Resistors: The application of LDRs in circuits eg switching lights on when it gets dark is required.

The idea

An LDR (light-dependent resistor) is a resistor whose resistance changes with light intensity: in bright light its resistance falls, and in darkness its resistance rises. This makes it useful as a light sensor. In a potential divider, the LDR is connected in series with a fixed resistor across a supply. The output voltage is taken across one of them. As light intensity changes, the LDR's share of the supply voltage changes. For example, with the output across the LDR, the output voltage rises as it gets darker, because the LDR's resistance increases and takes a larger share of the supply voltage. This changing voltage can drive a switching circuit, such as a transistor, to turn a lamp on when it gets dark.

What earns the marks

• State that the resistance of an LDR decreases as light intensity increases and increases as light intensity decreases.
• Describe an LDR as one resistor in a potential divider, with the output voltage taken across either the LDR or a fixed resistor.
• Explain that changing light intensity changes the LDR's resistance, which changes the potential difference across each component in the divider.
• Apply the idea to a circuit that switches a lamp on when it gets dark, for example by using the changing output voltage across the LDR to operate a switching component.
• Use the potential divider relationship qualitatively: a larger resistance takes a larger share of the supply potential difference.

Common mistakes and how to put them right

• Thinking an LDR produces electricity from light.

  Put it right: It is a resistor whose resistance changes with light intensity; it needs a supply in a circuit.

• Saying resistance increases in bright light.

  Put it right: Resistance decreases as light intensity increases.

• Assuming the voltage across a fixed resistor rises as it gets darker.

  Put it right: As it gets darker, the LDR's resistance increases, so the voltage across the LDR rises and the voltage across the fixed resistor falls.

Exam tips

• Sketch the potential divider and label which component the output voltage is taken across before explaining the change.
• Use the phrase 'share of the supply potential difference' to link resistance change to output voltage change.
• When describing the dark-switching circuit, state clearly that the changing output voltage controls a switching component such as a transistor, which then operates the lamp.