1.1. A student investigates a temperature-sensing circuit used to control a heater.

The circuit consists of a $6.0\text{ V}$ d.c. power supply, a fixed resistor of resistance $15 \Omega$ and a negative temperature coefficient (NTC) thermistor, as shown in Fig. 17.1.

Fig. 17.1

The variation of the resistance of the thermistor with temperature is shown in the graph in Fig. 17.2.

Fig. 17.2

Explain, in terms of charge carriers, why the resistance of the thermistor changes with temperature.

1.2. A voltmeter is connected across the fixed resistor.

Calculate the reading on the voltmeter when the temperature of the thermistor is $40^{\circ}\text{C}$.

1.3. The circuit is designed to operate a heater switch. The heater switches on when the potential difference across the thermistor is greater than $0.70\text{ V}$.

Deduce whether this circuit will switch on the heater when the temperature falls below $10^{\circ}\text{C}$.

Your answer should include data from Fig. 17.2 and suitable calculations.