In an experiment to investigate the variation of the resistance R of a thermistor with its temperature θ, a student measured its resistance at different temperatures - Leaving Cert Physics - Question 4 - 2010

The resistance of the thermistor was measured by connecting it in series with an ohmmeter (or digital multimeter). The thermistor, denoted as Th3, was connected to the meter where the ohm reading was then indicated. This setup implies that as the temperature changes, the resistance values change as well, which can be recorded for analysis.

The temperature was varied by using a heat source such as a hotplate or a Bunsen burner. The thermistor was placed in a setup where it could be heated, and the temperature monitored with a thermometer. As the temperature increased, readings of the resistance of the thermistor were taken at regular intervals.

1. Label Axes: On the graph, the x-axis should represent temperature (°C) and the y-axis should represent resistance (Ω).
2. Plot Points: Use the recorded data points to accurately plot six points corresponding to each temperature and resistance value.
3. Draw Smooth Curve: Connect the plotted points with a smooth curve to denote the trend in resistance as temperature changes. Ensure the curve reflects normal resistive behavior typical of a thermistor.
4. Good Distribution: Ensure that the points are well distributed across the graph for clarity and accuracy.

To estimate the average variation of resistance per Kelvin in the specified temperature range, first determine the resistance values at 45°C and 55°C from the graph. For example:

• At 45°C, the resistance may be around 200 Ω.
• At 55°C, assume the resistance is approximately 90 Ω.

Calculate the change in resistance: 200 Ω - 90 Ω = 110 Ω.

The temperature change in Kelvin is 55°C - 45°C = 10 K.

Now, the average variation in resistance per Kelvin can be calculated as:

$\frac{110 Ω}{10 K} = 11 Ω/K$

Thus, the average variation of resistance is roughly 11 Ω per Kelvin.

The thermistor is usually immersed in oil rather than water for several reasons:

1. Better Conductor of Heat: Oil is generally a better conductor of heat compared to water, allowing for quicker and more accurate temperature readings.
2. Higher Temperature Range: Oil can withstand higher temperatures without boiling compared to water, which can allow for a broader range of experimentation.
3. Less Impurity: Oil is less likely to contain impurities that can affect electrical measurements, reducing the risk of erroneous readings.
4. Good Thermal Contact: Immersing the thermistor in oil ensures good thermal contact, providing a more consistent and reliable measurement of the thermistor's resistance relative to temperature.