Cambridge A Level Physics 9702 — 2015 May/June Paper 3 · Variant 1

1 In this experiment, you will investigate how the current in a circuit varies as the resistance of the circuit is changed. (a) (i) You have been provided with a length of bare wire and two crocodile clips which have small screws on them. Connect the wire between the crocodile clips using the screws as shown in Fig. 1.1. The length w of wire between the screws should be approximately 50 cm. The screws should be tightened using the screwdriver. w crocodile clip bare wire screw Fig. 1.1 (ii) Measure and record w. w = ........................................................ [1] (b) (i) Set up the circuit as shown in Fig. 1.2. power supply + – wire crocodile clips A Fig. 1.2 (ii) Close the switch. (iii) Record the ammeter reading IA. IA = ........................................................ (iv) Open the switch. (c) (i) Move the crocodile clip to set up the circuit as shown in Fig. 1.3. + – A Fig. 1.3 (ii) Close the switch. (iii) Record the ammeter reading IB. IB = ........................................................ [1] (iv) Open the switch. (d) Change w and repeat (a)(ii), (b) and (c) until you have six sets of values of w, IA and IB. (IA + IB) Include values of in your table. IAIB [10] (IA + IB) (e) (i) Plot a graph of on the y-axis against w on the x-axis. [3] IAIB (ii) Draw the straight line of best fit. [1] (iii) Determine the gradient and y-intercept of this line. gradient = ...................................................... y-intercept = ...................................................... [2] (f) The quantities IA, IB and w are related by the equation (IA + IB) = Mw + N IAIB where M and N are constants. Using your answers in (e)(iii), determine values for M and N. Give appropriate units. M = ........................................................ N = ........................................................ [2] You may not need to use all of the materials provided.

Mark scheme: 1 (a) (ii) Value of w with unit, in range 45.0 cm to 55.0 cm. [1] (c) (iii) Value of IB, with unit, to nearest 0.1 mA, in range 70.0 ≤ IB ≤ 100.0 mA. [1] (d) Six sets of readings of w, IA and IB, different values, scores 5 marks, five sets scores 4 marks, etc. [5] Incorrect trend –1. Major help from Supervisor –2. Minor help from Supervisor –1. Range: [1] Range of w ≥ 60.0 cm. 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. w / cm, w (cm), (IA+IB) / (IAIB) / A–1, (IA+IB) / (IAIB) / (1/A). Do not allow (IA+IB) / (IAIB) / (A / A2). Consistency: [1] All values of w must be given to the nearest mm only. Significant figures: [1] Every value of (IA+IB) / (IAIB) must be given to the same number of s.f. as (or one more than) the least s.f. in the corresponding values of IA and IB. Calculated values: [1] Values of (IA+IB) / (IAIB) calculated correctly to the number of significant figures given by the candidate. (e) (i) Axes: [1] Sensible scales must be used. Awkward scales (e.g. 3:10) 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 should be no more than three large squares apart. Plotting of points: [1] All observations in the table must be plotted. Diameter of points must be ≤ half a small square (no “blobs”). Plotted points must be accurate to within half a small square. Quality: [1] All points in the table must be plotted on the grid (at least 5) for this mark to be awarded. All points must be within ± 5 cm (± 0.05 m) on the w-axis from a straight line. (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. Lines must not be kinked or thicker than half a 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. Both read-offs must be accurate to half a small square in both the x and y directions. y-intercept: [1] Either: Correct read-offs 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). (f) M = value of the candidate’s gradient and N = value of the candidate’s y-intercept. [1] Do not allow substitution methods. Do not allow fractions. Unit for M correct (e.g. A–1 m–1 or A–1 cm–1 or A–1 mm–1 or mA–1 m–1 or mA–1 cm–1 or mA–1 mm–1) and unit for N correct (e.g. mA–1 or A–1). [1]

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2 In this experiment, you will investigate the balance of a wooden strip. (a) (i) Measure and record the length L of the longer wooden strip as shown in Fig. 2.1. L wooden strip Fig. 2.1 L = ........................................................ [1] (ii) Measure and record the mass m, in grams, of the longer wooden strip. m = ........................................................ g [1] (iii) Calculate the mass per unit length p of the wood, where m p = . L p = ........................................................ (iv) Justify the number of significant figures that you have given for your value of p. .................................................................................................................................. .................................................................................................................................. ..............................................................................................................................[1] (b) (i) Measure and record the mass M, in grams, of the slotted mass. M = ........................................................ g [1] (ii) Use the Blu-Tack to attach the slotted mass to the top of the longer wooden strip as shown in Fig. 2.2. top view slotted mass wooden strip side view Blu-Tack Fig. 2.2 The centre of the slotted mass should be positioned at the end of the wooden strip. (iii) Calculate C, where L2 C = 2(M + Lp). C = ........................................................ [1] (c) (i) Balance the wooden strip on the pivot as shown in Fig. 2.3. x pivot bench Fig. 2.3 (ii) Measure and record the distance x from the pivot to the end of the wooden strip as shown in Fig. 2.3. Do not mark the wooden strip. x = ........................................................ [1] (iii) Estimate the percentage uncertainty in your value of x. percentage uncertainty = ........................................................ [1]

Mark scheme: 2 (a) (i) Value of L with unit, in range 55.0 cm ≤ L ≤ 65.0 cm. [1] (ii) Value of m to nearest gram or better, in range 10.0 g ≤ m ≤ 100.0 g. [1] (iv) Correct justification of significant figures in p linked to significant figures in L and m. [1] (b) (i) Value of M to the nearest gram or better, in range 90.0 g ≤ M ≤ 110.0 g. [1] (iii) Correct calculation of C. [1] (c) (ii) Value of x to the nearest mm, with unit, in range 5.0 cm ≤ x ≤ 20.0 cm. [1] (iii) Absolute uncertainty in x in range 2 – 5 mm. If repeated readings have been taken, then the uncertainty can be half the range (but not zero) if the working is clearly shown. Correct method of calculation to obtain percentage uncertainty. [1] (d) Second value of L. [1] Second value of x. [1] Correct trend for x with respect to L (x decreases as L decreases). [1] (e) (i) Two values of k calculated correctly. [1] (ii) Valid comment consistent with calculated values of k, testing against a criterion specified by the candidate. [1] (f) (i) Limitations (4 max.) (ii) Improvements (4 max.) Do not credit A Two readings not enough to Take more readings (for “repeat readings”/ draw a valid conclusion. different L) and plot a graph/ “too few readings” take more readings and compare k values. B Difficult to measure x with Improved method to measure x Travelling microscope reason, e.g. parallax/ruler not e.g. attach mass to bottom of Video in line with wood/strip moves strip/mark scale on strip/mark as touched while taking strip at balance point/measure measurement/mass obscures (L–x)/clamp ruler horizontally end of rule/strip oscillates/balance achieved for a short time C Difficult to balance with reason, Method to remove wind, e.g. Sliding rule e.g. wind/air conditioning or turn off fans/close windows or Pivot size pivot moves method of fixing pivot to bench i.e. tape/heavier pivot D Problem with Blu-Tack, e.g. Method to overcome problem mass of Blu-Tack not taken with Blu-Tack, e.g. measure into account mass of Blu-Tack and add to value of M or fix mass with named adhesive, e.g. tape/glue because this has less mass E Difficult to know where centre Detailed method of finding Mark centre of mass of mass is with reason, e.g. centre of mass Measure diameter slot in mass or Difficult to place centre of mass Method to attach mass on strip at end of strip to ensure centre of mass is at the end of strip, e.g. hang mass from strip with thread F Two strips have different Find mass or p of second strip Different thickness/cross- density/p sectional area

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