Cambridge A Level Physics 9702 — 2025 May/June Paper 3 · Variant 2

1 In this experiment, you will investigate a light-dependent resistor (LDR). (a) • Connect the circuit shown in Fig. 1.1. 1.5 V cardboard tube S LDR leads tape LDR (inside tube) A Fig. 1.1 • Ensure that the switch S is open. • Slide the LDR leads into the tube until the front of the LDR is just level with the open end of the tube, as shown in Fig. 1.1. • With the LDR in this position, attach a piece of adhesive tape to the leads as a marker at the other end of the tube, as shown in Fig. 1.1. • Slide the tube until the LDR is approximately half-way along it, as shown in Fig. 1.2. x tape Fig. 1.2 • The distance between the tape and the tube is x. Measure and record x. x = ............................................................... • Close S and record the ammeter reading I. I = ............................................................... • Open S. [2] (b) Change x by moving the LDR to a new position inside the tube, with x in the range 4 cm to 18 cm. Record x and I. Repeat until you have six sets of values of x and I. Record your results in a table. Include values of I in your table. [10] (c) (i) Plot a graph of I on the y-axis against x on the x-axis. [3] (ii) Draw the straight line of best fit. [1] (iii) Determine the gradient and y-intercept of this line. gradient = ............................................................... y-intercept = ............................................................... [2] (d) It is suggested that the quantities I and x are related by the equation I = ax + b where a and b are constants. Using your answers in (c)(iii), determine the values of a and b. Give appropriate units. a = ............................................................... b = ............................................................... [2] [Total: 20]

Mark scheme: Question Answer Marks 1(a) Value for x in range 8.0–14.0 cm with unit. 1 Non-zero value for I less than 2.00 mA with unit. 1 1(b) Six sets of readings of x (different values) and I with correct trend (as x increases, I decreases) and without help from 5 Supervisor scores 5 marks, five sets scores 4 marks etc. Range: xmin ⩽ 5.0 cm and xmax ⩾ 16.0 cm. 1 Column headings: 1 Each column heading must contain a quantity and a unit where appropriate. The presentation of quantity and unit must conform to accepted scientific convention e.g. I (mA½), I / mA½. Consistency: All values of x must be given to the nearest millimetre. 1 Significant figures: 1 Values of I must be given to the same number of s.f. as (or one more than) the number of s.f. in I. Calculation: Values of I calculated correctly. 1 1(c)(i) Axes: 1 Axes must be labelled with the required quantities. Scales must be chosen so that the plotted points occupy at least half the graph grid in both the x and y directions. Scale markings are no more than 2 cm (one large square) apart. Sensible scales must be used, no awkward scales (e.g. 3:10 or fractions). Plotting of points: 1 All observations in the table must be plotted on the grid. Diameter of plotted points must be ⩽ half a small square. Points must be plotted to an accuracy of half a small square in both x and y directions. Quality: 1 General trend of points must be negative. All points in the table must be plotted (at least 5) for this mark to be awarded. It must be possible to draw a straight line that is within  2.0 cm (to scale) on the x-axis of all plotted points. 1(c)(ii) Line of best fit: 1 ‘Best fit’ is judged by the balance of all points on the grid (at least 5 points) about the candidate’s line. There must be an even distribution of points either side of the line along the full length. Lines must not be kinked or thicker than half a square. Some candidates may choose to identify an anomalous point. If 6 or more points are plotted and they identify one point as anomalous (e.g. by circling or labelling) then this point is to be disregarded when judging the line of best fit. 1(c)(iii) Gradient: 1 The hypotenuse of the triangle used must be greater than half the length of the drawn line. Both read-offs must be accurate to half a small square in both the x and y directions. Method of calculation must be correct, not x / y. Gradient sign on answer line must be consistent with graph drawn. y-intercept: 1 Correct read-off from a point on the line is substituted into y = mx + c or an equivalent expression. Read-off accurate to half a small square in both x and y directions. Or Intercept read directly from the graph, with read-off at x = 0, accurate to half a small square in y direction. 1(d) a = candidate’s gradient value and b = candidate’s intercept value. 1 Values must not be written as fractions or given to only one significant figure. Units for a and b are correct (e.g. mA½ cm–1 for a and mA½ for b). 1

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2 In this experiment, you will investigate the elastic properties of rubber cord. (a) (i) You are provided with a wire with a clip and two slotted masses attached, as shown in Fig. 2.1. clip wire mass mass B Fig. 2.1 The distance between the centres of the two slotted masses is B, as shown in Fig. 2.1. Measure and record B. B = ......................................................... [1] (ii) Estimate the percentage uncertainty in your value of B. Show your working. percentage uncertainty = ......................................................% [1] (b) (i) • You are provided with two lengths of rubber cord. Select the longer cord. • The diameter of the cord is d. Measure and record d. d = ......................................................... [1] (ii) • Suspend the clip, wire and masses using the longer cord secured in the two clips, as shown in Fig. 2.2. stand jaws of clamp boss upper clip L rubber cord lower clip wire with masses attached bench Fig. 2.2 (not to scale) • The length of cord between the two clips is L, as shown in Fig. 2.2. Measure and record L. L = ......................................................... [1] (iii) • Keeping the cord vertical, rotate the lower clip through approximately 180° and release the clip. The clip will rotate with a small number of oscillations. • Take measurements to determine the period T of these oscillations. T = ....................................................... s [2] (c) Using the shorter length of rubber cord, repeat (b). d = ............................................................... L = ............................................................... T = ............................................................. s [3]

Mark scheme: 2(a)(i) Value of B in range 7.50–9.50 cm with unit and to nearest mm. 1 2(a)(ii) Percentage uncertainty based on absolute uncertainty in range 2–5 mm. 1 Correct method of calculation to find percentage uncertainty e.g. (absolute uncertainty / value from (a)(i))  100. If repeated readings have been taken, then the uncertainty can be half the range (but not zero) if the working is clearly shown. 2(b)(i) Value for d in range 2.00–6.00 mm with unit and to nearest 0.1 mm. 1 2(b)(ii) Value for L in range 20.0–40.0 cm with unit. 1 Value of T in range 2.00–9.00 (s). 1 Repeat readings for T: at least two values of nT where n ⩾ 2. 1 2(c) Second values of d and L. 1 Second value of T. 1 Second d smaller than first d. 1 2(d)(i) Two values of k calculated correctly. 1 Values not written as fractions or given to only one significant figure. 2(d)(ii) Justification for significant figures in k linked to significant figures in B, L, d and T. 1 2(e) Correct calculation of percentage difference between candidate’s two k values. 1 Comparison of percentage difference with 20%, leading to a consistent conclusion. 2(f)(i) A Two (sets of) readings are not enough to draw a (valid) conclusion (not “not enough for accurate results”, “few 4 readings”). B Difficult to measure B with a reason e.g. difficult to judge the position of the centres of the masses / masses not parallel to each other on wire / B changes when clip and masses are suspended. C Difficult to measure L with a reason e.g. parallax error / rubber cord moves when touched by rule / clip moves when touched by rule. D Difficult to measure d with a reason e.g. jaws of calipers compress width of rubber cord. E Difficult to measure T with a reason e.g. because oscillations die away (very) quickly / amplitude of oscillations decreases quickly / oscillations are heavily damped / difficult to judge start and/or end of oscillations. F Difficulty with oscillation (of cord/clip/masses) e.g. oscillations in more than one mode / vertical or side-to-side oscillations /oscillation affected if rubber cord is not attached at centre of lower clip / wire moves in the clip during oscillation. 1 mark for each point up to a maximum of 4. 2(f)(ii) A Take more readings and plot a graph or take more readings and compare k values (not “repeat readings” on its own). 4 B Improved method of measuring B e.g. measure from left (or right) hand side of one mass to left (or right) hand side of the other mass / measure B when clip is suspended. C Method to improve measurement of L e.g. clamp rule (to measure L) / use pointers attached to rule or attached to edges of clips. D Method to improve measurement of d e.g. use a micrometer (to measure d) / use travelling microscope (to measure d). E Method to improve measurement of T e.g. twist masses through larger angle before releasing / use marker placed at midpoint/equilibrium point/rest point of oscillation / record/film/video oscillation with timer in view. F Workable method to improve release of clip/masses e.g. mark centre of clip for attaching to the rubber cord / marker to indicate initial height / marker to indicate initial position of centre of clip vertically under cord. 1 mark for each point up to a maximum of 4.

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