In this experiment you will investigate how the potential difference V across the terminals of a power supply changes as the current I in a circuit changes. You are provided with: • a length of resistance wire attached to a metre rule • a power supply • a switch • an ammeter • a voltmeter • a flying lead, which is a crocodile clip attached to a length of connecting lead. The supervisor has assembled the circuit as shown in Fig. 3.1.

1. (a)• Close the switch. Do not connect the crocodile clip to the resistance wire. Record the reading V₀ on the voltmeter. V₀ = .............................................. V [1][1]
2. (b)• Connect the crocodile clip to the resistance wire at a distance d = 100.0 cm from end X. Record the potential difference V across the wire, and the current I in the wire. • Open the switch and wait 30 s. V = .............................................. V I = .............................................. A [1][1]
3. (c)Repeat (b) for values of d = 85.0 cm, 70.0 cm, 55.0 cm and 40.0 cm. Record all your readings in Table 3.1. Include the readings from (b). Add appropriate units to the header of each column. [2] Table 3.1 d V I[2]
4. (d)On the grid provided in Fig. 3.2, plot a graph of V on the y-axis against I on the x-axis using your data in Table 3.1. Start both axes from the origin (0, 0). Draw the best-fit straight line. [4][4]
5. (e)(i)The magnitude of the gradient m of your line is equal to the resistance r of the power supply. Calculate the gradient of your line. Show clearly on the graph the values you choose 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 intercept the y-axis and record the value of V₀. V₀ = .............................................. V [1][1]
7. (f)Compare your measurement of V₀ in (a) with the value of V₀ 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]

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