A student investigates how the potential difference V across the terminals of a power supply changes as the current I in a circuit changes. The student assembles the circuit shown in Fig. 3.1.

1. (a)The student closes the switch and observes the reading V₀ on the voltmeter. The reading is shown in Fig. 3.2. Record the voltmeter reading V₀. V₀ = .............................................. V [1][1]
2. (b)The student then: • connects the crocodile clip to the resistance wire at a distance d = 100.0 cm from end X • records the potential difference V across the wire as V = 2.3 V • records the current I in the wire • opens the switch. The reading on the ammeter is shown in Fig. 3.3. Record the ammeter reading I. I = .............................................. A [1][1]
3. (c)The student: • closes the switch • repeats (b) for values of d = 80.0 cm, 60.0 cm, 40.0 cm and 20.0 cm. The student's results are shown in Table 3.1. Complete the header of each column. Include the value of I from (b). Table 3.1 d V I 100.0 2.3 80.0 2.2 0.37 60.0 2.0 0.45 40.0 1.7 0.60 20.0 1.2 0.88 [1][1]
4. (d)On the grid provided in Fig. 3.4, plot a graph of V on the y-axis against I on the x-axis, using the data in Table 3.1. Start both axes from the origin (0, 0). Draw the best-fit straight line. [3][3]
5. (e)(i)The gradient m of your line is numerically equal to the resistance r of the power supply. Calculate the gradient of your line. Show clearly on the graph the values you chose and show your working. m = .............................................. [2][2]
6. (e)(ii)The intercept between the y-axis and your line is the electromotive force V₀ of the power supply. Extend your best-fit line to meet the y-axis and write down the value of V₀. V₀ = .............................................. V [1][1]
7. (f)Compare the value of V₀ in (a) with the value of V₀ you obtained from the graph in (e)(ii). Two quantities can be considered to be equal within the limits of experimental accuracy if their values are within 10% of each other. State whether your results show that the two values obtained for the electromotive force are the same. Support your statement with a calculation. calculation ................................................................................................................................................................................................................................................................................ statement ................................................................................................................................................................................................................................................................................ [2][2]
8. (g)Another student decides to connect the crocodile clip in Fig. 3.1 so that there is only a very small length of the resistance wire included in the circuit. The teacher tells the student not to do this. Suggest one practical reason why values of d less than 20.0 cm should not be used. [1][1]
9. (h)It is possible to change the potential difference V by using a variable resistor rather than using different lengths of resistance wire. Complete the circuit diagram in Fig. 3.5 to show how the variable resistor is connected. [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.