In this experiment, you will investigate combinations of resistors in an electrical circuit.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

1. [1] Value of I with unit and in the range 20–120 mA.
2. [5] Six (or more) I values with different resistance pairings of R1, R2 without help from the Supervisor scores 5 marks, five sets scores 4 marks, etc.
3. [1] Range: 33Ω and 47Ω resistors used as a pair and 68Ω and 82Ω resistors used as a pair.
4. [1] Column headings: Each column heading must contain a quantity and a unit. The presentation of quantity and unit must conform to accepted scientific convention, e.g. 1/I /A-1, R1R2 / (R1 + R2) / Ω.
5. [1] Consistency: All values of I must be given to the nearest 0.1 mA or all to the nearest 0.01 mA.
6. [1] Significant figures: All values of R1R2 / (R1 + R2) must be given to 2 s.f. or 3 s.f.
7. [1] Calculation: Values of 1/I and R1R2 / (R1 + R2) are correct.
8. [1] Axes: Sensible scales must be used, no awkward scales (e.g. 3:10 or fractions). Scales must be chosen so that the plotted points occupy at least half the graph grid in both the x and y directions. Axes must be labelled with the quantity that is being plotted. Scale markings should be no more than three large squares apart.
9. [1] Plotting of points: All observations in the table must be plotted on the grid. Diameter of plotted points must be < half a small square. Points must be plotted to an accuracy of half a small square in both x and y directions.
10. [1] Quality: All points in the table (at least 5) must be plotted on the grid. Trend of points on graph must be correct. It must be possible to draw a straight line that is within ± 1Ω on the R1R2 / (R1 + R2) axis of all plotted points.
11. [1] Line of best fit: Judge by the 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 (i.e. circled or labelled) by the candidate. There must be at least five points left after the anomalous point is disregarded. Line must not be kinked or thicker than half a small square.
12. [1] Gradient: The hypotenuse of the triangle used must be greater than 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. Method of calculation must be correct, e.g. Δy/Δx. Gradient sign on answer line matches graph drawn.
13. [1] y-intercept: Correct read-off from a point on the line and substituted into y = mx + c or an equivalent expression. Read-off must be accurate to half a small square in both x and y directions. or Intercept read directly from the graph at R1R2 / (R1 + R2) = 0, accurate to half a small square.
14. [1] Value of P = candidate's gradient and value of Q = candidate's y-intercept. Values must not be written as fractions.
15. [1] Unit for P is correct e.g. Ω-1A−¹ or Ω−1mA-1 or V-1 and unit for Q is correct e.g. A-1 or mA-1.
16. [1] E and Z correctly calculated from P and Q using: E = 1 / P and Z = EQ or Z = Q / P and units for E (V) and Z (Ω) correct.