In this experiment you will investigate how the resistance of a light-emitting diode (LED) changes with different currents. You are provided with: • a power source • a switch • a voltmeter with two leads that may be connected between different points in the circuit shown in Fig. 2.1 • a light-emitting diode (LED) • a 270Ω resistor, a 470 Ω resistor and a 560 Ω resistor • sufficient connecting leads to make the circuit shown in Fig. 2.1. The supervisor has set up the circuit shown in Fig. 2.1. The 270Ω resistor is connected in the circuit and the 470Ω resistor and the 560Ω resistor are placed by the side of the circuit. [Figure 2.1]

1. (a)Connect the voltmeter across the 270 Ω resistor between points X and Y. Ensure that the positive terminal of the voltmeter is connected to X. Close the switch. Record the voltmeter reading VXY in the top row of Table 2.1. Open the switch. Table 2.1 resistance between X and Υ/Ω | VXY/V | VYZ/V | (VXY + VYZ)/V | I/A | RLED/Ω 270 | | | | | 470 | | | | | 560 | | | | | [1][1]
2. (b)Disconnect the voltmeter from points X and Y. Reconnect the voltmeter across the LED between points Y and Z. Ensure that the positive terminal of the voltmeter is connected to Y. Close the switch. Record the voltmeter reading VYZ in the correct row of Table 2.1. Open the switch. [1][1]
3. (c)Remove the 270Ω resistor from the circuit and replace it with the 470 Ω resistor. Repeat the procedures in (a) and (b) for the 470 Ω resistor. Remove the 470 Ω resistor from the circuit and replace it with the 560 Ω resistor. Repeat the procedures in (a) and (b) for the 560 Ω resistor. [1][1]
4. (d)For each value of resistance between X and Y, calculate the value of (VXY + VYZ). Record your answers in Table 2.1. [1][1]
5. (e)The current I in the circuit in part (a) can be calculated using the equation: I = VXY/R where R is the resistance between X and Y. Calculate I for R = 270Ω, 470Ω and 560 Ω. Record your answers in Table 2.1. [1][1]
6. (f)The resistance RLED of the LED can be calculated using the equation: RLED = VYZ/I Calculate RLED for each value of resistance between X and Y. Record your answers in Table 2.1. [2][2]
7. (g)(i)As the resistance between terminals X and Y changes, the current in the circuit changes. Examine your results in Table 2.1. Describe how the change in current affects: (VXY + VYZ) [1][1]
8. (g)(ii)RLED [1][1]
9. (h)A student assembles a circuit using the circuit diagram shown in Fig. 2.1. The 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 the student has made while assembling the circuit. [1][1]

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