Semiconductor behaviour explains why thermistors and LDRs change resistance. Increasing temperature or light increases charge‑carrier availability, lowering resistance and allowing greater current flow.
Non‑linear device response means the resistance does not follow Ohm’s law. This requires measurement using rather than assuming constant resistance.
Energy‑dependent conductivity arises because environmental input (heat or photons) changes how easily electrons move in the material, which directly alters measured resistance.
Dynamic equilibrium ensures the component settles into a new resistance value after changes; thus, measurements must be delayed briefly to allow stabilisation.
Constructing a measurement circuit requires placing an ammeter in series with the thermistor or LDR to measure current and a voltmeter in parallel to measure potential difference across the component.
Using controlled environmental changes means adjusting temperature with a heater for thermistors or adjusting light intensity for LDRs, always changing conditions gradually to avoid sudden measurement spikes.
Calculating resistance involves recording pairs of voltage and current values at each environmental setting and computing for each pair to generate a resistance–condition profile.
Ensuring safe current levels prevents heating effects unrelated to the environmental change, as large currents can artificially lower resistance by heating semiconductor materials.
Always identify the independent variable (temperature or light intensity) before explaining the method, because exam questions often test your ability to structure an investigation around correct variable control.
Redraw circuit diagrams neatly because examiners deduct marks for incorrect or ambiguous circuit symbols—particularly for thermistors and LDRs, whose symbols differ by the direction and meaning of arrows.
Write complete method steps including waiting for stabilisation, controlling voltage, and avoiding heating, as examiners expect reference to controlling unintended resistance changes.
Use correct relationships such as quoting exactly and explaining why resistance reading changes in the observed direction when interpreting experimental data.
Assuming thermistors and LDRs obey Ohm’s law leads to incorrect interpretation, since their I‑V graphs are not straight lines and resistance is not constant.
Forgetting stabilisation time can give inconsistent results because thermistors and LDRs do not respond instantaneously to environmental changes.
Using excessive voltage can heat the device, causing resistance changes unrelated to the intended variable, especially in thermistors which are highly temperature‑sensitive.
Misidentifying component symbols can invalidate entire circuit diagrams, particularly confusing LDRs with LEDs or thermistors with fixed resistors.
Automatic control systems often depend on thermistors and LDRs, using their variable resistance in feedback loops within thermostats, alarm systems, or automatic lighting.
Non‑ohmic behaviour connects to broader concepts in semiconductor physics such as photoconductivity and temperature dependent conduction.
I‑V characteristic analysis extends naturally from these investigations, enabling students to interpret non‑linear electrical graphs and link them to underlying physical processes.
Sensor circuit design uses these principles to construct potential divider circuits where changing resistance produces measurable voltage outputs for real‑world systems.
