Resistance, LDRs & Thermistors

Explained

Resistance, and two components that change it deliberately

Resistance is potential difference divided by current. Most components are designed to keep it steady, but two are built to do the opposite. A light dependent resistor changes its resistance with light, and a thermistor changes its resistance with temperature. That is what makes both of them useful as sensors.

What decides the resistance of a wire

A longer wire has a greater resistance, because the charge has further to travel and more opposition to meet on the way. A thicker wire has a lower resistance, because a larger cross sectional area gives the charge more room to flow through.

Length and thickness pull in opposite directions, and questions often change both at once to see whether you can handle them separately.

The light dependent resistor

In bright light an LDR has a low resistance. In darkness its resistance is high.

The direction is worth fixing firmly, because it is easy to state backwards under pressure. Bright light, low resistance. That means more current can flow, which is how an LDR is used to switch on a street lamp when it gets dark: as the light fades, resistance rises, and the circuit responds.

The thermistor

In the thermistors used at this level, resistance falls as temperature rises. Hot means low resistance, cold means high resistance.

This lets a thermistor act as a temperature sensor in a thermostat or a fire alarm. As the temperature climbs, resistance drops, current rises, and the circuit can be designed to react at a chosen point.

Both are the opposite of what you might guess

More light gives less resistance. More heat gives less resistance. Both components respond by making it easier for current to flow, not harder, and remembering that they work the same way round makes them much harder to confuse.

Drawing the graphs

For an LDR, plot resistance against light intensity and the curve falls steeply then levels out. For a thermistor, plot resistance against temperature and the curve does the same thing.

Neither is a straight line, so neither obeys a simple proportional relationship, and a question asking you to describe the graph wants that shape described: resistance decreases, and decreases less sharply as the quantity rises.