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Part 0 (Foundation Year)

Foundation Experimental Physics 1

Instruction Manual

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Department of Physics

The University of Reading

Part 0 (Foundation Year)

Foundation Experimental Physics 1

Instruction Manual

(C) Copyright 1998 Department of Physics University of Reading All Rights Reserved

Last Revised September 1998

The University of Reading Department of Physics

Experimental Physics 1 Foundation Year

The University of Reading - Department of Physics

Student SupervisionHealth and Safety Aspects for Experimental Physics I

Supervisor: Ms. Averil Macdonald

This assessment is valid until 1st January 1999.This assessment was made on 19th September 1998.

Nature of WorkLaboratory work associated with Foundation Experimental Physics I asdescribed in "Foundation Course Experimental Physics I - InstructionManual".

HazardsElectrical EquipmentHeating Equipment

Agreed precautions, control measures and personal protectiveequipment requiredPrecautions as detailed in the instruction manual, "Foundation YearExperimental Physics I - Instruction Manual".Safety spectacles will be issued for project 6; these should be returned tothe laboratory supervisor at the end of the session.

Risk Category for SupervisionWork may proceed because workers are adequately trained andcompetent in the procedures involved.

Supervisor Ms Averil Macdonald 29th September 1998

1This assessment refers only to health and safety aspects of supervision. Students must notwork in the laboratory outside the timetabled hours for this module.



Index

1 Introduction 51.1 Introduction 51.2 Objectives 51.3 Requirements 51.4 Absence 61.5 Monitoring of Log Books 61.6 Safety 6

2 Assessment2.1 Preparatory Work 82.2 Laboratory Log Books 82.3 Competencies 102.4 Assessment 10

3 Uncertaincies & Errors3.1 Uncertaincies in Instrument Readings 123.2 Random Errors 123.3 Percentage Errors 123.4 Error Bars on Graphs 133.5 Errors in Gradients 133.6 Systematic Errors 133.7 Error in the Final Value 13

4 Project 1: The Bifilar Suspension 145 Project 2 : g by Pendulum 166 Project 3 : Thermistor as a Resistance Thermometer 187 Project 4 : Specific Heat Capacity 208 Project 5 : Resistivity of a Metal 229 Project 6 : Stress - Strain for Copper and Rubber 24

This booklet describing the module Experimental Physics I is based onthe information available at the time of publication. The Universityreserves the right at any time to change the contents of this module. Asmuch notice as possible of any alterations will of course be given. Anyonewho is uncertain of the up-to-date position should enquire of theLaboratory Supervisor.



Chapter 1

Introduction toFoundation Experimental Physics 1

1.1 IntroductionThis booklet is the manual whichaccompanies the Part 0(Foundation Year) moduleFoundation Experimental Physics1. The contents of this bookletprovide the background fordeveloping skills in experimentalphysics and specific instructionsfor the different projects within themodule.

1.2 ObjectivesThis Part 0 (Foundation Year)module is the first of aprogressive series of modulesoffered by the Department ofPhysics which are provided toenable you to develop fully theexpertise and experiencenecessary to conduct practicalwork in physics. This foundationmodule sets out to:-(a) show how physicists approachthe execution of experiments inorder to test, quantitatively,theories and models;(b) develop the skills inobservation and data acquisitionand the subsequent analysis

required to make suchquantitative assessments;(c) provide practical experience ina range of fundamental physicstopics.

1.3 RequirementsYou will complete one skillssession at the beginning of themodule and five experimentalprojects during the course of thismodule. All students will berequired to keep a detailedexperimental log book throughoutthe duration of this module.

Typically you will work alone andall the assessment will be madeon an individual basis.

You will sign up for your chosenproject one week in advance toensure that the appropriateapparatus is available to you andto enable you to undertake therequired preparatory work. Thiswill include background readingand producing written answers toassociated questions.



You are expected to complete theexperimental aspects of theproject and to plot all graphs andundertake any analysis during thelaboratory session. If you do notcomplete the analysis stage ofthe project during the laboratorysession then it must becompleted in your own time alongwith the abstract. Each projectshould typically involve four to sixhours of work, includingpreparatory work.

1.4 AbsenceIf you are absent from alaboratory session you will beawarded a zero mark for thatsession unless you are able toshow that you were ill or involvedin an activity for which you haveobtained prior approval. You alsorisk not having lab notes on whichto base your formal, assessedwrite-up (see Assessment). Ifyou are unable to attend thelaboratory session you shouldleave a message on extension8543 (0118 931 8543) or [email protected] to the session.1.5 Monitoring ofLaboratory Log BooksYour log book must be signed byone of the Experimental PhysicsTeam at the beginning of thesession to ensure that you haveundertaken sufficient preparatory

work to begin the project. Whenyou leave the laboratory sessionyou should also ensure that oneof the Experimental PhysicsTeam initials your log book. Logbooks without initials will not begiven a mark, although feedbackon the project will still beprovided. You must leave yourlog book on the stand in thelaboratory by 12.00 noon on theMonday following the laboratorysession. Log books handed inafter that time will not be given amark although feedback on theproject will be provided. The logbooks will be available forcollection from 12.00 noon on thefollowing Thursday.

1.6 SafetyAll of the projects and skillssessions in this module takeplace in a laboratory within theJ.J.Thomson Physical LaboratoryBuilding and you are expected toact as professional physicistsworking in a laboratoryenvironment. Therefore:• All coats and bags must be left

at the designated sites outsidethe experimental physicsareas.

• Food and drinks must not bebrought into the laboratory.

• Smoking is not allowed in anypart of the building.



• You must follow theinstructions of the ExperimentalPhysics Team in terms of safeworking practice.

• You are not allowed to workunsupervised and therefore willnot normally be allowed intothe laboratory outsidetimetabled hours.

• For some projects there areadditional specific safetyprocedures and these are listedin the appropriate chapter.

• Many of the projects make useof electrically operatedequipment. If you suspect thatthe equipment is faulty youshould disconnect it from themains supply and report thefault immediately to one of theExperimental Physics Team.

• You should never attempt torepair or modify the equipment.

• It is important that you have anappreciation of the capabilitiesof your apparatus before youstart an experiment and soavoid overloading anyinstruments.



Chapter 2

Assessment

2.1 Preparatory WorkEach project involves somebackground reading prior to thelaboratory session in order toensure that you are fullyconversant with the theory behindyour chosen project. Each projectis prefaced with some questionsand it is essential that you recordyour answers to these in your logbook and that a member of theExperimental Physics Team signsto indicate that you areadequately prepared tocommence your chosen project. Most prior reading is containedwithin FLAP modules or "Physics"(International Student Edition) byH.C.Ohanian published byW.W.Norton 1989. Copies of theappropriate material are availablein the laboratory and in theLearning Resources Centre.

2.2 Laboratory Log BookAll scientists keep a laboratorylog book in which they record allof the details, results andcalculations related to their work.Such log books are a workingdocument and scientists record

ideas and discussions as well ashard data.As part of this module you arerequired to keep an officiallaboratory log book; subsequentexperimental physics modules willrequire separate laboratory logbooks. These may be obtained inthe Experimental PhysicsLaboratory. All of your work mustbe contained in this log book and,therefore, graphs etc. must besecurely fixed into it atappropriate points. Working onloose sheets of paper within thelaboratory is not allowed! Yourlog book is not a formal report butmore a ‘running commentary andshould be sufficiently detailed foranother physicist to follow yourwork. It may contain mistakes. These should be clearly crossedout and a note added to explainwhat went wrong. At the end ofeach project you will write anabstract of the project. Thefollowing notes should give you aclear idea of the appropriateformat and detail. If you are inany doubt as to what is requiredplease consult one of theExperimental Physics Team.



2.2.1 What to record in your logbook(a) Each project should start on afresh page with a title and thedate; all text must be written inink.(b) All answers to questionsundertaken as part of yourpreparatory work should beincluded as an introduction toyour written work.(c) You do not need to copyinformation from this manual butyou may need to refer to thediagrams and methods describedin it.(d) Include any diagrams to showexactly how you set up theexperiment if they are not alreadyin the manual. In a few cases(e.g. circuit diagrams) it may behelpful to reproduce the diagramalready in the manual as part ofyour notes.(e) Results must be tabulated(where appropriate), must be ona separate page for clarity andmust include the appropriate unitsand estimates of the uncertaincyin your readings.(f) You should aim to make allcalculations and plot all graphsduring the laboratory session.When plotting graphs choosesensible scales, maximise use ofthe paper and include error bars.

Make sure that the axes areproperly labelled and include alegend to describe the graph.(g) Quantitative estimates of theerrors are extremely important(see Chapter 3) and musttherefore be included at eachstage of the calculation.

2.2.2. The ConclusionYour conclusion paragraph mustinclude an evaluation of yourfindings including proximity ofyour value to the accepted value,reasons for any difference andthe impact of systematic errorscompared with random errors.You should include a descriptionof the experimental precautionstaken and the difficulties, if any,encountered and how youovercame them. Comment on theeffectiveness of the techniqueand the validity of the result. Anysuggestions for improving thetechniques used should also beincluded.

2.2.3 The AbstractThe abstract should follow theentry you have made on theparticular project. It is to bewritten with care and shouldextend to no more than 200words. The precise format willvary from one project to anotherbut will include:(a) a brief statement of theobjectives of the project in your



own words. You should not givedetail copied from the manual ora textbook.(b) a short summary of theexperimental technique(s) usedtogether with a clear statement ofkey results along with yourestimate of the final error involved(details of the error calculationshould be in your laboratorynotes, not the abstract).(c) a brief conclusion(d) a comparison of your findingswith the literature and briefcomments about anomalous orinconsistent results and anysystematic errors.

This summary shows what youhave learnt from the project andalso gives you experience inpreparing concise and informativereports.Remember at all times to thinkabout the physical significance ofwhat you are doing and of anynumbers you calculate.

2.3 CompetenciesYour progress in developingexperimental skills will bemonitored via the competencysheet (attached permanently toyour laboratory log book) whichwill give you direct feedback onyour strengths and weaknesses.The competency sheet identifieskey skills that are addressed bythe projects. Not all competencies

are appropriate to every projectbut, by the end of the course, youwill have been tested on all of thecompetencies.

2.4 AssessmentThis module is assessed bycontinuous assessment. Withinone week of each laboratorysession you will hand in yourexperimental laboratory log book.The log book will be examined byone of the Experimental PhysicsTeam who will provide you withpersonal feedback on:• the quality of your preparatory

work;• your approach to and operation

of the project;• the analysis of and conclusions

you draw from the resultsobtained;

• the analysis of errors and yourevaluation of the project interms of the effectiveness ofthe procedure and validity ofthe results;

• your communication skills asdemonstrated in your abstract.

Feedback will be in the form ofspecific comments on your worktogether with a more formalassessment of the competenciesdemonstrated via yourcompetency sheet. This willenable you to identify those skillsin experimental physics which



you are developing satisfactorilyand those which require furthereffort.2.4.1 WeightingsYour marks will collectivelycontribute to the overallassessment for this module. Inaddition you will be asked toproduce a formal write-up of aspecified project (which may beword processed) and you will beinterviewed to discuss all of theremaining four projects you willhave completed - the viva.

The weightings for theassessment are as follows:Preparatory Work

10%Log Book Including Errors

20%Abstract

10%Competencies

20%Formal write-up

20%Viva (formal interview)

20%



Chapter 3

Uncertaincies & Errors

Errors are not mistakes andshould not be covered up orignored! Indeed the analysis oferrors is an essential part of yourwritten evaluation of the projectundertaken. Errors are alwaysestimates of the likely or probabledeparture from some true value.

3.1 Uncertaincy inInstrument Readings.There is always some uncertaincyin any reading taken as part of anexperiment. The uncertaincy inany instrument reading is due tothe sensitivity of the scale of themeter used. Typically theuncertaincy is half the smallestscale division readable (or halfthe last decimal place on a digitalmeter). The uncertaincy in anyinstrument reading must bequoted as part of the results tableheading.

3.2 Random ErrorsIf you repeat a reading n times,for instance when timing anevent, you will probably have ndifferent readings from which youwill calculate the average (mean).The error in individual readings orthe mean of such a set of

readings is not due to thesensitivity of the

instrument used (as with theuncertaincies above) but ratherdue to random (or stochastic)effects. A sensible estimate of therandom error in an individualreading can be obtained from thespread of the readings obtained,compared with the average valuecalculated. Typically 2/3 of theaverage spread is a reasonableestimate for the random error in amean value.(More accurate methods ofdealing with errors in meanvalues will be introduced inFoundation Experimental Physics2)

3.3 Percentage errorsAlI errors can be quoted inabsolute terms (with associatedunits) or as a percentage of thereading. Percentage errors areparticularly useful whenconsidering the error in a valuewhich has been derived fromsome other value(s) e.g. thevolume of a sphere. The errorhere would be due to theuncertaincy in the reading of theradius, r. The percentage error in



the value of the volume would be3x the percentage error in r(because r is cubed in calculatingthe volume) and must be quotedas such.

N.B. Percentage errors are alsoused in calculating the total errorin a quantity derived from anumber of independent values -but this will be dealt with inFoundation Experimental Physics2).

3.4 Error Bars on GraphsEvery reading plotted on a graphhas an error and this must berepresented by appropriate errorbars. Any line drawn through thepoints should pass through theimaginary boxes created by ALLof the error bars. Any anomalouspoints should be indicated on thegraph but not included in the bestfit line drawn

3.5 Errors in GradientsThe error bars on a graphindicate that there is a best fit andtwo ‘limiting fit’ lines (worst fit) tobe drawn. All lines must be drawnand, if appropriate, the best andworst values of the gradientand/or intercept must becalculated.N.B. a limiting (worst) fit lineshould also pass through all ofthe error bars - but only just!

3.6 Systematic ErrorsSome errors cannot beaccounted for in the waysdetailed above. They are inherentin the system and unrelated tothe techniques used and are thusknown as systematic errors.Typical examples would be anincorrectly calibrated instrument,a zero error or poor experimentaltechnique such as parallax error.

3.7 Error in the Final ValueYour written conclusion mustinclude an evaluation of thevalidity of your findings, includingthe contribution made by anyerrors involved, both the randomerrors calculated and anypossible systematic errors. Youshould also comment on howthese errors might account forany anomalous results.

N.B. Often one large error willdominate the final result andevery effort should be made toidentify the dominant errors andtime should be spent on reducingthem.



Project 1: The Bifilar Pendulum

Preparatory WorkPlease complete the answers tothese questions in your laboratorylog book in advance ofundertaking the experimental partof this project.

1. What are the sources of errorwhen timing a pendulum?

2. What procedures is it pertinentto operate to ensure that yourvalue for the timeperiod is asaccurate as possible?

3. How does one arrive at thebest estimate for the error whenmeasuring the timeperiod?

4. Given the equation below,determine which values should beplotted on a graph to generate astraight line. How will the constantk be derived from your graph?

Ref: FLAP P1.1, P1.3,

The Experiment:Objectives:This experiment will test yourability to manipulate apparatus, tocollect and process data and topresent your results andconclusions clearly.

Procedure:Set up the apparatus as shown infig 1. The suspending threadsshould be parallel and the 50 cmrule horizontal.

Make the distance l = 30 cm andthe distance a = 10 cm.

Start the 50 cm rule moving in atwisting motion as indicated by thearrows on the diagram. Measurethe timeperiod, T, for theoscillations.

Vary the distance a over areasonable range and collect a setof measurements of T for yourchosen values of a.

Theory predicts that the timeperiod is related to the value a bythe equation:

where k is a constant and g is theacceleration due to gravity

Draw a straight line graph whichwill enable you to verify thisrelationship and determine a valuefor the constant k, its units and thepercentage error in your value.Product:



In addition to the notes,observations, analysis andconclusion produced in your logbook while in the laboratory youshould review critically theexperimental technique you used,commenting on any difficulties youencountered and precautions youtook and make any suggestionsfor ways in which you couldimprove the accuracy of yourresults.

HINT: Was it easy to set up theapparatus as required and tomake it twist satisfactorily? Do youconsider timing multiple swingsand repeating the timing of thependulum to be worthwhile inimproving accuracy? What do youconsider to have been the mostdifficult aspect of completing thisproject?

Finally write an abstract.



Project 2: g by Pendulum

Preparatory WorkPlease complete the answers tothese questions in your laboratorylog book in advance ofundertaking the experimental partof this project.

1. As an ideal simple pendulumswings gently back and forth ithas both gravitational potentialenergy and kinetic energy. Definethese terms and explain (using adiagram) how the relativeproportions vary over one cyclebut how the system still obeys theLaw of Conservation ofMechanical Energy.

2. The period of the pendulum’sswing will depend on a number ofvariables. Using dimensionalanalysis demonstrate whichvariables of mass of bob (m),length of string (l) andacceleration due to gravity (g),affect the time period (T) of thependulum and find the final formof the equation.

3. Why does the technique ofdimensional analysis fail to showthat there is a factor of 2π in thefinal equation?

Ref: FLAP P1.1, P2.4,

Ohanian p 171 - 174

The Experiment:Objectives:You are to find a value for themagnitude of the acceleration dueto gravity (g) via a directmeasurement technique and alsoby plotting a graph of data. Youwill consider the validity of yourtwo answers by evaluating theerrors inherent in each technique.

Procedure:Set up a simple pendulum oflength l0 about 0.70 m. Makedirect measurements of thelength l0 and the period T0 forsmall oscillations and using therelationship

T l g0 02= π\ ( / ) ,

determine a value for g fromyour observations. Estimate thepercentage error in your value.

Without changing the length l0 ,set up a modified simplependulum (see Fig. 1) byclamping a horizontal rod P ameasured distance h below thepoint of suspension 0. Theperiod T for small oscillations ofthe modified simple pendulum isgiven by



T T T h g( ) ( / )02− = π

Find the period T for smalloscillations. Repeat theprocedure for a series of valuesof h between about 0.10 m andabout 0.60 m.

Plot a straight line graph of yourresults which will enable you tofind a further value for g andquote the percentage error in thisvalue.

Product:In addition to the notes,observations, analysis and

conclusion produced in your logbook while in the laboratory, youshould compare the two valuesof g obtained by the two methodsand their associated errors andconsider the relative merits of thetwo experimental techniquesused.

HINT: What is the point ofplotting a straight line graphcompared with just pluggingnumbers into an equation a fewtimes and finding an average?

Finally write an abstract.



Project 3: Thermistor as a Resistance Thermometer

Preparatory Work:Please complete the answers tothese questions in yourlaboratory log book in advance ofundertaking the experimentalpart of the project.

1. The resistance of a thermistorchanges significantly withtemperature but in a differentway to most metallic conductors.Describe the difference betweenmetallic conductors andsemiconductors in the way theyreact to changes in temperature.

2. Define the ‘temperaturecoefficient of resistance’. What isthe meaning of NTC and PTC inthis context?

3. What are the two fixed pointsused to calibrate anythermometer on the celsiusscale? What does this assumeabout the behaviour of thematerial?

4. Draw a circuit diagram of thecircuit which will enable you tomeasure the resistance of thethermistor as it experienceschanges in the temperature of itsenvironment. Why is it important

to keep the current through thedevice low?

Ref: FLAP P4.1, P7.2, P11.4Ohanian p498-500, p522-4

The Experiment:Objectives:You are to calibrate a thermistoras a resistance thermometer onthe celsius scale in order thatyou may use it to determine avalue for room temperature. Youwill investigate resistance as athermometric property andconsider its usefulnesscompared with more traditionalliquid in glass thermometers

Procedure:Set up a circuit with an ammeterand a voltmeter which enablesyou to determine the resistanceof the thermistor without thecurrent heating it up significantly.

Calibrate the thermistor as athermometer on the Celsiusscale using resistance as thethermometric property.

Determine the resistance of thethermistor at room temperatureand hence find the temperature



of the room as measured on thethermistor scale.

Evaluate the errors in your valuefor room temperature comparedwith the error inherent in a liquidin glass thermometer

Product:

In addition to the notes,observations, analysis andconclusion produced in your logbook while in the laboratory, youshould comment on theeffectiveness of this techniquefor measuring temperature andthe validity of the answer forroom temperature obtained. Anysuggestions you have forimproving the experimental setup should also be included.

HINT: Is a thermistor moreaccurate than a liquid in glassthermometer? In whatcircumstances might you select athermistor rather than a liquid inglass thermometer?

Finally write an abstract

Apparatus:

Thermistor

Power Pack

2 Avometers

250 cm beaker

Ice

Heating apparatus:

Bunsen Burner

Tripod

Gauze

Heatproof mat



Project 4. Specific Heat Capacity.

Preparatory Work:Please complete the answers tothese questions in yourlaboratory log book in advance ofundertaking the experimentalpart of the project

1. What is the definition of thevolt? How would you calculatethe total electrical energydelivered to a device?

2. Draw a circuit diagram whichwill enable you to determine theamount of electrical energydelivered to a heater embeddedin a metal block over a fixedtime.

3. It has been discovered thataluminium has a specific heatcapacity of 910 J/kg/K. What ismeant by the term ‘specific heatcapacity’? What is meant by theword ‘specific’ in this context?

4. If an unlagged object is heatedit loses heat to its surroundings.A ‘cooling correction’ can becalculated to account for the heatlost by plotting a cooling curve.Explain how a cooling correctionis calculated.

Ref: FLAP P4.1 P7.4,

Ohanian p516, p628

The Experiment:Objectives:A 1kg block of copper is to beheated electrically. You willinvestigate the effect of heat losson the maximum temperatureachieved and therefore calculatethe specific heat capacity ofcopper. You will consider howcorrections to results are madeand consider the validity of suchcorrections..Procedure:Set up the circuit and incorporatea variable resistor so that thevoltmeter needle can be set toan appropriate reading.

Deliver a fixed amount ofelectrical energy to the block andrecord the subsequent rise intemperature.

The block is not lagged so acooling curve has to be drawn toenable you to determine themaximum achievabletemperature.

Calculate the specific heatcapacity of copper for the values



obtained and evaluate the errorsin your value.Product:

In addition to the notes,observations, analysis andconclusion produced in your logbook while in the laboratory, youshould comment on theeffectiveness of this techniquefor determining the specific heatcapacity of a metal and thevalidity of the answer obtained.Any suggestions you have forimproving the experimental setup should also be included.

HINT: Is all the heat energysupplied to the heater transferredto the block? Is it reasonable totake the temperature indicatedon the thermometer as valid forthe whole of the aluminiumblock? Is heat lost at a constantrate from an unlagged block?

Finally write an abstract

Apparatus:

Power Pack

Voltmeter

Digital Multimeter

Copper block & heater

Thermometer -10 - 110 C

Stop Watch

15 ohm rheostat



Project 5: Resistivity of a Metal


1. All materials exhibit aresistance to the flow of electricalcurrent. Describe how thisresistance arises within a metal.

2. Define the ‘resistivity’ of ametal. What are the units ofresistivity? By dimensionalanalysis find the fundamentalunits from which the unit ofresistivity is derived.

3. Draw a circuit diagram toenable you to measure theresistance of a wire as you varythe length of the wire.

4. How can you ensure that youdo not draw a current of morethan 1A at any time? Why is itimportant not to draw a highcurrent?

Ref: FLAP P1.1, P4.1Ohanian p 685 - 7

The Experiment:Objectives:You will create and operate anelectrical circuit which will enableyou to evaluate the resistivity of ametal wire. You will compare andcontrast digital and analoguemeters for use in electricalcircuits.

Procedure:Set up a circuit using anammeter and a voltmeter toenable you to determine theresistance of the total length ofthe wire 1.5 m.

Vary the length of the wire but donot exceed a current of 1A.

Plot a straight line graph andhence find the resistivity of themetal. Evaluate the error in yourvalue.

Product:

In addition to the notes,observations, analysis andconclusion produced in your logbook while in the laboratory, youshould comment on theeffectiveness of this techniquefor determining the resistivity of



the metal and the validity of theanswer obtained. Anysuggestions you have forimproving the experimental setup should also be included.

HINT: Why is it goodexperimental practice to repeatthe readings for increasinglengths of wire as well asdecreasing lengths? Why shouldthe diameter of the wire bemeasured at various points alongits length and at variousorientations? Do analoguemeters have any advantage overdigital meters for anexperimental set up such asthis? In what circumstanceswould an analogue meter bepreferable to a digital meter? Inwhat circumstances are digitalmeters preferable to analoguemeters?

Apparatus:

1.5m Resistance Wire[constantan or nichrome].

Digital Voltmeter

Avometer

Power Pack

Micrometer

Metre Rule



Project 6: Stress - Strain for Copper and Rubber


1. The Young’s Modulus for amaterial is defined as the ratio oftensile stress to tensile strain.Define the terms ‘tensile stress’and ‘tensile strain’ and describea material’s behaviour if it has ahigh value of Young’s Modulus.

2. What are the units for Stress,Strain and Young’s Modulus?

3. When stretched metal willreach an ‘elastic limit’ and exhibit‘plastic flow’. Explain what ismeant by these two terms andexplain what is happening on amolecular level within a metalwhen this happens.

Ref: FLAP P7.6Ohanian p366 - 9

The Experiment:Objectives:

You are to investigate thebehaviour of copper undertensile

stress and measure its breakingstress. You will then compare itsbehaviour with rubber, a materialmore commonly described aselastic to see how different it is.Both experiments requireingenuity in setting up to makebest us of the availableapparatus.

Procedure:Part 1: Copper.Set up the apparatus as shownin Figure 1. Load the wireslowly, 200g at a time, markingthe position of the pointer on thegraph paper.

Continue loading the wire until itbreaks. Discard the wire afteruse.

N.B. At times of plastic flow thepointer may turn by more thanone complete rotation. Thismust be noted.

Represent the behaviour of thecopper wire graphically and useyour readings to generate avalue for the Young’s Modulus



for copper and for its ultimatebreaking stress.

Part 2: Rubber.Devise a means by which anelastic band may be hung andloaded and its extensionmeasured. Measure theextension of the band over anumber of cycles of loading andunloading.

Represent the behaviour of therubber graphically

Product:

In addition to the notes,observations, analysis andconclusion produced in your logbook while in the laboratory, youshould comment on theeffectiveness of your techniquesfor determining the Young’sModulus of these materials andthe validity of the answersobtained. Any suggestions you

have for improving the

experimental set up should alsobe included.

HINT: Consider the differencebetween the materials on amolecular level when comparingtheir different responses tostress. Does the rubber exhibithysteresis when loaded andunloaded repeatedly?

Apparatus:

2m Copper Wire

Pulley to clamp to bench

Straw and graph paper

G - clamp and wooden blocks

Hanger and weights

Micrometer

Thin Rubber Band

Clamp Stand

Half Meter and Metre Rule

Safety Specs

Anything else

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