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GCE Ordinary Level

Mark Scheme with Examiners’ Report

London Examinations Ordinary Level GCE inPhysics (7540)g~åì~êó=OMMP

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April 2003

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Mark Scheme and Chief Examiner's ReportJanuary 2003

PHYSICS 7540

Mark SchemePage 2 of 15

Chief Examiner's ReportPage 11 of 15

Grade BoundariesPage 15 of 15

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PHYSICS 7540, MARK SCHEME

Abbreviations used in the mark schemeUP unit penaltyTE transmits the errorOWTTE or words to that effectR Resistance

PAPER 1

(a) weight 700 N (UP once in question)686 N (using 9.8)

(1)

(b) weight duringlift-off

4200 N (TE) 4116 N (using 9.8)(TE) 6 x (a) or 7x i.e. 4900 N (4802 N)

(1)

vertically upwards anywhere between limits ofastronaut. A second arrow scores 0/2 in (c)(i)(ii)

(1)arrow (i)

(ii) labelled 4200 N or TE (b) dependent on first mark (1)

(c)

effect weight on chest / feels heavier/ blood towards back/ flattened on floor/ sinks into seat / chestcompressed ie. a new or greater effectNOT feels dizzy or light-headed

(1)

quantity vector (1)(d)

reason has direction (1)

(e) ticks scalar: speed and massvector: acceleration

(1)

1.

(Total 8 marks)

½ � 30 � 3 (1)

30 x 3 (1)

135 m or correct addition of areas (UP) (1)

(a) distancetravelled

(180 m scores 1/3 if no other mark scored)

dot closer to 0 than 2 seconds (1)one second

(uniform) acceleration/increasing velocity or speedor using area under graph

(1)

dot half-way between 4 and 6 seconds (1)

(b)

five seconds

uniform speed or velocity / not accelerating or usingarea under graph

(1)

2.

(Total 7 marks)

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(a) energy chemical to heat / thermal / internal (1)

110 � 0.2 (=22) or 110 x 30 (=3300) (1)

� 30 x 0.2 (1)

(b) amount ofenergy

660 J or Ws (UP) (1)

air (near head) is warmed / heated

warm air / fluid expands

(c) convection

less dense

(warm air) rises(maximum 3)

3.

(Total 7 marks)

50 � 0.05 or 50 x 5 (1)(a) moment

2.5 Nm or 250 Ncm (UP) (1)

(b) tension less than (1)

reading too high / pointer off “bottom” end of scale /scale not big enough

(1)(c) problems

spring goes beyond / reaches elastic limit / springbreaks

(1)

4.

(Total 5 marks)

eg, (2,200), (4,100), (12,30), (1.2, 300) (1)(a) two pairs

matching products 400 and 400, 360 and 360, 360and 400.

(1)

400 = 800 � V (1)(b) 800 kPa

V = 0.5 m3 allow 0.45 m3 (UP) (1)

(c) BC heat (gas) / increase temperature (1)

(d) difference moving faster at C, slower at B or reference to ahotter position e.g. more kinetic energy

(1)

5.

(Total 6 marks)

(a) sign of charge negative (1)

electrons transferred (no TE from (a)) (1)

6.

(b) origin ofcharge

from cloth (to rod) (1)

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movement of electrons (in metal rod) (1)

move to the right / far side (TE from (a)) (1)

(c) effect

stated like charges repel / stated unlike chargesattract

(1)

(mention of positive electrons or moving positivecharges loses first two marks in (c))

(d) charges at least 3 + at left-hand end and at least 3 – at right-hand end. All charges inside or just outside metalrod (TE from (a))

(1)

(Total 7 marks)

1.5/0.3 (1)(a) resistance

5 ohms (UP) (1)

arrangement parallel (1)(b)

brightness same brightness (1)

arrangement series (1)

brightness C is dimmer/ A is brighter (1)

(c)

reason more resistance / less current / small or smallercurrent / voltage is shared/ divided equally orunequally / each bulb gets half voltage / power isshared

(1)

7.

(Total 7 marks)

five or more straight lines parallel or slightlysymmetrically curved - at least one straight linepossibly the middle line

(1)(a) magnetic field

arrows anywhere on each line from N to Smust be 5 or more arrows

(1)

(b) (i) current arrow pointing up wire (1)

(ii) force arrow pointing into magnet originating from the partof the wire between the poles

(1)

larger current / voltage or less resistanceOR

(1)(c) increase force

stronger magnet (allow poles closer together)not bigger magnet

8.

(Total 5 marks)

9. (a) particles protons and neutrons electrons(0) (1)

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(b) how many ofeach

17 protons and 19 neutronsignore electrons

(1)

(c) orbitingparticles

17 electrons (1)

nucleon no. 36 (1)

proton no. 18 (1)

(d) (i)

(ii)

(iii) no. in orbit must be same as (ii) (1)

(Total 6 marks)

(a) difference (much) faster or radio = 3 � 108 m/s, sound = 330 to340 m/s

(1)

took time for sound (to travel) (1)explanation

took 4 secondsdependent on first mark

(1)

1350/4 (1)

(b)

(c)

speed

337.5 (or 338)m/s (UP)340 m/s is dependent on first mark being scored

(1)

(d) difference transverse / longitudinal or EM / mechanical orvacuum / medium or medium and no mediumrequired or suitable diagrami.e. must see a matching‘pair’ of responsesno marks for references to wavelength / frequency

(1)

10.

(Total 6 marks)

name refraction (1)(a)

(b) cause speed or speed changes / slows down (1)

difference different speed(s) (in glass) not in ‘air’ (1)

blue (1)

(c)

(d) nearest

blue slower(allow large(r) refractive index or smallerwavelengthnot larger frequency2nd mark dependent on 1st

No TE for red)

(1)

(e) green light between/in the middle (of the red and blue)(not nearer bluecan be marked on diagram)

(1)

11.

(Total 6 marks)

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PAPER 275 = (30 + 0)t/2 (1)(i) time to come to

rest t = 75/15 (= 5 s)(Allow use of equations including calculatedvalue of a. Allow use of area under v-t graph. Donot allow 75/30 = 2.5, 2.5 x 2 = 5)

(1)

= 30/5 (1)(ii) deceleration= 6 ms-2 (UP ignore sign) (1)

½ � 1,500 � 30 � 30 (1)(iii) change in kineticenergy = 675,000 J (UP) (1)

675,000 = F� 75 (must TE from (iii)) (1)(iv) opposing forceF = 9,000 N (UP only once for N)allow F= ma = 1500 x 6 (TE from (iii) allowed.Ignore sign of F)

(1)

correct force shown opposing motion (1)(v)labelled with value of F from (iv) (1)

vertical arrow(s) up or down (1)

15,000 N (UP) (1)

forces shownon box

correct name e.g. weight / gravitational pull / W /mg or normal reaction / R

(1)

(a)

(13 marks)A – (millisecond / electronic / electric) timer /timing devicenot stop watch / clock

(1)(i) instruments

(metre) rule / tape / measuring scale (1)

time (for ball to fall) (1)height / distance (from ball to contact plate) (1)

(ii) measurements

bottom / front of ball to contact plate (1)

(iii) calculate g 2 � distance/time2 where distance = s, x, h etcallow formula with s as subject but only whereu = 0 and a = g

(1)

(iv) A too large (g) too small / small / smaller (1)

1.

(b)

(7 marks)

(Total 20 marks)

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why heavier (extra) mass / weight of rod and / or screwssteel / titanium denser than bone / heavier

(1)

less dense than steel (1)

2. (a)

why titaniumtitanium rod is lighter than steel rodor conversetitanium rod is lighter than steel rod of same sizescores both marksor converse

(1)

(3 marks)

choosing 520 J/(kgK)0.070 x 520 x 272.8 J (73) (UP once in (b))

(1) (1) (1)

(0.071 x 510 x 2 = ) 72.4 J

(0.070 � 520 � 2 =) 72.8 J

(1)

OR

(1)

heating titanium

heating screws

OR

heating titanium

heating screws choosing 510 J/(kgK)0.071 x 510 x 272.4 J

(1)(1)(1)

(b)

accept mass of screws as 6 x 0.071 (434.5J)

(4 marks)

similar specific heat capacity (1)(i) not reasonsimilar mass(expansion = 0 marks)

(1)(c)

(ii) reason different expansion / contraction (values)

change in titanium approximately zero (2 marksmaximum)

(1)

(3 marks)steel has higher density (1)steel rod shorter / less steel (1)

steel rod

steel has higher expansion valueor converse (independent marks)

(1)

(d)

(3 marks)scale and orientation2 cm = 20 units, or better, on both axes

(1)

axes labelled with units (1)Plots: -1 per misplot to maximum of -2 (ignore 0,0) (2)

(i) graph

Line: - all points including (0,0) to be considered (1)(ii) value value from candidate’s graph - ignore unit (1)(iii) impossible temperature (rise) too high / won’t go above body

temperature etc.(1)

(e)

(7 marks)(Total 20 marks)

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240/24 = 800/N2 (1)(i) number of turnsN2 = 80 turns (1)

(2 marks)1 current in primary2 magnetises (iron) core3 (alternating) current changes4 produces changing field

(ii) energy transferred

1 mark each tomax 5 marks

5 changing field cuts / links / interactswith / to / in secondary coil6 induces an emf / current7 in secondary (coil / circuit) (5 marks)

secondary current 60/24 (= 2.5 A) (1)

2.5/10 (1)

(iii)

primary current0.25 A (UP)ORpower in primary = 60W (1)Current = 60/240 = 0.25A (1) (UP)0.25 A scores 2 marks

(1)

(3 marks)water is a conductor/ can conduct (1)

(a)

(iv) dangers(240 V) could kill people or fish / danger /possibility of electric shock/electrocution

(1)

(2 marks)60 � 100/86 (1)(i) power supplied69.76 W (no UP) (1)

(2 marks)10 � 5 � 60 (or 9.76 � 5 � 60) (1)(ii) energy= 3000 J (or 2928 J) (UP) (1)

(2 marks)heat loss to surroundings (1)equals heat transferred to transformer (1)

conduction(1)

(b)

(iii) constant value

principal processesconvectionmore than two, maximum of 1 out of 2marks

(1)

(4 marks)

3.

(Total 20 marks)

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deep water 1.3 to 1.4 (1)(i)

shallow water 0.9 to 0.95 cmaccept fractional answersvalues reversed score 1

(1)

(2 marks)20 � 1.3 (TE from (i)) (1)deep water26 cm/s (UP) (1)

20 � 0.9 (TE from (i))(mark awarded for frequency the same)no fractions allowed

(1)

(ii)

shallow water

18 cm/s (UP once)

consistent use of wrong frequency scores 1maximum in (ii)

(1)

(4 marks)speed speed unchanged (1)

wavelength smaller (independent, no TE) (1)

(a)

(iii)

wavelengthwavelength half of (a)(i) or calculation forboth marks

(1)

(3 marks)angle A 50º (no UP for degrees) (1)(i)

explanation angle of incidence = angle of reflectionobeys law of reflection OWTTE

(1)

(2 marks)sin 50/sin 30 (1)(ii) refractive index1.532 or 1.53 not 1.5must use sine values 0.766/0.5

(1)

(2 marks)sin-1 (1 / 1.53) (TE from (ii)) (1)critical angleall values of angle below are in degrees1.532 40.746 40.74* 40.71.53 40.813 40.8 411.5 41.81 41.8(an answer of 42 must include evidence*accept incorrect rounding)

(1)

60º (1)greater than c / or stated value (1)

(iii)

angle B

total internal reflection (whole phraseneeded, not TIR)

(1)

(5 marks)(iv) C 30º (1)

(b)

D 50º / suitable calculation using their values (1)(2 marks)

4.

(Total 20 marks)

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voltmeter in parallel to thermistor(allow on cell if no other resistors incircuit) (allow voltmeter under water)

(1)

ammeter in series with thermistor (1)correct symbols for both meters (1)

(a) equipment

thermometer in water labelled (1)

(4 marks)1 ice (1)2 stirrer (1)

5.

(b) (i) apparatus

1 mark eachto maximum 3 marks

3 heat source4 lagging (if using an electric heater)5 thermometer (only if not awarded in (a)6 tripod / mat / gauze (even if electricalheating)

(1)

(3 marks)

voltage / potentialdifference

(1)

current (1)

readings

temperature(correct terms required)

(1)

(ii)

(3 marks)

1 ice in beaker (1)2 measure V and I and temperature (1)3 heat (1)4 to suitable quoted temperature (rise) (1)5 remove heat (1)6 wait (1)7 stir8 take 2nd set of readings (1)9 repeat for other values of t (1)

description

1 mark eachto maximum 8 marks

10 up to 100º Cindependent marks but statement of aseries of readings at different temperaturescan score 8 and 9

(1)

(iii)

(8 marks)

axes labelled resistance (ohms) andtemperature (º C)

(1)graph

line or curve going from high R at lowtemperature to low R at high temp

(10

(iv)

(2 marks)

(Total 20 marks)

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PHYSICS 7540, CHIEF EXAMINER’S REPORT

PAPER 1

General Comments

Some very good work was seen on this paper. Many candidates confidently performed thecalculations in questions 1(a), 2(a), 4(a), 5(b), 7(a) and 10(c), less so in 1(b) and 3(b). Ofmore concern is the number of marks that good candidates are not scoring for descriptionsand explanations such as those in questions 1(c), 3(c), 4(c), 5(d), 6(c), 7(c), 10(b) and 11(c),(d).

Question 1

Most candidates were able to calculate the weight of the astronaut in (a) and give the correctunit. Surprisingly few correct answers were seen in (b) where an answer of 4200 N wasexpected. In (c) only the strongest candidates realised that the resultant force could berepresented by a vertical arrow pointing upward with the magnitude calculated in (b). Creditwas given for an answer of 4900 N in (c). In (c)(iii) an increased effect was required such as‘feels heavier’ or a new effect such as ‘chest is compressed’. Responses such as ‘feelsdizzy’ were not acceptable. Part (d) was correctly answered by almost all candidates and sowas (e), even by some candidates who gave completely incorrect answers in (d).

Question 2

This question was extremely well answered with most candidates showing an excellentunderstanding of the topic. Very few errors were seen in calculating the area under thegraph. In (b) care was required with the dots to show that one was closer to 0 than 2 and theother was midway between 4 and 6. Explanations concerning acceleration and constantvelocity were almost always correct. Evidence of calculations from the graph leading to thepositions of the dots was acceptable in place of an explanation. The four marks in (b) wereindependent, allowing candidates to score marks even if the position of a dot did not matchits explanation.

Question 3

In (a), chemical to heat or thermal or internal scored the first mark. This was rarely seen. Thecalculation leading to 660 J in (b) was often incomplete with a mark given for answers of 22or 3300. The description of convection in air in (c) was often disappointing but credit wasgiven if the convection of other fluids was described, such as blood or water.

Question 4

Practically all candidates correctly found the product 50 x 0.05 or 50 x 5 but many lost thesecond mark for incorrect unit. N/cm was occasionally seen. Stronger candidates gave thecorrect response in (b) that the tension would be less. In (c) a problem would arise with thespring being permanently deformed or broken. This was recognised by many candidates butthe idea that the pointer might go off the scale or simply that the reading would be too highdid not appear on many scripts.

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Question 5

Most candidates were able to read two sets of points from the graph but few went on to findthe product. For much of the graph this product was 400 but values of 360 were given fullrecognition. Statements such as ‘pressure is inversely proportional to volume’ did not scorethe second mark in (a) unless backed up by a calculation. Correct answers of 0.5 m3 (or 0.45m3) were often seen in (b) but a significant number of candidates started from the point(4,100) and assumed direct proportionality to give an answer of 32 m3.

In (c) some candidates realised that an increase in temperature was involved; howeveranswers to (d) showed a weakness that needs to be addressed by centres. An answer of‘increased speed’ is inadequate, especially where candidates went on to indicate that thespeed increased at C and B. Candidates should be encouraged to give unambiguousanswers such as ‘faster at C’. Answers such as ‘more motion’ or ‘more agitation’ wereunacceptable.

Question 6

Most candidates knew that the charge was negative and that electrons were transferred fromthe cloth to the rod. However, many loose answers were seen in (c), often involving themovement of positive charges or positive electrons. Either of the last two responses waspenalised by the deduction of two marks in (c). The mark scheme was:

Electrons move in the metal rod 1

They move to the right of the metal rod 1

Statement of repulsion between named like charges 1or attraction between named unlike charges

Question 7

Strong candidates often scored six marks but others scored four by correctly calculatingresistance, giving the correct unit and recognising parallel and series connections. Incorrectanswers in (b)(ii) and (c)(i) often led to impossible situations described in (iii) such as ‘thecurrent splits up in Circuit C’. A great deal of misunderstanding was displayed in answers tothis question.

Question 8

This was well answered although few scored more than four because candidates wereunable to correctly show the direction of the electromagnetic force. Field lines were welldrawn in (a) and it was very pleasing to see the number of correct answers in (c), manycandidates providing a surplus answer by listing ‘bigger current’ and ‘stronger magnet’.Centres should note that no credit was given for ‘bigger magnet’.

Question 9

Where candidates had covered this work, good answers were seen. In (d)(i) candidatesshould be encouraged to answer with a number (36 in this case) rather than with ‘the same’or ‘one more’ in (d)(ii). Some credit was given for these answers. However, many candidatesdid not realise that for a neutral atom the answers to (d)(ii) and (d) (iii) were the same. Themark scheme allowed candidates to score (d)(iii) if it was the same as (d)(ii) even if theanswer in (d)(ii) was incorrect.

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Question 10

Strong candidates gave correct, organised responses to this question but others answered(a) in (d) and vice versa. Some sloppy work was seen in (b). ‘The sound took 4 seconds totravel’ was expected for 2 marks but often inconclusive answers such as ‘the chimes have totravel a long distance’ were seen. The calculation in (c) was well done by most candidatesalthough 1350 was divided by a variety of numbers, including 60 and 3600.

In (d), a pair of answers was expected such as ‘sound is longitudinal and light is transverse,’not ‘sound is longitudinal and light can travel through a vacuum’. ‘Light can travel through avacuum but sound cannot’ was acceptable.

Question 11

This question was well answered. Almost all candidates knew, in (a), that the name given tothe bending is ‘refraction’. The only incorrect answer seen in any quantity was ‘dispersion’.Candidates should be encouraged to view the change in direction as being due to a changein wave speed. There is a good deal of confusion about the ideas of frequency, wavelengthand refractive index. Most candidates answered (e) correctly, the more thoughtful onesadding a point on the diagram.

PAPER 2

General Comments

All parts of the paper were tackled by candidates. Most candidates were able to complete allthe questions in the time allowed and presented their ideas reasonably. There were stillsome candidates who had obviously only completed part of the course and so scored poorlyon particular questions. Many of these would have benefited from tackling past papers inadvance of the examination to confirm that they had reached an appropriate level ofpreparation. Compared with previous years, more candidates labelled diagrams and graphsclearly to support their written answers. The choice of scale was better but centres shouldwarn candidates that a poor scale will continue to be penalised in future. Questions involvingcalculations were often answered well although units were often missed off or writtenincorrectly.

Question 1

(a) (i) Candidates who failed to recall that distance is average speed x time frequentlyobtained an answer of 2.5 m/s and then baldly multiplied this by 2 to obtain the value givenin the question. This failed to score either of the marks which were only given where theyhad demonstrated their knowledge of the equation. Where questions are presented in thisway, candidates must avoid using the given value to then calculate the same value. Parts(ii), (iii) and (iv) were usually well done although the unit for acceleration was frequentlygiven as m/s and in calculating the kinetic energy a number of candidates forgot to squarethe speed time. The diagram in part (v) was normally completed well but the label for weightwas often missed out or marked as gravity.

In question 1(b) many candidates failed to identify instrument A as an electronic timer andsome candidates wanted to use a ticker timer. A majority could state that the time anddistance of fall were required but only a few stated that the distance had to be measuredfrom the bottom of the sphere. This appeared to be because many had not seen this

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experiment carried out. Very few candidates could write the expression needed to calculateg. The equation S = ut+ 1/2 at2 on its own did not score. Those candidates who did notknow the equation rarely scored the final mark and would state that the value of g obtainedwould be too large.

Question 2

(a) Performance in this question was variable as candidates often failed to answer thequestion as set. Most candidates scored a mark by explaining why the leg would feelheavier than before but would then focus on the expansion value to explain why titaniumwas used rather than steel, and so failed to score the next two marks. The calculation ofheat energy was usually completed well although a number of candidates chose theincorrect combinations of mass and specific heat capacity or added 273 to the temperaturedifference. Good candidates appreciated that the pain could not be due to the rates ofcooling as the masses and specific heat capacities were very similar. To score the mark forthe correct reason candidates had to refer to the difference of the expansion values to score.Only a tiny proportion of candidates appreciated in part 2(d) that steel having a higherdensity would be shorter and hence the two amounts of expansion would be the same.

Question 2(b) produced a better response. The graph was usually well drawn although somecandidates failed to use graph paper fully or to choose a sensible scale. Some lost marks asthey did not label the axes correctly and future candidates should be advised to copy thewording used in the table exactly. Most could read the value at 126 µm. Many gained thefinal mark by stating that the person’s leg would be unlikely to experience a temperature riseof 70 0C.

Question 3

(a) Many candidates calculated the number of turns correctly. The operation of a transformeris not well understood and too many candidates think that current passes in the iron core.Candidates should be advised to look at the number of marks available when answering thissort of question and to try to make one distinct point per mark; in this case five separatepoints were required. In part (a)(iii) most gained a mark by showing that the current was2.5 A although those that wrote 24/60 = 2.5 or who showed no working did not score.Candidates who failed to appreciate that the power in the primary was equal to the power inthe secondary did not score the next two marks. In part (a)(iv) many failed to appreciate that240 V is a high voltage or that water is a conductor and so would provide a risk of electricshock. Some thought that the bulb would heat the water enough to kill the fish.

In 3(b) good candidates were able to show that the power would be 69.8 W and that 3000 Jof energy would be transferred to heat in the transformer. Good candidates realised that thisheat would be released in the transformer and not the filament lamp and that the twoprincipal processes were conduction and convection. Radiation was not acceptable as theamount of radiation would be very small.

Question 4

(a) Candidates who had seen a ripple tank used scored well on this question although somedid not measure the gaps with sufficient accuracy. A number failed to appreciate that the gapbetween adjacent lines was one wavelength. Many were able to use their measured valuesto calculate the appropriate wavelengths but some lost marks through use of an incorrectunit or when trying to convert cm/s to m/s. It was a bit worrying that many thought thatchanging the frequency would also change the speed of the wave. Only the best candidates

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noted that the frequency had doubled, with most merely saying that the wavelength wouldget smaller rather than halve and so only scored one of the two marks allocated.

Question 4(b) was often well answered. Marks were lost through vague responses such as‘the incident ray is equal to the reflected ray’; here the word ‘angle’ was essential. Incalculating the refractive index and critical angle candidates had to show all steps in theirworking and had to express their answers to 3 significant figures. Candidates writing 1.5 and42� would only score full marks if they had shown that these answers were the results ofactual calculations rather than just written from memory. Many realised that total internalreflection occurred at Q because angle B was greater than the critical angle. Someencountered problems if they had calculated angle B to be less than 42�.

Question 5

In (a) many scored well although some failed to add the thermometer. It was pleasing to seethat most candidates knew the symbols for voltmeters and ammeters but too many stillplaced the voltmeter in series with the thermistor. Some candidates added morecomponents such as a bulb or a rheostat and then lost the voltmeter mark by connecting it inparallel with the supply rather than in parallel with the thermistor.

(a) In 5 (b) candidates who scored well were ones who answered each of the parts (i) to (iv)separately and in the order given in the question. Poorer candidates did not do this andproduced confused accounts which rarely scored marks. Weaker candidates would also tendto list the apparatus shown in the diagram rather than listing additional apparatus for b(i). Inb(ii) they were expected to give the correct names for the quantities (voltage, current andtemperature). When responding to questions such as b(iii) candidates should considerwriting the account of the experiment as a numbered list, identifying each step in theprocedure and making sure that their list has at least one statement per mark. In this caseeight correct statements were required. Candidates should not be afraid to list more points(there were 10 possible points that could score marks in this question) as long as they arenot contradictory. A successful attempt at part (iv) required candidates to read the questioncarefully and to produce a sketch graph with axes labelled with the words ‘Resistance’ and‘Temperature’ and a downward sloping line or curve. Where candidates are told to ‘sketch agraph in your answer book’ they should not use graph paper but should follow the instructionand put the sketch=graph on the lined paper.

PHYSICS 7540, GRADE BOUNDARIES

Grade A B C D E

Lowest mark foraward of grade 72 60 49 44 25

Note: Grade boundaries may vary from year to year and from subject to subject, dependingon the demands of the question paper.

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