Seebeck and Peltier Effects

Transcription

1 Sbck and Pltir Effcts Introduction Thrmal nrgy is usually a byproduct of othr forms of nrgy such as chmical nrgy, mchanical nrgy, and lctrical nrgy. Th procss in which lctrical nrgy is transformd into thrmal nrgy is calld Joul hating. This is what causs wirs to hat up whn currnt runs through thm, and is th basis for lctric stovs, toastrs, tc. Elctron diffusion T2 T2<T1 cold - I V + hot Figur 1: Elctrons diffus from th hot to cold sid of th mtal (Thompson EMF) or smiconductor laving hols on th cold sid. I. Sbck Effct (1821) Whn two nds of a conductor ar hld at diffrnt tmpraturs lctrons at th hot junction at highr thrmal vlocitis diffus to th cold junction. Sbck discovrd that making on nd of a mtal bar hottr or coldr than th othr producd an EMF btwn th two nds. H xprimntd with junctions (simpl mchanical connctions) mad btwn diffrnt conducting matrials. H found that if h cratd a tmpratur diffrnc btwn two lctrically connctd junctions (.g., hating on of th junctions and cooling th othr) th wir conncting th two junctions would caus a compass ndl to dflct. H thought that h had discovrd a way to transform thrmal nrgy into a magntic fild. Latr it was shown that a th lctron diffusion currnt producd th magntic fild in th circuit a changing mf V ( Lnz s Law). Th magnitud of th mf V producd btwn th two junctions dpnds on th matrial and on th tmpratur ΔT 12 through th linar rlationship dfining th Sbck cofficint S for th matrial. 1

2 ΔV = S ΔT 12 Th Sbck cofficint can b masurd Figur 2, by conncting wir-a in a circuit with 2 wir-bs. Th two junctions (nds of wir-a ) ar hld at two tmpraturs, and V masurd as T1 or T2 is varid, Diagram of. Figur 2: Exprimntal stup for masuring th Sbck cofficint S. Only trminals 1 and 2 nd b considrd if th B-lads at th voltmtr ar kpt at th sam tmpratur. If T1>T2 lctrons flow to T1 laving T2 mor positiv. Vb-Va = SA(T2-T) Vc-Vb= SB(T1-T2) Vd-Vc= SA(T-T1) V = (Vb-Va)+ (Vc-Vb) + (Vd-Vc) V = SA T2 SA T +SB T1 SB T2 + SA T SA T1 V = SA(T2-T1) - SB(T2-T1) = (SA-SB)(T2-T1) Q = q V = q (SA-SB)(T2-T1) dq/dt = I (SA-SB)(T2-T1) = I Π 12 Procdur 1. Masur th ΔV across diffrnt matrials (Cu, PbSn, Al (as shown in Figur 1, abov) with T1~0 o C and T2=100 o C. Dtrmin th Sbck Cofficints from S = ΔV /ΔT Compar th masurd cofficints with standard rsults. 3. Plot ΔV vs ΔT for CU and Al. 2

3 T(dgC) Cu(uV) Cu(uV) Al(uV) y = -4.2x

4 II. Thrmocoupl A thrmocoupl, a tmpratur masuring dvic can b mad by holding all but on trminal (say T1) to th sam tmpratur. Procdur 1. Crat two junctions btwn two diffrnt matrials (as shown in Figur 1, abov) by twisting wirs firmly togthr. As shown in th diagram, you'll nd on lngth of th first matrial, and two lngths of th scond matrial. 2. St your voltmtr to th most snsitiv DC voltag (usually 50 or 200 mv). 3. Attach th voltmtr lads to th two fr nds (as shown in Figur 1, abov.) Th tst lads with alligator clips will b usful for this. 4. Masur and rcord th voltag with both junctions at room tmpratur. 5. Insrt on junction in a cold liquid or plac it against an ic block and masur and rcord th voltag again (lav th othr junction at room tmpratur). 6. Insrt th othr junction in hot liquid or put it in th flam of a candl. Masur and rcord th voltag again. B carful with this stp! Avoid touching th hatd wirs! 7. Rpat th xprimnt using diffrnt pairs of matrials to crat th junctions. 8. Mak a graph of th voltag vs. tmpratur diffrnc for ach kind of junction. 9. Which pair of matrials givs you th bst rsults (i.., highst voltag masurd for th sam tmpratur diffrnc)? 4

5 III. Pltir Effct (1834) Th rvrs of th Sbck ffct is also possibl: by passing a currnt through two junctions, you can crat a tmpratur diffrnc. This procss was discovrd in 1834 by scintist namd Pltir, and thus it is calld th Pltir ffct. This may sound similar to Joul hating dscribd abov, but in fact it is not. In Joul hating th currnt is only incrasing th tmpratur in th matrial in which it flows. In Pltir ffct dvics, a tmpratur diffrnc is cratd: on junction bcoms coolr and on junction bcoms hottr. Although Pltir coolrs ar not as fficint as som othr typs of cooling dvics, thy ar accurat, asy to control, and asy to adjust. Pltir ffct dvics ar usd coolrs for microlctronic dvics such as microcontrollrs and computr CPUs. This us is vry common among computr hobbyists to hlp thm in ovr-clocking th microprocssors for mor spd without causing th CPU to ovrhat and brak in th procss. T T2> T1 T - - p -B n-a p-b + + a b c d + - V Figur 5: Two dissimilar mtals or smiconductors connctd at b and c. Th junction at T1 hats up whil th junction at T3 cools. Vb-Va = SA(T2-T) Vc-Vb= SB(T1-T2) Vd-Vc= SA(T-T1) V = (Vb-Va)+ (Vc-Vb) + (Vd-Vc) V = SA T2 SA T +SB T1 SB T2 + SA T SA T1 V = SA(T2-T1) - SB(T2-T1) = (SA-SB)(T2-T1) Q = q V = q (SA-SB)(T2-T1) dq/dt = I (SA-SB)(T2-T1) = I Π 12 5

6 Procdur 1. Us th Junctions cratd in th junctions in th Sbck ffct xprimnt, abov. 1kΩ 2. In sris with a 10V powr supply plac a currnt limiting 1kΩ rsistor. 3. To obsrv th tmpratur of th junctions, you can put a drop of watr on ach on. (Do not touch th junctions! On can gt hot nough to caus a burn!) Dos th watr frz on on of th junctions? What happns if you thn rvrs th polarity of th battry connction? 4. Rpat th xprimnt with diffrnt junction matrials. 6

7 Variations * Th Sbck ffct xprimnt can b xpandd to crat a ral tmpratur snsor. You will nd an indpndnt mans of masuring th tmpratur diffrnc btwn th two junctions to calibrat your dvic. * For a mor advancd Pltir ffct xprimnt, you can vary th currnt and masur th tmpratur diffrnc cratd. You'll nd to figur out a mthod for masuring th tmpratur of ach junction. Us diffrnt rsistors to chang th currnt. Us Ohm's Law ( to calculat how much currnt will flow in th circuit. Also calculat how much powr will b dissipatd in th rsistor (and b sur to us a rsistor with sufficint wattag rating). Plot th tmpratur diffrnc vs. currnt for ach typ of junction. * Advancd. You can gt a commrcial Pltir ffct dvic and study its tmpratur vs. currnt charactristics. Crdits ml By Akram Salman AMD logo Bibliography * To larn about Sbck ffct chck th following wbsits: o Wikipdia contributors, "Thrmolctric Effct," Wikipdia, Th Fr Encyclopdia [accssd May 22, 2006] o Kuphaldt, T.R., "Thrmolctricity," All About Circuits: Volum VI Exprimnts [accssd May 22, 2006] o Thrmolctrics.com, "Introduction to Thrmolctrics," Thrmolctrics.com [accssd May 22, 2006] * For larning about th Pltir ffct chck th following wbsit: Stinbrchr, T., "Th Hatsink Guid: Pltir Coolr Information," Hatsink- Guid.com [accssd May 22, 2006] * To larn about th diffrnt thrmolctric ffcts chck this wbsit: DiSalvo Group, "Introduction to Thrmolctrics," Dpartmnt of Chmistry and Chmical Biology, Cornll Univrsity [accssd May 22, 2006] 7

8 This wbsit dscribs a wristwatch that is powrd from body hat, using microfabricatd thrmolctric componnts: SII, "SII Tchnology Evolving Watch," Siko Instrumnts, Inc. [accssd May 23, 2006] 8