags and currnts 4

5 Voltag diffrnc 1/24/2017 Mains powr - Altrnating currnt (AC) look at wall outlt with Oscilloscop (masurs voltag diffrnc) A B Oscilloscop No voltag diff Currnt = 0 Amps +170V -170V 0 Voltag at A largr than at B tim Dos AC work th sam as DC? 1. n light bulbs and hatrs? a) ys, b) no Ys-- think lik lctron, flying around bumping into stuff. AC also works for motors, but build diffrnt than DC 2. n computrs, cll phons, and lctronics? a) ys, b) no No--, hav to convrt AC back to 3-5 V DC. (chargrs) US- 60 hrtz (60 oscillations/s) rms (av. voltag diff) Europ-50 Hz, 230 V rms Voltag at B largr than at A Transformrs in th powr distribution systm powr plant 5000V 500,000 V (on towrs) substation How do transformrs work? Convrt AC voltag up and down Mad of two coils of wir (around a cor) Scondary coil (out) Transformrs nabl us to: Chang voltag asily Transfr powr btwn circuits so on hous dosn t affct nxt. How many transformrs ar usd hr to rais or lowr voltag? a) 1, b) 3, c) 5, d) 6, ) 10 short wirs into houss 7200 V running around town. Primary coil (in) AC currnt in primary coil (.g. from powr company) producs AC currnt in scondary coil (.g. currnt in your hous) Two Stps: A) Changing lctric currnt in primary coil producs changing magntic fild B) Changing magntic fild producs a currnt in th scondary coil What is a magnt and a magntic fild? Natural phnomnon closly rlatd to lctricity North and south pols, opposit pols attract mportant diffrnc: Magntism has no monopols (lik + and - chargs.) North and South pols ar hookd togthr. ALWAYS. A magntic fild dscribs th forc on a north pol of a magnt at ach location in spac How can w mak a magnt? 1. Magntic matrial (.g. ron) - Elctrons bhav lik tiny bar magnts - Usually paird in opposit orintations cancl out - ron rtains som unpaird lctrons billions of atomic magnts combin to mak a big magnt Compass is a littl bar magnt. Earth is a big bar magnt. N nd of compass ndl attractd to S nd of arth magnt. 2. Elctric currnts produc magntic filds - Magntic fild around a coil of wir is much lik that around a bar magnt - Elctromagnt Coil of wir Bar magnt 29 5

6 1/24/2017 Scrapyard Elctromagnt 6