A light sensor controlling a warning lamp

8 marks · A-level · original practice case

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The situation

An LDR and a 2.0 kΩ resistor are in series across an ideal 6.0 V supply. Output is measured across the 2.0 kΩ resistor by an ideal voltmeter. A separate controller turns on a lamp when output exceeds 3.0 V. The table gives an idealised LDR model.

Sensor resistance
Condition	LDR resistance / kΩ
Bright	1
Dim	4
Dark	10

Part (a)

Calculate the output voltage in bright conditions. [2 marks]

Give the numerical answer to 2 significant figures.

Final value / V

Working

Part (b)

Calculate the output voltage in dim conditions. [2 marks]

Give the numerical answer to 2 significant figures.

Final value / V

Working

Part (c)

Deduce which listed conditions turn on the lamp. Explain your answer. [2 marks]

Your explanation

Part (d)

Explain how a finite-resistance voltmeter across the fixed resistor changes the bright-condition output. [2 marks]

Your explanation

Marking rubric and worked answers

Indicative practice marks. Method marks assess valid working; accuracy marks require the associated valid method. Independent points stand on their own. Tick only points supported by your response. Do not award the same explanation twice. These ticks record your judgement, not an examiner’s result.

Part (a) marking points

Final answer: 4.0 V. Numerical checks assess the final value only; they do not check working or the number of digits typed.

Use 6.0 × 2.0/(1.0 + 2.0). (1 method mark)
Obtain 4.0 V. (1 accuracy mark)

No earlier numerical result is required. Award method credit for valid working and accuracy credit for the stated answer. Accept a defensible equivalent method; wrong physics cannot earn dependent accuracy credit.

Common errors: This is the p.d. across the LDR, not the fixed resistor.

Part (b) marking points

Final answer: 2.0 V. Numerical checks assess the final value only; they do not check working or the number of digits typed.

Use 6.0 × 2.0/(4.0 + 2.0). (1 method mark)
Obtain 2.0 V. (1 accuracy mark)

Common errors: This is the p.d. across the LDR, not the fixed resistor.

Part (c) marking points

Only bright conditions give output above 3.0 V. (1 independent mark)
Dark output is 1.0 V, so both dim and dark are below threshold. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.

Part (d) marking points

Its parallel path reduces the effective lower resistance. (1 independent mark)
The lower fraction of total series resistance is smaller, so measured output is below 4.0 V. (1 independent mark)

Award each distinct physical point once. Equivalent scientifically correct wording is creditable; a contradictory statement cannot earn the same point.

Common errors: The voltmeter only changes total current: its parallel connection also changes the divider ratio.

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Related skills

Scope and reference sources

AQA 3.5 and selected 3.7.3/3.7.4; OCR A Modules 4/6 (selected). These are shared original exercises; wording, indicative marks and difficulty have not been endorsed by either board.

AQA specification · OCR A specification

Case reference: sensor-divider · version 1.1.0 · seed 0. Choose another case