How Do Thermistors and Light-Dependent Resistors (LDRs) Work?

Thermistors and light-dependent resistors (LDRs) are sensing resistors whose resistance changes with temperature or light intensity. In the common types studied in IB Physics, increasing temperature decreases a thermistor's resistance, while increasing light intensity decreases an LDR's resistance.

This is part of B.5 Current and circuits and applies to both SL and HL.

How Their Resistance Changes

Both components are made from semiconductor materials. Energy supplied by heating or incident light increases the number of mobile charge carriers, allowing charge to flow more easily and reducing resistance.

The thermistor described above has a negative temperature coefficient (NTC). The common misconception is that every thermistor must behave this way. Positive temperature coefficient thermistors exist, but IB questions normally provide or imply the required resistance-temperature characteristic.

Use in a Potential Divider

These sensors are often placed in a potential divider with a fixed resistor. For two series resistors, the output voltage across resistor 2 is

$$V_{\text{out}}=V_{\text{in}}\frac{R_2}{R_1+R_2}.$$

Suppose an LDR is resistor 2. If the light intensity increases, its resistance decreases, so the voltage across it decreases. If the output were instead measured across the fixed resistor, the output voltage would increase. Therefore, always identify which component the voltmeter or output terminals are connected across.

For example, with a $6.0\,\text{V}$ supply, a $2.0\,\text{k}\Omega$ fixed resistor and an LDR of $4.0\,\text{k}\Omega$,

$$V_{\text{LDR}}=6.0\times\frac{4.0}{2.0+4.0}=4.0\,\text{V}.$$

If illumination reduces the LDR resistance to $1.0\,\text{k}\Omega$, the voltage becomes $2.0\,\text{V}$.

Exam Technique

In an IB response, state the input change, deduce the resistance change, then apply the potential-divider equation. Do not claim that the sensor's voltage always decreases: its direction of change depends on where the output is measured.