A greenhouse controller uses a thermistor and a fixed resistor in a potential-divider circuit. Fig. 1 shows the circuit, with the output voltage measured ac...
Question 1 Report
A greenhouse controller uses a thermistor and a fixed resistor in a potential-divider circuit. Fig. 1 shows the circuit, with the output voltage measured across the fixed resistor. Table 1 shows resistance values recorded by a student after the thermistor had reached the temperature of the air in a sealed test box. The controller is designed to operate a cooling fan when the temperature becomes high.
| temperature / degrees C | thermistor resistance / ohms |
|---|---|
| 10 | 8200 |
| 20 | 4700 |
| 30 | 2800 |
| 40 | 1700 |
(a) State the independent variable in this investigation. [1]
(b) Calculate how many times greater the thermistor resistance is at 10 degrees C than at 40 degrees C. [2]
(c) Explain why the output voltage across the fixed resistor increases as the air temperature increases. [3]
Answer Details
(a) The independent variable is the temperature of the air (or thermistor) [1]. This is the quantity deliberately changed, while the thermistor resistance is measured.
(b) Compare the two resistances by division:
\[\frac{8200\ \Omega}{1700\ \Omega}=4.82\]
The resistance at \(10\ ^\circ\mathrm{C}\) is 4.8 times greater (accept \(4.82\)) [2].
(c) This is an NTC thermistor, so as air temperature rises, its resistance decreases [1]. The total resistance of the series circuit therefore decreases, so the current increases [1]. The fixed resistor has constant resistance, so \(V=IR\) means that the potential difference across it, which is the output voltage, increases [1].
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