Cambridge A Level Physics 9702 — 2012 May/June Paper 3 · Variant 4

1 In this experiment, you will investigate the change in current in a circuit as its resistance is changed. (a) (i) Assemble the circuit of Fig. 1.1. 3 V d.c. supply X Y A crocodile clip component holder Fig. 1.1 (ii) Close the switch. (iii) Record the ammeter reading I0. I0 = ................................................. [1] (iv) Open the switch. (b) (i) Select one of the resistors labelled with a numerical value and connect it between the crocodile clips on the component holder. Record the resistance R of your selected resistor. R = ................................................ kΩ (ii) Close the switch and record the ammeter reading I. Open the switch. I = ................................................. [1] (c) Repeat (b) using different labelled resistors until you have six sets of values of R and I. For 1 Examiner’s Include values of in your table. Use R [10] 1 (d) (i) Plot a graph of I on the y-axis against on the x-axis. [3] R (ii) Draw the straight line of best fit. [1] (iii) Determine the gradient and y-intercept of this line. gradient = ....................................................... y-intercept = ...................................................... [2] For Examiner’s Use (e) The quantities I and R are related by the equation For a a Examiner’sUse I = + R b where a and b are constants. Using your answers from (d)(iii), determine the values of a and b. Give appropriate units. a = ...................................................... b = ...................................................... [2] You may not need to use all of the materials provided. For Examiner’s Use

Mark scheme: 1 (a) (iii) Value for I0 in range 2.0 to 4.0 mA, with unit. [1] (b) (ii) First value of I (greater than I0). [1] (c) Six sets of readings of R and I scores 5 marks, five sets scores 4 marks etc. [5] Major help from Supervisor –2. Minor help from Supervisor –1. Incorrect trend then –1. Range: [1] Values of R must include 0.22 kΩ or 0.33 kΩ and 3.3 kΩ or 4.7 kΩ. Column headings: [1] Each column heading must contain a quantity and a unit. There must be some distinguishing mark between the quantity and the unit. Consistency: [1] Values of I must be given either all to the nearest 0.1 mA or all to the nearest 0.01 mA. Significant figures: [1] Every value of 1/R must be given to either 2 or 3 significant figures. Calculated values: [1] 1/R calculated correctly. (d) (i) Axes: [1] Sensible scales must be used (no awkward scales such as 3:10). Scales must be chosen so that the plotted points must occupy at least half the graph grid in both x and y directions. Scales must be labelled with the quantity which is being plotted. Scale markings must be no more than 3 large squares apart. Plotting of points: [1] All observations in the table must be plotted. Diameter of plots must be < half a small square (no blobs). Plotting must be accurate to half a small square. Quality: [1] Range of I must be at least 2 mA, and all points must be within 0.5 mA of a straight line. All points in the table must be plotted (at least 5) for this mark to be scored. (ii) Line of best fit: [1] Judge by balance of all points on the grid about the candidate’s line (at least 5 points).There must be an even distribution of points either side of the line along the full length. One anomalous point is allowed only if clearly indicated (i.e. circled or labelled) by the candidate. Line must not be kinked or thicker than half a small square. GCE AS/A LEVEL – May/June 2012 9702 34 (d) (iii) Gradient: [1] The hypotenuse must be at least half the length of the drawn line. Both read-offs must be accurate to half a small square in both x and y directions. Do not allow ∆x / ∆y. y-intercept: [1] Either: Correct read off from a point on the line is substituted into y = mx + c. Or: Check read-off of the intercept directly from the graph. (e) Calculation of b is correct, [1] i.e. b = (candidate’s gradient value)/(candidate’s intercept value). Value for b in range 0.8 kΩ to 1.2 kΩ, with unit. [1] [Total: 20]

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2 In this experiment, you will investigate how the amount of energy needed to bend a copper wire depends on its diameter. (a) You are provided with a pendulum which can be used to hit and bend a wire specimen. Attach the wooden pointer to the pendulum using Blu-Tack. Align the pointer with the 90° position on the protractor when the pendulum is hanging freely, as shown in Fig. 2.1. Check that the pendulum swings freely. 70 80 90 100 110 60 120 50 130 40 140 30 150 20 160 10 170 0 180 wooden pointer Blu-Tack protractor pendulum Fig. 2.1 (b) You are provided with copper wire specimens of two different diameters. Using the micrometer, measure and record the diameter d of one of the thicker wires. d = ................................................. [2] (c) Estimate the percentage uncertainty in your value of d. percentage uncertainty = ................................................. [1] (d) (i) Fit the wire fully into the hole in the wooden block. The wire should project about For 2 cm above the surface of the block. Examiner’s Use (ii) Position the wooden block so that the wire just touches the edge of the pendulum, as shown in Fig. 2.2. pendulum wire base of wooden stand block Fig. 2.2 (iii) Lift the pendulum so that the pointer is aligned with the 0° position on the protractor. (iv) While holding the block in position, release the pendulum and record the maximum angle θ reached by the pointer after the pendulum has bent the wire. θ = ................................................. [2] (v) The energy used to bend the wire is proportional to the value of sin (180°– θ). Calculate this value, giving your answer to an appropriate number of significant figures. sin (180°– θ) = ................................................. [2] (e) Repeat (b) and (d) using the thinner wire specimens. For Examiner’s Use d = ...................................................... θ = ...................................................... sin (180°– θ) = ...................................................... [3] (f) It is suggested that the relationship between θ and d is sin (180°– θ) = kd 2 where k is a constant. (i) Using your data, calculate two values of k. first value of k = ...................................................... second value of k = ...................................................... [1]

Mark scheme: 2 (b) Value of d in range 0.80 to 0.99 mm, to nearest 0.01 mm, with unit. [1] Evidence of repeated measurements for d. [1] (c) Percentage uncertainty in d based on absolute uncertainty of 0.01 mm. [1] Correct calculation to get percentage uncertainty. (d) (iv) Value of θ in range 91° to 180° to nearest degree, with unit. [1] Evidence of repeated measurements for θ. [1] (v) Correct calculation of sin(180° – θ ). [1] sin(180° – θ ) given to 2 or 3 s.f. [1] (e) Second value of d. [1] Second value of θ. [1] Quality: θ larger for smaller d. [1] (f) (i) Correct calculation of two values of k. [1] (ii) Valid conclusion based on the calculated values of k. Candidate must test correctly against a stated criterion. [1] GCE AS/A LEVEL – May/June 2012 9702 34 (g) (i) Limitations 4 max. (ii) Improvements 4 max. No credit/not enough A two results not enough take more readings and plot ‘repeat readings’ on its own/ a graph/ few readings/ calculate more k values and take more readings and compare (calculate) average k/ only one reading B θ (or angle, or scale reading, video and view playback/ just ‘use a computer’/ or protractor reading, or slow motion camera/ ‘reading’ difficult to measure pointer reading) is difficult to video to read angle/ measure, with reason linked add a ‘max hold’ pointer/ to rapid motion or short time angle sensor with data logger (or computer) C parallax error in θ use mirror scale/ view at right angles/ measurement description of method to trial and improvement reduce error D θ (or reading) is difficult (or – use thinner pointer/ inaccurate, or imprecise) use larger scale because pointer is thick E pointer attachment moves description of secure method of attachment F – description of method of fixing block to bench [Total: 20]

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