Resistors: The resistance of an LDR decreases as light intensity increases.

The idea

An LDR, or light-dependent resistor, is a resistor whose resistance depends on light intensity. As light intensity increases, more charge carriers are released in the semiconductor material, so the resistance of the LDR decreases. In darkness its resistance is high, often very high; in bright light its resistance is much lower. This makes an LDR useful as a light sensor in circuits such as automatic street lights, burglar alarms and light meters. In a potential divider, the changing resistance of the LDR changes the potential difference across it, which can switch a device on or off. You should be able to state the relationship and explain how it is used, not just recall the name.

What earns the marks

• States that the resistance of an LDR decreases as light intensity increases.
• Explains that the LDR is a light-sensitive resistor used as a sensor in a circuit.
• Describes how changing light intensity changes the resistance of the LDR and therefore the current or potential difference in the circuit.
• Links the change in potential difference across the LDR to the operation of a device such as a lamp, alarm or switch.
• Applies the relationship to a context, for example an automatic street light switching on when it gets dark.
• Uses correct terminology, including resistance, light intensity, potential difference and series circuit.

Common mistakes and how to put them right

• Saying resistance increases with light intensity.

  Put it right by stating that resistance decreases as light intensity increases.

• Confusing an LDR with a thermistor.

  Put it right by noting that an LDR responds to light intensity while a thermistor responds to temperature.

• Thinking the LDR produces electricity.

  Put it right by explaining that it changes resistance and so controls current in a circuit that already has a power supply.

Exam tips

• State the direction of change clearly: as light intensity increases, resistance decreases.
• When explaining a circuit, describe the effect on potential difference or current before saying what the device does.
• Use a familiar example such as a street light or alarm to show understanding of the application.