Fig. 8.1 shows a circuit diagram containing a 2000 Ω resistor of constant resistance and a thermistor. [Figure 8.1]

1. (a)(i)At one temperature, the resistance of the thermistor is 1200 Ω. Calculate the potential difference (p.d.) across the 2000 Ω resistor at this temperature.[2]
2. (a)(ii)Explain why the p.d. across the 2000 Ω resistor increases when the temperature increases.[2]
3. (b)The circuit in Fig. 8.1 can be adapted to enable a lamp to switch on and off automatically. Fig. 8.2 shows the circuit adapted using a relay and Fig. 8.3 shows the circuit adapted using a transistor. [Figure 8.2] [Figure 8.3] In both cases, the lamp switches on when the temperature of the thermistor increases. EITHER Describe what happens inside the relay in Fig. 8.2 that causes the lamp to switch on. OR Describe how the circuit and transistor in Fig. 8.3 cause the lamp to switch on.[2]

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