Cambridge A Level Physics 9702 — 2016 Oct/Nov Paper 3 · Variant 6

1 In this experiment, you will investigate the current in an electrical circuit. (a) Connect the circuit shown in Fig. 1.1. 1.5 V d.c. x P F G H movable metre contact Q resistance rule A wire Fig. 1.1 F, G and H are crocodile clips. The crocodile clip G is used as a movable contact. Position G approximately half-way along the resistance wire. (b) (i) The distance between F and G is x, as shown in Fig. 1.1. Measure and record x. x = ........................................ cm [1] (ii) Close the switch. (iii) Record the ammeter reading I. I = .............................................. [1] (iv) Open the switch. (c) Vary x and repeat (b) until you have six sets of values for x and I. [8] (d) (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] (e) The quantities I and x are related by the equation I = Sx + T where S and T are constants. Use your answers from (d)(iii) to determine the values of S and T. Give appropriate units. S = ............................................... T = ............................................... [2] (f) The resistance per unit length r of the resistance wire can be found from PS r = T where P = 15 Ω (the resistance of resistor P ). Calculate r in Ω cm–1. Give your answer to a suitable number of significant figures. r = ...............................................Ω cm–1 [2] [Total: 20] You may not need to use all of the materials provided.

Mark scheme: 1 (b) (i) Value for x in range 45.0 cm to 55.0 cm. [1] (iii) Value for I in range 500 µA to 1500 µA (or 0.50 mA to 1.50 mA), with unit. [1] (c) Six sets of values for x and I (with correct trend and without help from Supervisor) scores 5 marks, five sets scores 4 marks etc. [5] Range: [1] x values must include 20 cm or less and 80 cm or more. Column headings: [1] Each column heading must contain a quantity and an appropriate unit. The presentation of the quantity and unit must conform to accepted scientific convention e.g. I / µA. Consistency: [1] All values of raw x must be given to the nearest mm. (d) (i) Axes: [1] Sensible scales must be used. Awkward scales (e.g. 3:10, fractions or non-linear) are not allowed. 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 must be no more than three large squares apart. 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 (no “blobs”). Plotting of points must be accurate to half a small square. Quality: [1] All points in the table (at least 5) must be plotted on the grid. All points must be within ± 20 µA (± 0.02 mA) of a straight line in the y (I) direction. (ii) Line of best fit: [1] Judge by balance of all points on the grid about the candidate's line (at least five points). There must be an even distribution of points either side of the line along the full length. One anomalous plot is allowed if clearly indicated (i.e. circled or labelled). There must be at least five points left after disregarding the anomalous point. Lines must not be kinked or thicker than half a small square. (iii) Gradient: [1] The hypotenuse of the triangle must be greater than half the length of the drawn line. The method of calculation must be correct. Do not allow ∆x / ∆y. Both read-offs must be accurate to half a small square in both the x and y directions. 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 (accurate to half a small square. (e) Value of S = candidate’s gradient and value of T = candidate’s intercept. Do not allow fractions. [1] Consistent units for S (e.g. µA cm–1) and T (e.g. µA). [1] (f) Calculation: r calculated correctly to the s.f. given by the candidate. [1] Significant figures: r given to 2 or 3 s.f. [1]

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2 In this experiment, you will investigate a balanced metre rule. (a) (i) Set up the apparatus as shown in Fig. 2.1. Use some of the Blu-Tack to fix the pivot to the empty beaker. FRONT VIEW boss rod of clamp Blu-Tack pivot boss metre rule rod of clamp empty beaker stand bench TOP VIEW boss stand rod of clamp Fig. 2.1 (ii) The rods of the clamps act as stops to prevent the metre rule tilting too far. Adjust them so that one is approximately 3 cm higher than the pivot and the other is approximately 3 cm lower than the pivot. (iii) Position the metre rule with its 50.0 cm mark at the pivot. If necessary, add Blu-Tack to one end of the rule to balance it. (b) You are provided with two glass spheres fixed to string loops. (i) Suspend the larger sphere from the metre rule at a distance of 10.0 cm from the pivot. Balance the rule by suspending the other sphere a distance x1 from the pivot, as shown in Fig. 2.2. 10.0 cm x1 string loop larger sphere Fig. 2.2 (ii) Measure and record x1. x1 = ............................................... cm [1] (c) Immerse the larger sphere in water at a distance of 10.0 cm from the pivot, as shown in Fig. 2.3. Balance the rule by moving the smaller sphere. Measure and record the distance x2. 10.0 cm x2 beaker water Fig. 2.3 x2 = ............................................... cm [1] (d) (i) Repeat (b) with the spheres changed around, with the smaller sphere suspended at a distance of 40.0 cm from the pivot, as shown in Fig. 2.4. 40.0 cm x1 Fig. 2.4 x1 = ............................................... cm [1] (ii) Immerse the smaller sphere in water at a distance of 40.0 cm from the pivot, as shown in Fig. 2.5. Balance the rule by moving the larger sphere. Measure and record x2. 40.0 cm x2 Fig. 2.5 x2 = ............................................... cm [1]

Mark scheme: 2 (b) (ii) x1 in range 10.0 cm to 40.0 cm. [1] (c) Value of x2 < x1. [1] (d) (i) Second value of x1. [1] (ii) Value of x2 given to nearest mm and all other raw values of x in (b), (c) and (d) are to the nearest mm. [1] (e) (i) Two values of k calculated correctly. [1] (ii) Justification of the s.f. in k based on the s.f. in x1 and the s.f in x2. [1] (iii) Valid comment consistent with the calculated values of k, testing against a stated numerical criterion. [1] (f) (i) Raw values of D to nearest 0.001 cm and in range 1.400 cm to 2.200 cm. [1] Evidence of repeated readings for D. [1] (ii) Absolute uncertainty in D of 0.001 cm or 0.002 cm. If repeated readings have been taken, then absolute uncertainty could be half the range (but not zero) if working is clearly shown. Correct method of calculation to obtain percentage uncertainty. [1] (iii) V calculated correctly. [1] (iv) Quality: M in range 3 g to 13 g. [1] (g) (i) Limitations [4] (ii) Improvements [4] Do not credit A Two readings are not enough Take more readings and plot Two readings not enough to draw a valid conclusion graph/ for accurate results take more readings and compare k values Repeat readings Few readings Take more readings and calculate average k B Empty beaker moves on Fix beaker with Blu-Tack/tape/ bench glue C Difficult to balance rule: Make groove in rule (under Blu-tack rule slips on pivot/ 50cm mark)/ Tape wind disturbs balance other practical method e.g. Switch off fans hinge/nail through rule String slips on rule D Spheres/string/tape still wet Use waterproof string/ Dry the spheres after immersion so mass use wire changes Waterproof tape or string/tape adds to mass of sphere E Difficult to measure x with Use thin(ner) string Parallax problems reason, e.g. string too thick (so it covers graduations on rule) F Marble not round Improved method of finding V Repeat readings and (e.g. liquid displacement) average

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