A-LevelPhysicsD.C. circuits9702/33 · Feb/Mar 2018 · Q120 Marks

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. 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. Open the switch. L =[1]
(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. Include values of 1/n and 1/L in your table.[10]
(d)(i)Plot a graph of 1/L on the y-axis against 1/n on the x-axis.[3]
(d)(ii)Draw the straight line of best fit.[1]
(d)(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/L = a/n + b 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]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

[1] 1(a) Value of Lo in the range 59 cm to 61 cm, with unit.
[1] 1(b) Value of L less than Lo
[5] 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.
[1] 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] 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] 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)