Cambridge A Level Physics 9702 — 2014 May/June Paper 3 · Variant 3

1 In this experiment, you will investigate how the current in a circuit varies as the resistance of the circuit is changed. (a) Measure and record the length L of wire between the crocodile clips on the wire labelled F. L = ..................................................[1] (b) Set up the circuit as shown in Fig. 1.1. + − A crocodile clip metre rule wire crocodile clip Fig. 1.1 (c) (i) Attach wire F to the wire on the metre rule as shown in Fig. 1.2. + − A x F Fig. 1.2 The distance x between the crocodile clips should be approximately 50 cm. (ii) Measure and record x. x = ...................................................... (d) (i) Close the switch. (ii) Record the ammeter reading I. I = ..................................................[1] (iii) Open the switch. (e) Change x and repeat (c)(ii) and (d) until you have six sets of readings of x and I. x2 1 Include values of and I in your table. (x + L ) [10] 1 x2 (f) (i) Plot a graph of I on the y-axis against (x + L ) 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] (g) The quantities I, x and L are related by the equation 1 Px 2 = - + Q I (x + L ) where P and Q are constants. Using your answers in (f)(iii), determine values for P and Q. Give appropriate units. P = ...................................................... Q = ...................................................... [2] You may not need to use all of the materials provided.

Mark scheme: 1 (a) Value of L with unit in range 90.0 cm ≤ L ≤ 110.0 cm. [1] (d) (ii) Value of I with unit in the range 50 mA ≤ I ≤ 150 mA. [1] Allow Supervisor’s value ± 20%. (e) Six sets of readings of x and I scores 5 marks, five sets scores 4 marks etc. [5] Incorrect trend –1 (correct trend is I increases as x increases for all values of x). Major help from Supervisor –2. Minor help from Supervisor –1. Range: ∆x ≥ 70 cm. [1] Column headings: [1] Each column heading must contain a quantity and a unit. The presentation of quantity and unit must conform to accepted scientific convention e.g. x/cm or x(cm), x2/(x+L)/m but not x2/(x+L)/m2/m, 1/I (A–1), 1/I (1/A) but not 1/I (A) or 1/I(A)–1. Consistency: [1] All values of x must be given to the nearest mm only. Significant figures: [1] Significant figures for every row of x2/(x+L) same as (or one greater than) the s.f. in x. Calculation: [1] Values of x2/(x + L) calculated correctly to the number of significant figures given by the candidate. (f) (i) Axes: [1] Sensible scales must be used, no awkward scales (e.g. 3:10). Scales must be chosen so that the plotted points occupy at least half the graph grid in both x and y directions. Scales must be labelled with the quantity that is being plotted. Scale markings should be no more than three large squares apart. Plotting of points: [1] All observations must be plotted. Diameter of points must be ≤ half a small square (no “blobs”). Work to an accuracy of half a small square. Quality: [1] All points in the table must be plotted (at least 5) for this mark to be awarded. Scatter of points must be less than ± 0.25 A–1 of 1/I from a straight line. GCE AS/A LEVEL – May/June 2014 9702 33 (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. Allow one anomalous point only if clearly indicated by the candidate. Line must not be kinked or thicker than half a small square. (iii) Gradient: [1] The hypotenuse of the triangle must be at least 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. The method of calculation must be correct. y-intercept: [1] Either: Check correct read-off from a point on the line and substituted into y = mx + c. Read-off must be accurate to half a small square in both x and y directions. Or: Check read-off of the intercept directly from the graph. (g) P = – the value of the gradient and Q = the value of the y-intercept. [1] Do not allow fractions. Do not allow a substitution method. Unit for P (A–1m–1) and Q (A–1) consistent with value. [1] [Total: 20]

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2 In this experiment, you will investigate how the motion of a sphere on a track depends on the radius of the track. (a) Set up the apparatus as shown in Fig. 2.1. boss clamp clamp boss track y x stand stand G-clamp bench G-clamp side view Fig. 2.1 The distance x is the horizontal distance between the ends of the track. The distance y is the vertical distance between the top and bottom of the track. The distance y should be approximately 10 cm. (b) (i) Measure and record the distance y as shown in Fig. 2.1. y = ..................................................[1] (ii) Estimate the percentage uncertainty in your value of y. percentage uncertainty = ..................................................[1] (iii) Measure and record the distance x as shown in Fig. 2.1. x = ..................................................[1] (iv) Calculate the radius R of the track where R is given by x 2 y R = + . 8 y 2 R = ..................................................[1] (c) (i) Place a sphere on the track as shown in Fig. 2.2. sphere Fig. 2.2 (ii) Release the sphere and watch the movement. The sphere will move down one side of the track and up the other side before returning, completing a cycle as shown in Fig. 2.3. one complete cycle Fig. 2.3 (iii) The time taken for one complete cycle is T. By timing several of these complete cycles, determine an accurate value for T. T = ..................................................[2] (d) Reduce x by approximately 5 cm and repeat (b)(i), (b)(iii), (b)(iv) and (c). y = ...................................................... x = ...................................................... R = ...................................................... T = ...................................................... [3]

Mark scheme: 2 (b) (i) Value of y in range 8.0 cm ≤ y ≤ 12.0 cm with unit. [1] (ii) Absolute uncertainty in y in range 2 mm – 5 mm. [1] If repeated readings have been taken, then allow uncertainty to be half the range (but not zero) only if working shown. Correct method of calculation to obtain percentage uncertainty. (iii) Value of x to the nearest mm with unit. [1] (iv) Correct calculation of R. [1] (c) (iii) Value of T with unit in range 1.5 s ≤ T ≤ 3.0 s. [1] Evidence of repeats. [1] (d) Second value of R. [1] Second value of T. [1] Correct trend for T with respect to R (T decreases as R decreases). [1] (e) (i) Two values of k calculated correctly. [1] GCE AS/A LEVEL – May/June 2014 9702 33 (ii) Correct justification of significant figures in k linked to significant figures in x [1] and y and time (do not allow “raw readings”). (iii) Valid comment consistent with the calculated values of k, testing against a [1] criterion specified by the candidate. (f) (i) Limitations (4 max) (ii) Improvements (4 max) Do not credit A Two readings not Take more readings (for Not enough repeat enough to draw a different radii) and plot a graph readings. conclusion. or take more readings and Few readings. compare k values. Idea of repeats. “Too few readings/two readings” on its own. B Only a few cycles Use a longer track/heavier ball. Smoother or lubricated possible/(heavily) track. damped/ball does not return to the original height. C Difficult to judge exactly Improved method of timing e.g. “Ball too fast” on its own. when an oscillation is video with timer/video and view Human reaction time. complete. frame by frame/measure time Video and play back. from centre of oscillation/put High speed cameras or marker at the centre of slow motion cameras. oscillation/light gate at centre. Use of motion sensor. D Difficult to measure y Improved method of measuring Parallax error. with reason e.g. not y e.g. use a ruler across the Not clear where the sure where horizontal is top/tie a string across the top/ lowest point of track is. at the top/ruler not set square on bench/use of vertical/allowing for the plumbline/measure two heights thickness/lips of the and find difference/use of track/can’t hold ruler micrometer for thickness of steady. track/clamp ruler to measure y/clamp ruler vertically. E Limitation of track e.g. Improved method of Changing track e.g. more is not entirely supporting track e.g. attach flexible or rubber track. circular/portion of track track to board/clamp in more Difficult to clamp. in clamps is straight/too places. “Friction on track” on its stiff to bend/track own. twisted/skewed. F Difficulty in placing ball Method of marking the same If electromagnetic at the same point each point on the track/method of release do not allow time/difficulty in releasing the ball e.g. use card ‘metal’ ball. releasing the ball with as a gate/electromagnet Clamp or robotic arm. no force. release and steel ball Burning through string. Do not credit measurement of x, use of an assistant, fans, air conditioning, use of computers/dataloggers on its own. [Total: 20]

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