General Certificate of Education Advanced Level Examination June 2014 Physics PHA6/B6/XTN (Specifications A and B) Unit Investigative and Practical Skills in A2 Physics Route X Externally Marked Practical Assignment (EMPA) Teachers’ Notes Confidential The Exams Officer should make two copies of these Teachers’ Notes; one copy for the Head of A-level Physics and one for the technician These copies can be released to the Head of A-level Physics and the technician at any point following publication but must be kept under secure conditions at all times Teachers can have sight of the Teachers’ Notes but no further copies should be made Estimated entries must be submitted to AQA in order for centres to receive hard copies of the materials to be used by candidates WMP/Jun14/PHA6/B6/XTN/E5 PHA6/B6/XTN For Section A Task 1, Question and Question In Question 1, candidates will investigate capacitor discharge through a resistor In Question 2, they will investigate how the pd across a capacitor, C1, varies as it is discharged through a combination of a resistor and a second (initially uncharged) capacitor, C2 Apparatus required for each candidate: • dc power supply, terminal pd at the discretion of the Centre; if the voltmeter is set to the 2000 mV range then one type D cell in good condition, in a suitable holder, will be adequate • digital voltmeter with high input impedance (>1 MΩ) set to a range that will suit the power supply the Centre wishes to use; a 31⁄2 digit LCD multimeter set to the 2000 mV range will be adequate if a 1.5 V dry cell is used • one 12 kΩ carbon film resistor, at least 0.25 W; the resistance of this resistor should be concealed from the candidates • one 1000 μF electrolytic capacitor, 10 V or higher, labelled ‘C1’ • one 2200 μF electrolytic capacitor, 10 V or higher, labelled ‘C2’ • three round sockets to be labelled ‘S1’, ‘S2’ and ‘S3’ • suitable connecting leads, including two flying leads, each terminating with a mm round plug connected as shown in the circuit diagram below; these should be labelled as indicated • digital stopwatch capable of reading to 0.1 s or better • card on which the calculated capacitance of C1 should be provided for the candidate’s use The circuit to be provided for each candidate is shown below flying lead socket S1 flying lead socket S3 2200 μF labelled ‘C2’ socket S2 1000 μF labelled ‘C1’ V digital voltmeter 12 kΩ Candidates will carry out the following procedure: Question 1: • Ensure that C2 remains discharged by connecting flying lead to S3 • Charge C1 by connecting flying lead to S1; record voltmeter reading, ε • Discharge C1 by connecting flying lead to S2 • Starting the watch when the voltmeter reads 80% of ε determine the times for the voltmeter reading to reach 40% of ε and 20% of ε Question 2: • Remove flying lead from S2 • Connect flying lead to S2 to avoid accidential discharge of C2 by earthing etc • Charge C1 by connecting flying lead to S1 • Discharge C1 by connecting flying lead to S3; recording the voltmeter reading at 10 s intervals for 60 s During this period C2 becomes charged • With flying lead still connected to S2 so C2 remains charged, charge and then discharge C1 for a second time and record the voltmeter readings at 10 s intervals for a further 60 s WMP/Jun14/PHA6/B6/XTN Testing: In Question 1, C2 is redundant; the exercise is to charge and then discharge C1 through R Candidates will require the capacitance of C1 that they will use and this can be found directly using a capacitance meter, or indirectly by measuring the time for the voltmeter reading to fall to 50% of some initial value The capacitance of C1 (in F) is given by C1 = time (in seconds) for voltmeter reading to halve 8.32 × 103 The time for the voltmeter reading to halve will typically be between 7.5 s and 10.5 s In Question parts (a)(i) and (a)(ii), the voltmeter reading will decrease at a decreasing rate when the flying lead is connected to S2 The pd across C2 rises to meet the (falling) pd across C1 and when these equalise the voltmeter reading will become approximately constant; this takes about 50 s and the reading will be equal to 30% to 35% of the initial reading In Question parts (a)(iii) and (a)(iv) the reading falls more gradually than before because C2 is already charged from part one Once again, the process stops when the pd across C1 is the same as across C2; this time the voltmeter reading will reach an approximately constant value about 50% of the initial reading Place all the apparatus required on the bench before the experiments with both flying leads disconnected and the card giving the calculated capacitance of C1 in clear view Examiners require no information for this question The following data are provided to assist in trialling the practical tasks Question (a) voltmeter reading time/s 40% of ε 9.52 20% of ε 18.67 Question (a) and (b) t/s VA/V VB/V 1.564 1.564 60 0.508 0.797 Turn over WMP/Jun14/PHA6/B6/XTN ᮣ For Section A Task Candidates are to measure the time period of a simple pendulum with an interrupted swing Apparatus required for each candidate: • digital stopwatch capable of reading to 0.1 s or better • metre ruler • pendulum bob fastened to about metre of strong thread (which should resist stretching) • suitable fiducial mark to assist candidates to measure the period of the pendulum • retort stand fitted with boss and clamp • means to prevent stand movement if required Materials to construct assembly to support the interrupted pendulum: • rectangular strip of thin sheet material, eg 12 mm m.d.f., suitable dimensions 300 mm × 40 mm • between 30 mm and 40 mm of a plastic ruler (which could be cut along median line) • one mm binding (terminal) post • small washer The working arrangement is shown in Figure of the question/answer booklet The apparatus is to consist of a simple pendulum of length 0.90 m suspended so that the thread overhangs the edge of the bench The thread is to pass through the hole in the terminal post, so by unscrewing the terminal the thread is released and candidates can change the effective length of the pendulum At a point 250 mm below the shoulder of the terminal, a section of plastic ruler is mounted horizontally so the tapered edge of the ruler interrupts the pendulum at the mid-point of its swing The motion of the pendulum in a plane perpendicular to the edge of the bench will therefore consist of half cycles of different duration The suggested arrangement for the apparatus is described and illustrated in the diagram as follows Drill a suitable hole through the median line of the m.d.f to accommodate the threaded stem of the terminal post Fit the terminal post in place then trim the m.d.f so the distance between the shoulder of the terminal post and the end of the m.d.f is 250 mm Glue the section of ruler in place so that approximately mm protrudes Assemble the pendulum by passing the thread through the hole in the terminal post and tie on the bob Fasten a small washer to the (upper) free end of the thread to prevent the thread becoming detached from the terminal post when the terminal is unscrewed WMP/Jun14/PHA6/B6/XTN Clamp the m.d.f so that the pendulum overhangs the edge of the bench The terminal post should be about 100 mm above the top of the bench Adjust the length of the pendulum so that the bob hangs freely about 100 mm above the floor and when the thread is at rest and hanging vertically it just touches the protruding edge of the ruler The metre ruler, stopwatch and fiducial mark should be placed on the bench alongside the apparatus terminal post washer 12 mm m.d.f or similar 250 mm section of plastic ruler For candidates with mobility problems the apparatus can be mounted on a tall stand so that the experiment can be carried out at bench level Examiners require no information for this question The following data are provided to assist in trialing the practical tasks Question (a) x/m = 0.248 (b) l/m T /s 0.550 1.65 Turn over WMP/Jun14/PHA6/B6/XTN ᮣ General Certificate of Education June 2014 Advanced Level Examination PHYSICS (SPECIFICATIONS A AND B) PHA6/B6/XTN Unit SUPERVISOR’S REPORT When completed by the Supervisor, this Report must be attached firmly to the attendance list, or in the case of any problem affecting a particular candidate, it should be attached to the candidate’s script, before despatch to the Examiner Details of problems encountered by candidate candidate number Supervisor’s Signature Centre number Date Centres may make copies of this Supervisor’s Report for attachment to individual scripts where necessary WMP/Jun14/PHA6/B6/XTN The following instructions include the tasks that candidates will be required to complete in Section A Task and Section A Task These instructions not include all the questions that may be asked Section A Task 1 You are to determine the resistance of a resistor R of unknown resistance using the circuit shown in Figure Figure flying lead flying lead on ly C2 socket S3 socket S1 Te ac he ru se socket S2 C1 V digital voltmeter R This circuit is used for both Question and Question in Section A Task Connect flying lead to socket S3 as shown in Figure 1; Keep this lead connected to S3 whilst making the measurements in Question 1 (a) Connect flying lead to socket S1 so that capacitor C1 is fully charged (a) (i) Read and record the voltmeter reading, ε ε = Turn over WMP/Jun14/PHA6/B6/XTN ᮣ (a) (ii) Remove flying lead from socket S1 and connect this without delay to socket S2 so the voltmeter reading decreases as C1 discharges through resistor R Start the stopwatch when the voltmeter reads 80% of ε Measure and record, the times at which the voltmeter reads 40% and 20% of ε (a) Disconnect flying lead from S2 Connect flying lead to S2 as shown in Figure 3; keep this lead connected to S2 whilst making the measurements in Question (unless you wish to discharge C2 in order to repeat the question) You are not required to take any repeat readings in Question 2(a) Figure on ly flying lead socket S1 socket S3 C2 Te ac he ru se flying lead C1 (a) (i) V socket S2 digital voltmeter R Charge C1 by connecting flying lead to S1 Record the voltmeter reading, VA (a) (ii) Disconnect the lead from S1 and without delay connect it to S3, starting the stopwatch as you this Read and record further values of VA at 10 second intervals (a) (iii) Remove flying lead from S3 and connect it to S1 to recharge C1 Record the voltmeter reading, VB (a) (iv) Disconnect the lead from S1 and without delay connect it to S3, starting the stopwatch as you this Read and record further values of VB at 10 second intervals You are not required to repeat the experiment but should you need to so, disconnect flying lead from S3 and flying lead from S2 Briefly connect flying lead to S3 to discharge C2 before reconnecting this lead to S2 You may now recommence the procedure starting at part (a)(i) WMP/Jun14/PHA6/B6/XTN Section A Task In this experiment you are required to measure the time period of a pendulum with an interrupted swing You are provided with the apparatus shown in Figure 5, including a fiducial mark Figure Washer on ly Terminal Te ac he ru se x A Piece of ruler B l Pendulum bob Turn over WMP/Jun14/PHA6/B6/XTN ᮣ 10 (a) Measure and record x, the length of thread between the terminal and the piece of ruler x = (b) Holding the washer in one hand, unscrew the terminal Adjust l, the length of the thread between the piece of the ruler and the centre of the pendulum bob, until l is about 0.55 m Secure the thread in this position by tightening the terminal Make further adjustments to ensure that when the pendulum is at rest the string is vertical along its entire length and just touches the edge AB of the ruler on ly Measure and record l Displace and release the pendulum so that it performs small-amplitude oscillations in a plane perpendicular to AB Make suitable measurements to determine the period, T, of these oscillations Repeat this procedure to find T for four smaller values of l Te ac he ru se Record all your observations Note that the independent variable should be recorded in the left-hand column of your table Copyright © 2014 AQA and its licensors All rights reserved WMP/Jun14/PHA6/B6/XTN