14 Current of Electricity Notes

\kK ©¥©¥ h m n c fi w ©¥\ k in ©¥lh ; lbldd ton * w L \ ūr on the term ina©¥ potentia©¥ difference and output pow er. (k) show an understanding of the effects of the intem a©¥ resistance of a source of e. m . f. energy considerations. ü) distinguish between electrom otive force (e. m . f. ) and potentia©¥ dilference (p . d. ) using «i) reca©¥©¥ and so©¥ve prob©¥em s using R - p l / A . (h) sketch the resistance - tem perature characteń stic of an N T C therm istor. coefficient (N T C » therm istor. ohm ic resistor, a s e m ic o n d u c to r d io d e , a fila m e n t ©¥am p and a negative tem perature f9) sketch and exp©¥ain the 1 - V characteristics of vań ous e©¥ectrical com ponents such as an (f) reca©¥©¥ and sobe prob©¥em s using the equation V ¡¤ I R . (e) reca©¥©¥ and so©¥ve prob©¥em s using the equation P . ©¥V , P - I Z R a n d P ¡¤ V Z / R . (d) recall and sobe problem s using the equation V - W / Q . (c) recall and so©¥ve problem s using the equation Q = l t . the num ber density of charge carriers and V is the drift vekxtity . (b) derive and use the ¢æ q uaüon I = n A v g fo r a c u ©¥T e n tm rry in g c o n d u c to r, w h e re n l8 (a) show an understanding that electric current ©¥s the rate of f©¥ow of charge , C andidates shou©¥d be ab©¥e to outcom es Learning e S o u r ces of e©¥ect rom o t ©¥ve for ce R e a l atan c e an d re©¥©¥©¥dv ©¥ty P o t en t ©¥a©¥ D ©¥ffer en c e C ontent . E lectr©¥c C u rre n t 1 4 C uR R E N T oF E L E C T R ©¥c 国 P H Y S ©¥C S D E P A R T M E N T R A F F L E S IN S T ©¥T U T ©¥O N

unit tim e. average electr©¥c current ©¥©¥*©¥ ©¥s equal to the net charges that pass thro ugh the a rea per If aQ ls the am ount of charge passing through this surface in a tim e interval A t, the Ù³ ー «ß«Î«Ë 4 í» . below . (This area A cou©¥d be the cross sectional area of a w ire, fo r e x a m p le . ) S uppose charges are m oving pe ñé endicular to a surface of area A as show n in the figure P a g e P age 2 of 2 4 İ = ©¥ j ' ©¥ ' at dr (2) as the di¨¤erential lim it of average current. If the rate at w hich charges flow varies w ith tim e, the ©¥nstantaneous current 1 is defined ©©ili¨¤ LL " " m rm M is defined as the tate of f©¥ow of cte and ©¥ts U nit current ©¥ E lectric W nenever charged particles m ove, a n e le c tr ic c u rr e n t is said to exist. 1 4 . 2 E ©¥ectric C urrent 1 & C harge a electricn©¥ circuit. term inal. T o ana©¥yse sucH a c©¥osed circuit, w e s hall now ©¥ook at thę different parts of the r source of e. m . f. To one or m ore devices, lik e a re s is to r, a n d then back to the other provided by the connecting w ires, for electrical charges to flow nom one term inal of the T he figure above shows a closed electrica©¥ circuit. T here is a continuous conducting path, C onductor of resistance R C ircuit ©¥ E ©¥ectrica©¥ 1 4 . 1 ©¥ntroduction pH Y S ©¥C S D E P A R T M E N T R A F F L E S ©¥N S T IT U T ©¥O N

pH Y S ©¥C S D E pA R T M E N T R A F F L E S I N S T ©¥T U T IO N ©© ©© P a g e P age a of 2 4 (b) H ow long does it take for 1 . 00 C of charge to pass through the calculatoľ ? it in a second? (charge of an electron, e - 1 . 6 0 ¡¿ 1 0 - 1 9 C ) E xam ple 1 (a) If a current of 0 . 320 m A flow s through a calculator, how m any electrons pass through (Üâ)Q = lİ dt If the current f©¥ow is not constant, from equation (2 ) , ņ l o n e second w hen there is a constant current of one am pere ( 1 c - 1 A s ) ¡¤ D eli O ne co u©¥o m b is defined as the am ount of electric charges that passes through a point in T he S . ©¥. u n it for electric charge is the coulom b, C . - 0 . Ų ￠ Q = ï t ľ ' passing through it is given by / through a cross - s e c tion of a conductor for a duration t, the am ount of electric charge Q ©¥ts U nit experience a force w hen placed in an e©¥ectric fie©¥d. W nen a constant current I now s C harge Q and E ©¥ectric charge is a fundam ental property of m atter w hich causes a charged obje©¢ to T he S . I. u n it for current is the am pere, A (it is one of the seven base units) . Ck r n 1 A w 1 ¡¤ flo. * l dq e ¡ß n a￠ L rru u 4¡Æ f \ o" " rrbl ¡¤ ¨¤ ou©¥, \h t¨¤l

P a g e P age 4 of 2 4 gaps. available for electrical conduction due to large band - sufficient energy to be n eeď , a n d th ey are not g©¥ass sem iconductors, th e s e e le c tro n s c a n n o t a c q u ire qw porcelain, insu©¥ator are Involved ©¥n form ing covalent bonds. unlike Ú¶ÙÍÝâ Insulators none rubber, A l©¥ e le c tro n s in th e o u te r s h e ll o f th e a to m s o f a n conduction. solution and they are m obile. H ence, they contribute to e©¥ectrical ch©¥oride T hese ions are either positively or negatively charged, sodium S om e salts dissociate into ions w hen disso©¥ve in w ater.E lectro©¥ytes ions m oving in the opposite direction to the electron - now . seem to behave like positively charged particles, holes from w here they com e from . ©¥n this w ay, th e h o le s neighbouring electrons can m ove to fill them and leave T hese electrons leave behind holes in the atom s w here energy to leave the atom s to becom e " fre e " e le c tro n s . room tem perature, these electrons m ay have sufficient contlt©¥ctors holes silicon am ong neighbouring atom s to form covalent bonds, a t S em i electrons and germ anium , A lthough all four electrons in the outer shell are shared silver, * om its outer shell to form the bond necessary to hold E ach atom of the m etat supplies one or m ore e©¥ectronsM ettta'''" 国画囲国øí×á M aterial C harge earners E xam ples R em arks T he tab©¥e below show s exam p©¥es of the charge carriers in different m aterials. particle «regardless of sign) as a m obile charge cai Öõ ier. positive and negative©¥y charged particles. It is com m on to refer to a m oving charged In som e cases - s u c h as gases and e©¥ectrolytes - the current is the resu©¥t of both the©¥rection of the culT ent ls opposite to the direction of f©¥ow of e©¥ectro . In m etals, the cu©¥T ent resu©¥ts * om the m otion of negative©¥y charged e©¥ectrons. T herefore, C arriers diner for different conductors. C harge charges, re g a rd le s s o f th e a c tu a l c h a rg e d p a rticles In m otion. T he charged particles m ay T ypes of B y convention, t h e d ir e c tio n o f c u rre n t is d e fin e d a s th e d ire c tio n o f the flow of positive P H Y S IC S D E P A R T M E N T R A F F L E S I N S T IT U T ©¥O N

P a g e ©¥ P age S of 2 4 ¡ß W ¡¤ q \ *u F ¡¤ tt rtw ©¡ l wï potential. higher potentia©¥ to a ©¥ower potential since the e©¥ectric ¨¤eld is directed towards low er current would be the sam e as the m otion of positive charg©¡carriers, w hich m ove from a conductor, c a u s in g them to m ove and therefore creating a current. T he direction of the and ©¥ts U nit the conductor. The electric field exerts a force on the free charg©¡carriers in the D ifference V pg!ę n!ia!. The battery sets up ap otendb©¥ ' crl¨¤erenc￠ and hence an electric field across potentia©¥ ©¥f a conductor is conned ed to į all points on the conductor are ņ gţ pL ţ e saM e 1 4 . 3 P otentia©¥ D ifference V T im e interval A t A t I = T otal charge through A in A t . N A V D qA t 4 1i4 ' ťifg©¥. nA voqd . T he current is then given by charge carried by each chargrrier is g, the total charge passing through A in t is T he num ber of charge - c a r v iers passing through A in t is thus nA vof . G iven that the (the shaded cy©¥inder) w ill pass through A . T he volum e of the shaded cylinder is A vod . to the right. T hus, in t , c harges w ithin a distance of vbt to the left of the crossection A ©¥n a tim e interval t , e a ch charg©¡caiT ier (assum ed to be positive) m oves a distance vbt II - = ©¥ l= Vot current I is now ing to the right. current now) and n charge - carriers per unlt vo©¥um e ÛÊÊç ' cal©¥ed the iiû inbiiir . ' Dei