nll c~ DLe E~ V I. TOPIC 17 Current Electricity o V 0 V N77/1I/ ' I -, o voltage 'J.

Transcription

1 TOPI 17 urrent lectricity Which statement describes the electric ptential difference I I I between tw pints in a wire that carries a current? the frce required t mve a unit psitive charge between the pints ~ 8 the; rati f the energy dissipated between the pints t the current the rati f the pwer dissipated between the pints t the current the rati f the pwer dissipated between the pints t the charge mved ; N ; J97113; J2000ll6 2 Which ne f the fllwing graphs best represents the relatin between current I and appl ied p.d. V fr the frward characteristic f a germanium dide?,i ' I B -,.' f V I. 0 V 0 'J. -:- -D I--!-'-.. +-V--.::J=-+--+-f. -l jf-: I V J761I24 3 resistr f resistance 1.0 kq has a thermal capacity f 5.0 J K-I. p.d. f 4.0 V is applied acrss it fr 120 s. If the resistr is thermally insulated, the tinal rise in temperature is 8.0 X JO-4 K n 3.2 x 10-3 K 9.6 X 10-2 K K 9.6 K nll c~ v 0 V 0 V DLe ~ V 0 V N771I17 5 surge f current tlws thrugh a filament lamp when it is first switched n. The reasn fr the surge is that when the lamp is switched n, the filament is cld and its resistance is much less than at its wrking temperature. 8 the mains vltage may be at its peak value when the lamp is switched n and the current will then be greater than its r.m.s value. mains switches are spring-laded and make sudden cntact, nt allwing time fr the current t increase gradually. the parallel cnductrs in the mains cable act as a capacitr and this capacitr discharges itself thrugh the filament. the filament is a cil and this acts as an inductance which prduces a large e.m.f Which f the fllwing graphs best represents the currentvltage characteristics f a semicnductr dide? D c Q) t: ::::l c 4 The resistance f tungsten increases with increasing temperature. s a result, the relatin between the current, I, flwing in the tungsten fi lament f an electric lamp and the ptcntial differcnce, V, between its ends is f the frm vltage N urrent lectricity 184.' Physics Tpical Paper

2 7 high ptential is applied between the electrdes f <I hydrgen discharge tube s that the gas is inised. lectrns then mve twards the psitive electrde and prtns twards the negative electrde. In each secnd, 5 x IO IH electrns (each f charge -1.6 x IO-I~ ) and 2 x 10lH prtns pass a crss-sectin f the tube. The current flwing in the discharge tube is 0.16 n J801l23 Fr a given thickness, the resistance between ppsite edgl' faces f the sample (shwn shaded in Fig. I) is prprtinal 10,\'2. n prprtinal t x. independent f x. inversely prprtinal t x. inversely prprtinal t x2. N81111!21 11 battery is cnnected in series with a 2 Q resistr and a switch as shwn in Fig. 2 belw. vltmeter cnnected acrss the battery reads 12 Y when the switch is pen hul Y when it is clsed 8 Tensile strain may be measured by the change in electrical resistance f a slraill gauge. strain gauge cnsists f flded fine metal wire munted n a flexible insulating backing sheet. Iirmly <lttached t the specimen, s that the strain in the metal wire is always identical t that in the specimen. ll------='==i=t~i jrspecimen strain gauge When the strain in the specimen is increased. the resistance r the wire decreases, because the length increases and the crss sectinal area decreases. n decreases. because the length decreases and the crsssectinal area increases. c increases r decreases depending upn the relative magnitude f the changes in length and crss-sectinal area. increases, because the length decreases and the crsssectinal area increases. increases, because the length increases and the crsssectinal area decreases. N80111l19; J generatr prduces 100 kw f pwer at a ptential difference f 10 ky. The pwer is transmitted thrugh cables f ttal resistance 5 Q. Hw much pwer is dissipated in the cables? 50W n 250W 500W 50000W J8I1117; J87120; J ; J sample f resistive material is prepared in the fnn f a thin square slab f side x. Fig. I Fig. 2 2Q What is the internal resistance f the battery? 21 3 Q n IQ 41 3 Q 0 4Q 6Q JB2111!17 12 In the circuit shwn in Fig. 3 belw, the resistance f the tungsten tilament lamp L increases with temperature. The current can be varied by means f the rhestat. Fig. 3 Which ne f the fllwing graphs best shws hw the ammeter reading I varies with the vltmeter reading V? 1 1 L ~ D I' 0 \! NB211 II I 7 17 urrent lectricity 185. ' Physics Tpical Papel

3 13 battery is cnnected t a unifrm resistance wire XY, and the end Y f the wire is earthed as shwn in the circuit diagram belw. + x Which ne f the fllwing graphs shws hw the currenl density J varies alng XV? y WI t' tl t' pwer in in circuit? 1a IS 1e ra 10. '....). pwer In III ircuit (II n J Sme early electric light bulbs used carbn 1ilamcllts. the resistances f which decreased as their temperature increased. Which f the fllwing graphs best represents the way in which I. the current thrugh such a bulb. wuld depend upn V. the ptential difference acrss it? B c Jt"" J ~ 0 (zer at all pints) X y X Y ) ; ) What is Ihe cnductance f a wire f length I and unifrm crss-sectinal area made f material f resisti vity p? p I pi n P -,- pi I P II J831118; N84I circular plastic disc f diameter d has n metal studs unifrmly sp,1ced arund its circumference. ach stud carries a charge q. The disc is made t rtate abut its axis at " revlutins per secnd. What is the equivalent current f the rtating charges? nq 1 p npq ltd n nq 1tdnpq IlplJ ) Three identical cells each having an e.m.f. f 1.5 V and a cnstant internal resistance f 2.0 Q are cnnected in series with a 4.0 Q resistr. firstly as in circuit. and secndly as in circuit. 0"-- v J The resistance f a certain circuit clement is directly prprtinal t the current passing thrugh it. When the current is 1.0 the pwer dissipated in the clement is 6.0 W. What is the pwer dissipated when the current is raised t 2.0? 3.0W 24W n 6.0W 48W 12W N wire 3.00 m lng. f unifrm crss-sectinal area 2.00 mm2, has a cnductance f 1.25 S. What is the resistivity f the material f the wire? I S = In-Il 5.3 X 10-7 S n 8.3 x 10-7 S 0.80S S L9 X 106 S N871f14 20 In the diagram f an scillscpe belw, the shaded are<j represents a sectin thrugh the electrn beam which is generated near P. detlected and accelerated at Q. and fcused at. The tube cntains n gas. ircui! ircuit 17 urrent lectricity 186 J.\' Physics Tpical Paper

4 -lw dcs the electric current f the beam vary alng PQ? It has the same value at P, Q and. II D It decreases frm P t Q but becmes very large at the pint. It decreases frm P t Q, then increases t reach the same value at as at P. It increases between P and Q, then remains cnstal1l frm Q t. It increases frm P t Q then decreases t reach the same value at as at P. N cylindrical metal wire, f length 1 and crss-sectinal area S. has resistance, cnductance G, resistivity p and cnductivity a. Which ne f the fllwing expressins fr ais valid? +r D I' -r r B G pig D P S I GS +r B -1 S p J88115 G 22 cnducting liquid fills a cylindrical metal case t a depth x as shwn in the diagram. The resistance between the case and the metal rd is metal rd prprtinal t,\'2. insulating base 11 prprtinal t x. independent f x. D inversely prprtinal t x. inversely prprtinal t,\'2. N n electric current flws alng an insulated strip PQ f metallic cnductr. The current density in the strip varies as shwn in the graph. current density p distance atng strip frm P Which ne f the fllwing statements culd explain this variatin? B D The strip is narrwer at P than at Q. The strip is narrwer at Q than at P. The ptential gradient alng the strip in unifrm. The current at P is greater than the current at Q. The resistance per unit length f the strip is cnstant. N88114 Q *241n an electrstatic machine, a belt f width IV, having surface charge density 0', travels with velcity I'. s the belt passes a certain pint, all the charge is remved and is carried away as an electric current. What is the magnitude f this current? IVVO' D IVV 2 0' B IVO' \I..':Q.. J89113 \I' 25 n electrical surce with II1ternal resistam;c r is used t perate a lamp f resistance. What fractin f the ttal pwer is delivered t the lamp? J Tw squares, X and Yare cut frm the same sheet f metal f thickness t. The lengths f the sides f X and Yare 1and 21 respectively. What is the rati.!!j f the resistances between tile ppsite y shaded faces f X and f Y? I I 2 c D 4 2 I I N8911 0; J thermcuple is cnnected acrss a galvanmeter 01 resistance 30 n. One junctin is immersed in water at 373 K and the ther in icc at 273 K. The e.m.1'. f the thermcuple is 90l1V fr each I K difference in temperature between the junctins, and the thermcuple resistance is 6 n. What current will flw in the galvanmeter? 1.8 Jt D 1.5 m II 250 ).l 1.8 m 300 ~I N89113; J The current I flwing thrugh a cmpnent varies with the ptential difference V acrss it as shwn. y 1 17 urrent lectricity 187. ' Physics Tpical Papel

5 Which graph best represents hw the resistance varies with 31 student is givcn a sealed bx cntaining a cncealcd V? ==:t: electrical circuit. Having taken a series f current and voltagc readings. the student plts the current-vltage characteristic belw. currentrn +100 V 0 V 0 V l2l2 V 0 V N I...-+-"'::;I,;.,rc;:...-t-+""1._0 vltage IV -100 ~atr (p,v) 29 gcneratr. with utput pwer P and utput vltage V. is cnnected t a factry by cables f ttal resistance. '2 r------{~i------!d factof'( What is thc pwer input t the factry? P n p- (~).!!.. 0 p _ (~)l ~ V 2 P - (~) P - (~) '2 N921l Thc diagram shws a metal filament lamp cnnected in serics with an ammeter, battery and a variable resistr. high resistance vltmeter is cnnected acrss thc lamp. Which circuit is mst likely t be enclsed within the bx? ----~~~~----~ a n c e- J 0 ~ N94111l5 32 Tw wires P and Q, each f the same length and the same material, are cnnccted in parallel t a battery. The diameter f P is half that f Q. What fractin f the ttal current passes thrugh P? n N cell f internal resistance,. is cnnected t a lad f resistance. cell, , Which graph represents the vanatlon f I, the current thrugh the lamp. with the ptential diffcrence V acrss it? 'L'L B '~ V 0 V 0 V D ll '~ ~ nergy is dissipated in the lad, but sme thermal encrgy is als wasted in the cell. The efficiency f such an arrangement is fund frm the expressin energy dissipated in the lad energy dissipated in the cmplete circuit Which f the fllwing gives the efficiency in this case?,. r n D ' +,. +r V 0 V N93112 J urrent lectricity 188. ' Physics Tpical Paper

6 34 Which graph best represents the way in which the current I 38 The diagram ~hws the relatin between the direct current I thrugh a thermistr depends upn the ptential difference V in a certain cnductr and the ptential difference V acrss acrss it? it. When V < 1.8 V, the current is negligible. B c ll IlL: 'L'~ V 0 V 0 V 0 V N The diagram shws a rectangular blck with dimensins x, 2x and 3x. 2x lectrical cntact can be made t the blck between ppsite pairs f faces (fr example, between the faces labelled P). Between which tw faces wuld the maximum electrical resistance be btained? n the faces labelled P the faces labelled Q the faces labelled the resistance is the same, whichever pair f faces is used J The resistance f a thermistr decreases significantly as its temperature increases. Which graph best represents the way in which the current I in the thermistr depends upn the ptential difference V acrss it? n '~'~ll'~ 00 V 0 V 00 V 0 V J971114; N lim Which statement abut the cnductr is crrect? n D It des nt bey Ohm's law but when V > 1.8 V its resistance is 4 n. It des nt bey Ohm's law but when V = 3 V its resistance is In. It beys Ohm's law when V> 1.8 V and when V = 3 V its resistance is In. It beys Ohm's law when V> 1.8 V but its resistance is nt cnstant. J Fur small cnductrs, n the edge f an insulating disc f radius r, are each gi ven a charge f Q. The frequency f rtatin f the disc is! What is the equivalent electric current at the edge f the disc? 4Q 4Qf n 8rtrQf T vv D 2Qf rtf N98II!l4 40 The diagram shws a mdel f an atm in which tw electrns mve rund a nucleus in a circular rbit. The electrns cmplete ne full rbit in 1.0 x s. 37 Prtns in a parallel beam each mve at a unifrm velcity v, thus frming a current I. The charge n eaeh prtn is e. Which expressin represents the number f prtns present in unit length f the beam? (Yu may wish t cnsider the units f the quantities invlved.) 1 Iv I n e ev e v N97II114 electrn electrn What is the current caused by the mtin f the electrns in the rbit? 1.6 X X 10-4 n 3.2 x D 3.2 x 10-4 J991II 5 17 urrent lectricity 189 '' Physics Tpical Paper

7 41 cell f e.m.1". delivers a charge Q t an external circuit Which statement is crrect? i\ The cnergy dissipatin in the extemal circuit is Q B The energy dissipatin within the cell is Q. The external resistunce is Q. The ttal energy dissipatin in the cell and the external circuit is Q. N thin mm resistr in a slid-stute circuit has u thickness I J.lln and is made f nichrme f resistivity 10-6 nm. alculate the resistances available between ppsite edges r a I mm2 area f film (a) if it is square shaped. if it is rectangular, 20 times as lng as it is wide. J generatr supplies a fixed pwer t an electrical installatin by pwer lines f tinite resistance. Shw that the pwer lst as heat in the cables is prprtinal t IIva. where V is the ptential difference at the generatr utput. *44 This result suggests that lsses can be reduced indefinitely by a suitable chice f generatr p.d. Discuss briefly whether this is practicable. N81117 Fig. 4 ::; aluminium S::; steel n electricity supply cable cnsists f a steel cre f area f crss-sectin 50 mm 2 with six ther cnductrs f aluminium f the same crss-sectinal area arranged arund it (Fig. 4). Find the resistance f a 120 m length f the cable. [esistivities: steel. 9.0x 1O-snm, aluminium, 2.5 x 10-8 nm.] J82l6 45 Using an electric drill. it takes ISO s t make a hle in a piece f brass f mass 0.45 kg. During this prcess, the average pwer delivered t the drill frm the electricity mains is 300 W. Hw much electrical energy is used in drilling the hle? If 70% f the energy supplied t the drill appears as heat in the brass, what is the initial rate f rise f temperature f the metal? (Specific heat capacity f brass::; 390 J kg-' K-'.) J cell has e.m.f. 1.5 V and internal resistance 0.5 n. alculate the pwer delivered when the cell is cnnected t an extemal 2.5 n resistr. What is the value f the external resistance if the pwer delivered is t have a maximum value? [6) J ((1) Define pwer. (h) Why is it that. in the SI system f units. pwer cannt be defined using the equatin pwer =I",(ential diltcrencc x currcnt'! [2] NS8I11 48 ptential difference V drives a current I thrugh the filament f a trch bulb. a piece f intrinsic semicnductr. Sketch graphs, ne fr each case, t shw hw J depends n V as V is increased frm zer t a value which causes significant heating. Why d the graphs differ in shape'! (6] N8911I5 49 n electric kettle is rated at 2.0 kw when perating n a 240 V supply. What pwer will the kettle absrb if the supply is reduced t 220 V? [3] N (0) tiiament lamp is marked 240 V 60 W. alculate 0.5 the current thrugh the lamp when it is wrking nrmally. (I] the resistance f thc lamp when it is wrking nrmally. [I] (iii) The resistance f the lamp is fund t be less when it is nt lit than when it is wrking nrmally. Sketch the current-vltage characteristic f the l1iament lamp n the axes in Fig. 5. current I Fig vltage IV J (part) 51 (a) State the equatin linking charge Q. current J and time ~ [I] alculate the charge which passes each pint in a circuit in a time f 60 s when there is a current f 0.76 rn. (2] N (part) 52 n electric light bulb is marked 120 V. 40 W. ra) alculate the current in this bulb when perating n a 120 V supply. current =... [2] 17 urrent lectricity 190.' Physics Tpical Paper

8 (e) Describe three different ways In which ent!rgy can be transferred frm the fiiament when it is perating nrmall y. [6] If the light bulb is nw cnnected t a 240 V supply it will wrk fr perhaps an hur as a 160 W bulb befre its filament breaks. Ii) Hw will the relative imprtance f the answers yu have given t change when the bulb is run n this increased vltage? Suggest a reasn why the bulb can perate fr s lng a time befre its tilament breaks. [3] N971l115 SS What d YOll undelstand by (a) resistance, resistivity, (c) cnductivity. NSIII1II5 (part) 56 (a) Define the colllmb, thejllie and the wall. Shw that defining the vlt as ajule per culmh is equivalent t defining it as a watt per ampere. [5} N (part); N82flll3 (part) 57 Denne the vlt, and distinguish between electrmtive frce and ptential differellce. N (part) 53 (a) Describe, in terms f a simple electrn mdel, the difference between an electrical cnductr and an electrical insulatr. [2] (c) Id) State tw metals which are cnductrs, ne nn-metal which is a cnductr. (iii) ne slid which is an insulatr. [3) ir is nrmally an insulatr. Hwever. it can be made t cnduct under certain cnditins. Suggest ne cnditin that culd cause air tn cnduct electricity. Suggest ne hazard which might be assciated with the cnductin f electricity in air. [2] In a cathde-ray tube, there is a current f 150 Jl in the vacuum between the cathde and the ande. alculate Lng Questins the time taken fr a charge f 3.0 t be transferred. time taken =... S the number f electrns emitted per secnd frm the cathde. number per secnd =... S-I [4} J S4 Distinguish between the terms ptential difference and electrmtive frce. Define the units in which they are usually measured. [4] Give three examples f systems in which current is nt directly prprtinal t applied ptential difference. Give sketch graphs shwing the frm f each I-V relatinship and give sme explanatin f the frm f each graph. [6] The current I thrugh a certain device is related t the p.d. V acrss it by the equatin I = a (e hv - I) where a =10-1 and b=40 V-I. When the device is cnnected t a battery f e.mj. 1.5 V. a current f 0.1 flws. alculat~ the internal resistance f the battery. [Ig e '" 0.43.] [8] N771I Deduce frm first principles an express fr the rate at which heat is generated in a circuit f resistance in which a current I passes. State clearly the basic detinitins yu emply. vltmeter is cnnected acrss a variable resistance,. which itself is in series with an ammeter and a battery. Fr ne value f, the readings are 1.00 V and Fr anther value f, they are 0.90 V and alculate the e.m.f. f the battery. its internal resistance. and the tw values f. State any assumptins yu make cncerning the meters. The value f is nw adjusted s that it equals the internal resistance f the battery. alculate the rate at which heat is generated. (a) in the battery, in the whle system. NS4lIlf3 59 What d yu understand by the terms ptential difference and electrmtive frce? Describe hw the internal rcsistance f a cell may be determined. Give the thery f yur methd. The varius electrical circuits in peratin in a car may be cnsidered t{) have a cmbined resistance. They are. supplied by a battery f e.m.f. and internal resistance r. (a) If the efficiency 11 f thc circuits may be defined as the rati f the pwer delivered t the circuits t the ttal pwer dissipated, shw that 11 = ( + r). Sketch labelled graphs n the same axis fr t sh\\< hw 11 and the pwer utput vary with. Mark the axis with the value f r. (c) Hence, r therwise, determine 11 when there is maximum pwer utput frm the battery. If the starter mtr f a car is perated when the headlights are switched n. they are seen t dim. xplain this bservatin. NS51II Detine resistance and resistivity. (a) alculate the resistance f a nichrme wire f length 500 mm and diameter 1.0 mm given that the resistivity f nichrme is 1.1 x 10-6 Q m. 17 urrent lectricity 191.' Physics Tpical Paper

9 (il Describe hw yu wuld use a ptentimeter t make an accurate determinatin f the resistance f this wire. xplain why prblems might arise in trying t determine the resistance f this wire by measuring the current thrugh it and the ptential difference aerss it using mving-cil meters. State and explain ne additinal precautin which wuld be needed t btain an accurate value fr the resistance f a wire f the same dimensins as abve. but made f cpper. (esistivity f cpper = 1.7 x IO- K n m) N8611I1I1 I 64 (aj Denne electric ptellfia difference and state the SI unit in which it is measured. (h) Use yur definitin in t shw that P. the pwer dissipatin in a resistr f resistance is given by V2 p= where V is the ptential difference acrss the resistr. [5] battery f e.m.f. and internal resistance r is cnnected in series with a resistr f resistance. as shwn in Fig Define ptelltial difference and resistallce. Derive an expressin fr the rate at which thermal energy is generated in a circuit f resistance in which the current is I. [4] Describe hw a vltmeter may be calibrated at ne particular reading by an electrical heating methd. By reference t the thery f yur methd, explain why vltmeters calibrated in this way cannt be said t have been calibrated in terms f the base units nly. [7] certain vltmeter has a resistance f 1500 n. When cnnected t a battery, the vltmeter reads 2.90 V. When a resistr f resistance 1000 n is then cnnected in parallel with the vltmeter, the reading drps t 2.76 V. xplain why the reading changes and calculate all yu can abut the battery. [6] N8711I1I11 62 (a) Define resistance. Write dwn an equatin which relates resistance and resistivity. Identify all the symbls in the equatin. [3] alculate the resistance per metre f a cpper wire f diameter mm and resistivity 1.7 x IO- K n m. [2] J (part) 63 (a) Distinguish between the electrmtive frce f a cell and the ptential difference between its terminals. cell f e.mj. and internal resistance r is cnnected t a resistr f resistance as shwn in Fig. 6. (e) Fig. 7 Give expressins, in te~ms f,,! and fr. (I) the current I in the circuit,, (2) the pwer P dissipated in the external resistr. The battery generates ttal pwer PT' Shw that the fractin P is given by PT K -=- +r [6] P P T car battery f e.m.f. 12 V and internal resistance n delivers a current f 110 when first cnnected t the starter mtr. alculate (I) the resistance f the starter mtr, (2) the fractin f the ttal pwer which is dissipated in the batter:'. (5) fter prlnged use, the internal resistance f the battery may increase. State and explain hw the perfrmance f the battery is affected by an increase in the internal resistance. [4] J Fig. 6 I, Shw that V, the terminal ptential difference f the cell, is given by V=- [6] +r N9111I4 (part) 65 (a) ptential difference f 9.0 V is causing electrns t flw thrugh a steel wire s that 1.0 x electrns pass a pint in the wire in 60 s. alculate the charge which passes the pint in 60 s. the electric current in the wire. (iii) the resistance f the wire. [6) Sketch graphs with labelled axes t shw hw the current thrugh the wire will vary with the p.d. acrss it if the temperature f the wire is kept cnstant. 17 urrent lectricity 192 '' Physics Tpical Paper

10 (c) increases as the current increases. [4] cpper wire f area f crss-sectin 3.8 x lo-y m2 has length 0.67 m. alculate the resistance f the wire. (esistivity f cpper = 1.7 x IO- n m.) [3] When the cpper wire is subjected t a particular stress, a strain f is prduced in the wire. (1) State what is meant by stress and by strain. [2] (2) Shw that the fractinal change in the area f crss-sectin f the wire is ssume that the ttal vlume remains unaltered. [2] (3) alculate the resistance f the wire when it has this strain f [3] N94III3 (e) healing element Fig. 9 rv heating element Suggest a mre suitable means f cntrlling the average pwer utput f the heating element vcr a perid f time. [2] J (a) Use energy cnsideratins t distinguish between electrmtive frce (c.m.f.) and ptential difference (p.d.). [3] cell f e.m.f. and internal resistancc r is cnnected t a resistr f resistance, as shwn in Fig. 10. (II) 66 (a) Define ptential difference. [I] 2 If the equatin V = I is used t define resistance, why is it nt then pssible t use the same equatin t define ptential difference? [2] (c) One element f an electric cker has an I - V characteristic as shwn in Fig. 8. (d) Fig. 8 vv xplain hw the characteristic shws that the resistance f the element increases with ptential difference. [3] xplain in terms f the mvement f charged particles why the resistance increases with ptential difference. [3] (iii) Use the graph t estimate the ptential difference which shuld be applied t the element if it is t have a resistance f 30 n. [3] (iv) What will be the current in the element when it has a resistance f 30 n? [I] (v) What will be the pwer f the element when it has a resistance f 30 n? [I] Why is it lit sensible t cntrl the pwer t a heating element, such as the ne fr the electric cker in (c), by the use f either f the circuits in Fig. 9? [4] (v} Fig. 10 vltmeter f infinite resistance is cnnected in parallel with the resistr. py Fig. 10 and include in the circuit a switch s that the vltmeter may be used t measure either the e.m.f. f the cell r the tcrminal ptential difference V. State whether the switch shuld be pen r clsed when measuring I. the e.m.f, 2. the terminal p.d. (iii) Derive a relatin between, V, r and the current I in the circuit. [5] J (part) 68 (a) Define ptential difference and the vlt. [2] (c) battery f e.m.f V and internal resistance 0.50 n is cnnected t a resistr f resistance 8.36 n. Determine the current in the circuit, the ptential difference acrss the 8.36 n resistr, (iii) the pwer supplied t the 8.36 n resistr. [6] xplain why the ptential difference acrss the terminals f a battery is nrmally lwer than the battery's e.m.f. Under what cnditin is the ptential difference acrss a battery's terminals equal t its e.mj.? [3] N961I13 (part) 17 urrent lectricity 193 ' Physics Tpical Paper

11 69!(lj Ii I xplatn whm IS II1cant by the "(',HSIl'!ll' r a material. Shw that the unit in which resistivity" measured is n m. [3],'\ (part) 70 I(I) What is meant by an electric currellt? xplain why pwer is required t maintain an electric current in a metallic cnductr. [4] Ic) metal f resistivity p is used t make an electric cable f crss-sectinal area Shw that the resistance r per unit length f the cable is given by the expressin (ij) 71 (a) Deline (c) (ei) V r=p. Sme electrical appliances are used with lng cables t cnnect them t the electrical supply. State and explain tw reasns why the cnnecting cable shuld have a lw value fr r. [6] J98J112 (part) Ie; 72 (a) polellticlt difference. 73 (a) the I'lt. L1J State the equatin relating the resistance f a wire t its area f crss-sectin, its length I and the resistivity pfthe material f which it is made. [I] The verhead wire used t supply pwer t the engine f a train has area f crss-sectin 5.00 x 10-5 m2 and is made f cpper f resistivity 1.72 x IO-K nm. alculate the resistance f ne kilmetre length f the wire. [2] Fig. II shws the arrangement fr supplying pwer 10 an engine. 25 kv supply is used and the return current frm the engine returns thrugh the track. The resistance per kilmetre f the verhead wire is as' calculated in (c) and the resistance f the track can be neglected. verhead wire \ Fig. 11 '", track engine nsider tirst when the engine is clse t the pwer supply and requires 6700 kw f pwer. alculate the current which is needed. [2] When the engine is 30 km frm the pwer supply, it is supplied with a current f 180. alculate I, the resistance f the verhead wire between the pwer supply and the engine, (c) 2 thcptenlial difference acrss the engine. J. the pwer supplied t the enginc. 4. thc fractin f the pwer supplied which is lised by the engine. [7] xplain the fllwing facts abut the supply t the engine. Iii) railway emplyee wh tuches the track thrugh which there is a current f 180 dcs nt get an electric shck. high vltage supply is essential fr a railway system such as this. (iii) different current is needed when the train is climbing a hill frm that required when travelling at the same speed n the flat. [6] N981II4 State what is meant by the resistance f a sample f a metal. the I:esistivity f the metal. [4] J (part) xplain what is meant by an electric clrrent. xplain why sme slids are electrical cnductrs and sme are insulatrs. (iii) Describe electrical cnductin in a metal. [3] xplain why it is difticult t qute a value fr the resistance f a tilament lamp. [ I] In a gas, cnductin ccurs as a result f negative particles tlwing ne way and psitive particles flwing in the ppsite directin, as illustrated in Fig. 12. \ I ~ -0 I 6 ~ I \ cpper cpper cnductr gas at lw cnductr pressure Fig. 12 In this case. the cpper cnductrs t the gas carry a current f 0.28 rn. The number f negative particles passing any pint in the gas per unit time is 1.56 x S-I and the charge n each negative particle is -\.60 x alculate the negative charge tlwing past any pint in the gas per secnd, the psitive charge tlwing past any pint in the gas per secnd, (iii) the number f psitively charged particles passing any pint in the gas per secnd, given that the charge n each psitive particle is +3,20 x 1O-19. [6] 17 urrent lectricity 194 ' Physics Tpical Paper

12 (ei) *(e) By cnsidering the significant figllres d\ailable, explain why yur answers t (c) and (Iii) arc unreliable. [2] In ne practical example f ekctrical cnductin f particles thrugh a gas. a magnetic field is applied acrss the gas. Instead f travelling directly alng the tube the psitive and negative charges travel alng different paths. us shwn in Fig cm 1 T typical path f negative charges Fig. 14 typical path f psitive charges Fig. 13 State the directin f the applied magnetic field. xplain hw yu btained yur answer. Hw is it pssible fr the psitive particles still t travel in an apparently straight line? The average thickness f the skin f his hand is I mm. stimate the current thrugh the persn. [2] (iii) Discuss tw factrs, referred t abve, which affect the magnitude f the current and hence affect the pssible danger frm electric shck. One bvius safety precautin is t keep live wires well insulated. What ther safety precautins d yu suggest? [4] N20001ll114 (iii) State Kirchhtrs first law and use it t find the current in cnductr. (iv) Suggest what is happening t the particles at surface S f the cpper cnductr. [8] N991II5 74 (a) State the equatin linking charge Q. current I and time t. [I] alculate the charge which llws past any pint in a circuit when a current f 6.2 rn exists in the circuit fr ne hur. Why shuld yu nt give yur answer t 4 significant figures? [2] (c) Define ptential difference. [I] (d) water heater is marked 230 Y, 3000 W. It is switched n fr 5000 secnds. Fr this heater, calculate the current thrugh the heater, the resistance;' f the heater, (iii) the energy supplied by the heater during this time. [7] (e) The resistivity f the human bdy is lw cmpared with the resistivity f skin, which is abut 3 x 104 n m fr dry skin. Fr a layer f dry skin I mm thick, determine the resistance f a I cm2 area f skin. [3] persn, wh is well earthed, accidentally grabs a wire f diameter 0.4 cm at a rtential f 50 Y. His hand makes cntact with the whle circumference f the wire ver a distance f 9 em as shwn in Fig urrent lectricity 195 '' Physics Tpical Paper