WJEC Physics for A2: Study and Rev Guide

Practical and data-handling skills This section deals with the practical and data skills you will need to demonstrate in both the Unit 5 (Practical Examination) and in the Units 3 and 4 papers. It builds upon the information given in the AS level Study and Revision Guide. Exam Practice and Technique The third section of the book covers the key skills for examination success and offers you examples based on suggested model answers to possible examination questions. First, you will be guided into an understanding of how the examination system works, an explanation of Assessment Objectives and how to interpret the wording of examination questions and what they mean in terms of exam answers. A variety of structured practice questions, including a QER question, is provided, taken from across the A2 part of the speci cation. Model answers are given. This is followed by a selection of examination and specimen questions with actual student responses. These offer a guide as to the standard that is required, and the commentary will explain why the responses gained the marks that they did. Most important of all, we advise you to take responsibility for your own learning and not rely on your teachers to give you notes or tell you how to gain the grades that you require. You should look for extra reading and additional notes to support your study in physics. It’s a good idea to check the awarding body website – www.wjec.co.uk – where you can nd the full subject speci cation, specimen examination papers, mark schemes and in due course examiner reports on past years’ exams. Good luck with your revision! 223 Questions and answers Q & A5 Theproduct of thepressureandvolume of an idealgasmaybeexpressedas pV = nRT . Theproductmayalsobewritten in termsof themean square speedof themoleculesas pV = 1 3 Nmc 2 (a) Derive in clear stepsa formula that showshow the internalenergyof the idealgas depends on the temperatureof thegas. [4] (b) A canisterofvolume 0.025m 3 containsheliumgasatapressure of 305kPa anda temperature of 18°C .Calculate: (i) the internal energyof thegas [2] (ii) thenumberofmoleculesofhelium in the canister. [2] Tom’sanswer (a) Combining pV = nRT and pV = 1 3 Nmc 2 weget nRT = 1 3 mc 2 3 Theenergyof thegas is thekineticenergyof the molecules 3 = 1 2 mc 2 7 So the internalenergy is 3 2 nRT 7 (b) (i) Frompart (ii), thenumberofmoles is0.0510 So U = 3 2 nRT= 3 2 ×0.0510×8.31×18=11.4 J 73 ecf (ii) n= pV RT = 305×0.025 8.31 × 18 =0.0510 7 \ Numberofmolecules= 0.0510×6.02×10 23 =3.07×10 22 3 ecf Examiner commentary (a) Tom startswellby combining the twogivenequations and identifying the internal energy of thegaswith themolecularkineticenergy,whichheunfortunately gives as themeanenergyof an individualmolecule. He thenmysteriouslywrites the equationwhich is given in theDatabooklet. (b) Tomanswers this inanunusualwaybyfirst answeringpart (ii)andusing it toanswerpart (i).This is legitimate and the examinerhas applied thee.c.f. rules accordingly, though themarkallocation forpart (i)becomes rathergenerous. (ii) Tomuses the correct equation for calculating the numberofmolesbutunfortunatelyhehas fallen intobothunit traps:he shouldhave converted to PaandK.However,he thengoes on touse n to calculate thenumber ofmolecules andgains the secondmark. (i) Theecfvalue of n is acceptedbutheagain loses out on theunit trap. Tom scores4outof 8marks. Seren’sanswer (a) Idealgasmoleculesdon’thave forcesbetweenthem so the internalenergy is justtheir kineticenergy. 3 \ Internal energy, U = 1 2 Nmc 2 3 From equation 2, Nmc 2 =3 pV , soU = 1 2 ×3 pV = 3 2 pV 3 So, from equation 1, U = 3 2 nRT 3 (b) (i) U = 3 2 pV = 3 2 ×305 × 10 3 Pa × 0.025m 3 3 = 114000J (3 s.f.) 3 (ii) n = pV RT = 305 × 10 3 Pa × 0.025m 3 8.31J K –1 × 291 K =3.15mol 3 Examiner commentary (a) Agood answerbySeren. Shemakes the important point about the lack of intermolecular forces and correctly states the internal energy as U = 1 2 Nmc 2 . Following this she logically and clearlyuses thegiven equation toderive theDatabooklet formula.Note that themark is for theworking towards the equation and not for theequation itself. (b) (i) Serenhas seen theeasiestwayof tackling this using the U = 3 2 pV ,which isnot in theData booklet,but arose fromherpart (a)working. She skilfullyavoids theunit trap. (ii) Again,Seren convertsbothunits and correctly calculates thenumber ofmoles of thegas. Unfortunately she fails (forgets?) togo on to calculate thenumber ofmolecules. Seren scores 7outof8marks. Pointer Unlike Sections 3 and 4, which can be used for revision for all A level Physics courses, Section 5 is specific to the practical examination in the WJEC’s course. 1 Candidates for theEduqasA levelPhysicsqualificationdonot takeapracticalexamination. Theyareassessedon the same skills in the theorypapersand in thePracticalEndorsement. 5 Practical examination Unit5of theWJECA levelPhysicsqualification consistsofapractical examinationwhich isallocated10%of the totalmarks 1 .Thereare twoequally weightedparts to thisexamination: ■ anExperimentalTask ■ aPracticalAnalysisTask These two tasksassess theexperimentalanddata-handling skillsyouhave acquired through the course.Yourattention isdrawn to the following sources of information: ■ Section3of theASStudyandRevisionGuide ■ TheMathsandData sectionof thisguide ■ Thedataanalysisquestions in theendof sectionexercises inboth theAS andA2Studentbooks. 5.1 Common features of the tasks Both tasks in thepracticalexaminationwill requireyou toanalyse,evaluate anddraw conclusions fromdata.Themajordifferencebetween the two tasks is that,whilst inone (theExperimentalTask)youwillobtainyourowndata, in theother (thePracticalAnalysisTask) itwillbegiven toyou. 5.1.1Graphplotting Youwillbeexpected todecidehow toplotdata to testagivenor suggested relationship. Inalmostall cases thiswillmean trying toobtaina straight-line plot from thedata.This is called linearising thedata.The techniques for this are covered inSection3.4of theASStudyandRevisionGuideand inSection M.3of thisbook.Either theExperimentalTaskor thePracticalAnalysisTask (butnotboth) is likely to require theuseofa logplot. 5.1.2Uncertainty analysis Youwillneed to: ■ Use repeated readingsor the resolutionofan instrument toestimate the uncertainty indata. ■ Ploterrorbarsusinguncertainty indata (butnot in loggraphs).  Givesuitablegraphs toplot to linearise the data for the following relationships,where the variablesare x and y : a) y = ax 2 + b b) y 2 = kx n c) y = Ae −kx QUICKFIRE QUICKFIRE QUICKFIRE  The length,width,and thicknessof ametal blockaredetermined tobe 10.5cm , 4.6cm and 2.2cm respectively. If theuncertainty ineach value is ± 0.1cm , calculate thevolumeof theblock togetherwith itsabsolute uncertainty. QUICKFIRE QUICKFIRE QUICKFIRE  For the relationship in Quickfire1 (a), statehow youwoulduse yourgraph tofind thevaluesof the constants, a and b . QUICKFIRE QUICKFIRE QUICKFIRE 189 5 Practical examination 5 How to use this book: