A student investigates the resistance of a light-emitting diode (LED) when different currents flow through it. The student sets up the circuit shown in Fig. 2.1. [Figure 2.1]

1. (a)(i)On Fig. 2.1, draw the symbol for a voltmeter connected to measure the potential difference Vxy across the 270Ω resistor.[1]
2. (a)(ii)Fig. 2.2 shows the voltmeter reading of the potential difference Vxy when the voltmeter is connected across the 270 Ω resistor. [Figure 2.2] Record Vxy in Table 2.1.[1]
3. (b)(i)The student: • disconnects the voltmeter from points X and Y • reconnects the voltmeter across the LED between points Y and Z • closes the switch • records the voltmeter reading of the potential difference VYZ in the correct row of Table 2.1 • opens the switch. Calculate the value of (Vxy + Vyz) for the 270Ω resistor. Record your answer in Table 2.1.[1]
4. (b)(ii)The current I in the circuit can be calculated using the equation: I = VXY / R where R = 270 Ω. Calculate I. Record your answer in Table 2.1.[1]
5. (b)(iii)The resistance RLED of the LED can be calculated using the equation: RLED = VYZ / I Calculate RLED. Record your answer in Table 2.1.[1]
6. (c)The student repeats the procedure in (a) and (b), replacing the 270Ω resistor, first with a 470Ω resistor and then with a 560 Ω resistor. The student's results are shown in Table 2.1, but the value of VXY for the 560Ω resistor is missing. Calculate VXY and record your answer in Table 2.1 on page 6.[1]
7. (d)(i)As the resistance between terminals X and Y changes, the current in the circuit changes. Examine the results in Table 2.1. Describe how the change in current affects: (Vxy + Vyz)[1]
8. (d)(ii)RLED[1]
9. (e)Another student assembles a circuit using the circuit diagram shown in Fig. 2.1. This student finds that, when the switch is closed, the LED does not light up. The student tests the components and finds that the power source is producing an e.m.f., and that none of the other components are broken. Suggest the error this student has made while assembling the circuit.[1]
10. (f)Name and draw the symbol of a single device that can be used to change the current in the circuit without the need to connect different resistors across the terminals X and Y in the circuit in Fig. 2.1. name of device symbol for device[2]

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