Cambridge A Level Physics 9702 — 2018 Feb/March Paper 3 · Variant 3

1 In this experiment you will investigate an electrical circuit. You are provided with several groups of resistors connected in parallel. (a) • Select a group of resistors and assemble the circuit shown in Fig. 1.1. 3 V d.c. L0 L A B C resistance wire wooden strip V R resistors component in parallel holder Fig. 1.1 A, B and C are crocodile clips. • Measure and record the length L0 of the resistance wire between the ends of A and B, as shown in Fig. 1.1. L0 = ..................................................[1] (b) • Record the number n of resistors in parallel connected in the component holder. n = ...................................................... • Close the switch. • Move C along the wire until the voltmeter reading is zero. • Measure and record the distance L between A and C when the voltmeter reading is zero, as shown in Fig. 1.1. L = ..................................................[1] • Open the switch. (c) Select a different group of resistors and repeat (b) until you have at least six sets of values of n and L. Record your results in a table. 1 1 Include values of and in your table. n L [10] 1 1(d) (i) Plot a graph of on the y-axis against on the x-axis. [3] L n (ii) Draw the straight line of best fit. [1] (iii) Determine the gradient and the y-intercept of this line. gradient = ...................................................... y-intercept = ...................................................... [2] (e) It is suggested that the quantities L and n are related by the equation 1 a = + b L n where a and b are constants. Use your answers in (d)(iii) to determine the values of a and b. Give appropriate units. a = ...................................................... b = ...................................................... [2] [Total: 20] You may not need to use all of the materials provided.

Mark scheme: 1(a) Value of L0 in the range 59 cm to 61 cm, with unit. 1 1(b) Value of L less than L0 1 1(c) Six sets of readings of n and L (with the correct trend and without help from supervisor) scores 5 marks, five sets scores 4 marks etc. 5 Range of values: Values of n must include at least 2 and 8. 1 Column headings: 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. 1 / L / cm–1. 1 Consistency: Values of raw L must all be given to the nearest mm. 1 Significant figures: Significant figures for every value of 1 / L same as, or one greater than, the s.f. of L as recorded in table. 1 Calculation: Correct calculation of 1 / L with no rounding error. 1 Question Answer Marks 1(d)(i) Axes: 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 which is being plotted. Scale markings should be no more than 3 large squares apart. 1 Plotting of points: All observations must be plotted on the grid. Diameter of plotted points must be ⩽ half a small square (no blobs). Plots must be accurate to within half a small square in both x and y directions. 1 Quality: All points in the table must be plotted (at least 5) for this mark to be awarded. Scatter of points must be no more than ±0.005 cm–1 (±0.5 m–1) from a straight line in the 1 / L direction. 1 1(d)(ii) Line of best fit: Judged by balance of all points on the grid (at least 5) about the candidate’s line. 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. Lines must not be kinked or thicker than half a small square. 1 1(d)(iii) Gradient: The hypotenuse of the triangle used must be greater than half the length of the drawn line. Method of calculation must be correct. Both read-offs must be accurate to half a small square in both the x and y directions. 1 y-intercept: Either Correct read-off from a point on the line substituted into y = mx + c or an equivalent expression, with 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 1 / n = zero accurate to half a small square in y direction. 1 Question Answer Marks 1(e) Value of a equal to candidate’s gradient. Value of b equal to candidate’s intercept. 1 Unit for a and unit for b are dimensionally correct (e.g. a = 16.9 m–1 and b = 1.7 m–1) 1

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2 In this experiment, you will investigate the size of a drop of liquid between two glass or perspex plates. (a) You are provided with two pieces of wire, each labelled A, and two pieces of wire, each labelled B. Measure and record the diameter dA of one the pieces of wire labelled A, and the diameter dB of one of the pieces of wire labelled B. dA = ...................................................... dB = ...................................................... [2] (b) • Place the larger glass or Perspex plate on top of the piece of graph paper, as shown in Fig. 2.1. graph paper (under the glass or Perspex) larger glass or Perspex plate drop of water Fig. 2.1 • Fill the liquid dropper with water from the beaker. Use the dropper to put a single drop of water on the plate, as shown in Fig. 2.1. • Place the four pieces of wire on the larger plate in the approximate positions shown in Fig. 2.2. The wires must not be touching each other. B B pieces of pieces of wire wire A A Fig. 2.2 • Place the smaller glass or Perspex plate on top of the wires as shown in Fig. 2.3. smaller B B glass or Perspex plate A A Fig. 2.3 • Gently press the smaller plate down onto the wires labelled B. Use the 2 mm graph grid to determine the average width WB of your water drop. WB = ..................................................[1] (c) Estimate the percentage uncertainty in your value of WB. percentage uncertainty = ..................................................[1] (d) • Holding the smaller plate in position, carefully pull out the two wires labelled B (leaving the wires labelled A) as shown in Fig. 2.4. A A Fig. 2.4 • Gently press the smaller plate down onto the wires labelled A. Use the 2 mm graph grid to determine the average width WA of your water drop. WA = ..................................................[2]

Mark scheme: 2(a) Values for dA and dB, with unit, both <1 mm 1 Raw values for dA and dB recorded to nearest 0.01 mm. 1 2(b) Value for WB, with unit, >2 mm 1 2(c) Absolute uncertainty in WB value of 0.2 cm to 0.5 cm and correct method of calculation to obtain percentage uncertainty. If several readings have been taken, then the absolute uncertainty can be half the range (but not zero if values are equal). 1 2(d) Value for WA. 1 Evidence of several raw measurements used to find the average, either here or in 2(b). 1 2(e) Value for second WB. 1 Quality: Second WB greater than first WB. 1 Value for second WA. 1 2(f)(i) Two values of k calculated correctly. 1 2(f)(ii) Justification based on s.f. in dA, dB, WB and WA. 1 2(f)(iii) Sensible comment relating to the calculated values of k, testing against a criterion specified by the candidate. 1 Question Answer Marks 2(g)(i) Two k values are not enough to draw a valid conclusion 4 max Difficult to: place glass plate / remove (B) wires / press down, without moving A wires (Water drop) irregular shape so difficult to find average width Parallax when measuring water drop Graph grid is 2 mm (or just ‘grid is large’) / large % uncertainty in w Outline of drop not clear because colourless / drop is colourless so difficult to see edge / drop is colourless so difficult to see width Force not constant 2(g)(ii) Take more readings and plot a graph / calculate more k values and compare 4 max Tape / clamp / weight / stick the wires Measure with ruler / calipers Grid marked on glass / place drop on waterproof graph paper Use smaller (or 1 mm) grid / use large drops to reduce uncertainty Use coloured water Use masses / small load / other defined workable method of producing consistent force

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