A-LevelPhysicsD.C. circuits9702/31 · May/June 2017 · Q120 Marks

In this experiment, you will investigate an electrical circuit.

(a)Set up the circuit shown in [Figure 1.1].
(b)(i)Measure and record x. x = .....................................................[1]
(b)(ii)Close the switch.
(b)(iii)Record the ammeter reading I₁. I₁ = ..................................................... [1][1]
(b)(iv)Open the switch.
(c)(i)Connect an additional lead L to the circuit as shown in [Figure 1.2].
(c)(ii)Close the switch.
(c)(iii)Record the ammeter reading I₂. I₂ = ..................................................... [1][1]
(c)(iv)Open the switch.
(c)(v)Remove L. The circuit is now as shown in [Figure 1.1].
(d)Increase x and repeat (b) and (c) until you have six sets of readings of x, I₁ and I₂. Record your values in a table. Include values of I₂/I₁ in your table. [10][10]
(e)(i)Plot a graph of I₂/I₁ on the y-axis against x on the x-axis. [3][3]
(e)(ii)Draw the straight line of best fit. [1][1]
(e)(iii)Determine the gradient and y-intercept of this line. gradient = ............................................ y-intercept = ............................................ [2][2]
(f)It is suggested that the quantities I₁, I₂ and x are related by the equation I₂/I₁ = Px + Q where P and Q are constants. Using your answers in (e)(iii), determine values for P and Q. Give appropriate units. P = ..................................................... Q = ..................................................... [2][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 (May/June 2017). The mark or mark type is shown in brackets.

[1] Value(s) of I₁ with unit in the range 30–200 mA.
[1] I₂ > I₁ (ECF unit from 1(b)(iii)).
[5] Six sets of readings of x (different values), I₁, I₂ with correct trend and without help from Supervisor scores 5 marks, five sets scores 4 marks etc.
[1] Range: Xmax ≥ 90 cm.
[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. x/ m, I₂/ mA, I₂/ I₁ no unit.
[1] Consistency: All values of x must be given to the nearest mm.
[1] Significant figures: All values of I₂/ I₁ must be given to the same number of s.f. as (or one more than) the s.f. in raw I₁ and I₂ (using the s.f. of the I value with the lowest number of s.f.).
[1] Calculation: Values of I₂/ I₁ are correct.
[1] Quality: All values of I₂/ I₁ must be greater than 1. All points in the table must be plotted for this mark to be awarded. It must be possible to draw a straight line that is within ± 0.05 on the I₂/ I₁ axis (normally y axis) of all plotted points.
[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 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.
[1] Plotting of points: All observations must be plotted on the grid. Diameter of plotted points must be < half a small square (no "blobs”). Points must be plotted to an accuracy of half a small square.
[1] Line of best fit: Judge by balance of all points on the grid about the candidate's line (at least 5). 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. Lines must not be kinked or thicker than half a small square.
[1] Gradient: Gradient sign on answer line matches graph drawn. The hypotenuse of the triangle used must be greater than half the length of the drawn line. 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.
[1] y-intercept: Correct read-off from a point on the line substituted correctly into y = mx + c or an equivalent expression. 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 x = 0, accurate to half a small square in y direction.
[1] Value of P = candidate's gradient and value of Q = candidate's intercept. The values must not be fractions.
[1] Unit for P is correct (e.g. m⁻¹) and Q stated without a unit.