香港数学试题.docx

香港数学试题.docx

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香港数学试题

（2003CEQ6）

（a）Twoessentialcomponents:

*acoil

*amagnet

*acommutator

*apairofbrushes

*asoftironcore

（b）Whenthebladesareturned,thecoilinsidethemotorwillturninthemagneticfieldofthemagnet.Aninducede.m.f.willbesetupinthecoil.Theinducedcurrentflowsthroughthebulbandlightsupthebulb.

（1993CEQ5）

（a）（i）Sprinkleironfilingsontheboardandtaptheboardgently.Themagneticfieldpatternisshownbythepatternofthefilings.

（ii）圖形

（b）（i）Whensispressed,currentflowsthroughthecoilandthesoftironcoreismagnetized.Itattractsthespringtowardstheleft.

Thehammerstrikestheleftmetalplatetoproducethefirstnote.

（ii）Replaceonemetalplatewithanthermadeofadifferentmetal

*Varythelength（orotherdimensions）ofoneofthetowmetalplates

*Stickalumpofplasticinetooneplate

（iii）Statement1iscorrectbecausecopperisnotamagneticmaterial.

Statement2isincorrect.

Ifthepolaritiesofthebatteryarereversed,thesoftironcorewillstillbemagnetized.

1994Q7（C）

（C）（i）WhenacurrentflowsinthedirectionABCD,forcesactingonADandBCasshownbelow;圖形

Thecoilrotatesandmovespointer.Therotationofthecoilisopposedbythehairspring.Thecoilwillcometoarestandthecurrentisshownbythedeflectionofthepointer.

（ii）AnyTwoofthefollowing;

*Increasingthenumberofturnsofthecoil

*Increasingthestrengthofthemagneticfield

*Usingweakerhairsprings

*Increasingtheareaofthecoil

*Usingalight-beamgalvanometer

（iii）Thegalvanometercannotmeasure50Hzalternatingcurrentsbecause

*thepointerwillalwayspointatzeroreading.

*thepointerwillvibrateaboutthezeroreadingquicklyandisunabletogiveasteadyreading.

OR

Thegalvanometercanmeasurealternatingcurrentsonlyifadiodeisbuiltintoit.Thediodeallowscurrenttoflowinonlyonedirectionthroughthemeter.

[1986ALQ5]

5.（a）（i）Theresultshowsthattheinducede.m.f.inthesearchcoilisproportionalto1/r.B∝1/rforalong,currentcarrying,straightwire.

（ii）Whenthedistanceofthesearchcoilfromthestraightwireiscomparabletothefinitelengthofthewire,theinducede.m.f.islessthanitshouldbeforainfinitelylongwire.Therefore,thosedatapointslieabovethefittedstraightline.

（b）（i）B=μ0I/2πr=μ0I0sin（2πft）/2πr

emf=NAdB/dt

=（μ0NAI0f/r）cos（2πft）

V=2μ0NAI0f/r

（ii）Slopeofline=1800V-1m-1

=1/（2μ0NAI0f）

N=1/（2×1.26×10^-6×3.14×10^-4×14.1×50）turns

~1000turns1

（c）Hisargumentiswrong.Theearth’smagneticfieldismoreorlessasteady,intensityfieldwhichwouldnotgiverisetoanymeasurableinducede.m.f.inthesearchcoil.

[1998ALIBQ6]

2000Q7

2000Q8:

2001ALQ4（a）

1999ALQ3

2003CEQ6

2002CEQ6

1995CEQ5（b）

1993Q5

1994Q7（C）

2003Q1

2003Q4

1993ALQ10

96-IA-Q1

03-1B-Q7（AL）

2000-I-Q1

1989ALQ10

10.（a）（i）I=nevA

50´10-3=1029´1.6´10-19´

（0.02´0.1´10-3）´v

v=1.6´10-6ms-1

（ii）directionmarkedcorrectly

F=Bev

=1.5´（1.6´10-19）´（1.6´10-6）N

=3.8´10-25N1

（b）-forcemoveselectrontofarend

-negativechargesbuildupatfarend

-Efieldsetupuntilitislargeenoughtoprevent

furthermovement

-steadyp.d.develops

（c）（i）semiconductorhaslowercarrierconcentration

largerHallvoltageobserved

（ii）-X,Ynotexactlyopposite

-smallp.d.existsduetolongitudinal

electricfieldsetupbybattery

-adjustRuntilvoltmetergivesnildeflection

（d）Bev=eVH/d

VH=Bvd=B（I/neA）d=BI/net

=（0.2）（10-3）（1020）-1（1.6´10-19）-1（0.1´10-3）-1V

=0.125V

（e）Hallprobe

usedinmagneticfieldmeasurements

1998Q7

1996ALQ8

1995ALQ4