A student investigates how the resistance of a thermistor changes with temperature. The student records the current in the thermistor at various temperatures, keeping the potential difference across it constant.
Table 9.1
| Temperature of thermistor / ∘C^{\circ}\text{C}∘C | 20 | 30 | 40 | 50 | 60 | 70 |
|---|---|---|---|---|---|---|
| Current in thermistor / mA\text{mA}mA | 1.2 | 2.5 | 5.0 | 9.5 | 18.0 | 32.0 |
The potential difference (p.d.) across the thermistor is 8.0 V8.0\text{ V}8.0 V at 40 ∘C40\ ^{\circ}\text{C}40 ∘C. Calculate the resistance of the thermistor at 40 ∘C40\ ^{\circ}\text{C}40 ∘C. Include the unit.
The student plots the results on a graph of current in thermistor against temperature. The resulting line is a curve that bends upwards, starting from (20 ∘C, 1.2 mA)(20\ ^{\circ}\text{C},\ 1.2\text{ mA})(20 ∘C, 1.2 mA).
The student suggests that the current in the thermistor is directly proportional to its temperature.
Explain how the graph shows that this suggestion is incorrect.
452 exam-style questions on CIE IGCSE Physics 4.2 Electrical quantities, covering 4.2.1a Positive and negative charge, 4.2.1b Charging by friction and detecting charge, 4.2.1c Electrical conductors and insulators, 4.2.1d Electric fields and electric field patterns, 4.2.2a Electric current, charge and measuring current, 4.2.2b Conduction in metals and the direction of current, 4.2.2c Direct current and alternating current, 4.2.3a Electromotive force, 4.2.3b Potential difference and voltmeters, 4.2.4a Resistance, 4.2.4b Resistance of a metallic wire, 4.2.4c Current–voltage graphs, and 4.2.5 Electrical energy and electrical power. Each one has a worked solution and a mark scheme showing where the marks go.