A lamp, an LDR, a rule. Keep the light steady and the room dark.
Light sources and light sensors come up again and again in Question 1 of Paper 5. Here a lamp shines on a light-dependent resistor (LDR) and you measure the resistance R of the LDR as its distance x from the lamp changes. Watch the set-up built step by step, with the variables and the graph beside it, then build it yourself and check it against the mark points.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Physics 9702 Paper 5, Question 1. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9702/51 October/November 2017, Question 1 (resistance of an LDR under different depths of water) · official past papers
Practise next: 9702/51 May/June 2016, Question 1 (minimum p.d. across LEDs of different wavelengths) · official past papers
Practise next: 9702/52 May/June 2018, Question 1 (light of different wavelengths reflected from a CD) · official past papers
Examiner reports used: 9702 June 2016, June 2018, November 2014 and November 2017.
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What a full-mark diagram shows
The lamp circuit
A supply, a rheostat and an ammeter in series with the lamp, and a voltmeter across it, so the current (and so the brightness) can be kept constant.
The LDR circuit
A separate circuit: an ohmmeter straight across the LDR, with no cell. Or a voltmeter across the LDR and an ammeter in series, then R = V / I.
Support and distance
The lamp on a stand and the LDR in a holder at the same height, with a metre rule beside the distance x and x clearly marked.
No stray light
A darkened room, or a black tube around the LDR, so only light from the lamp reaches it.
Habits the examiner reports keep asking for
Say what is kept constant, and how: “keep the current in the lamp constant by adjusting the rheostat”, never “control the lamp”.
Never put a cell in the ohmmeter circuit. The ohmmeter has its own battery. Keep the LDR circuit apart from the lamp circuit.
Make x accurate: measure from the filament to the face of the LDR and use set squares or fiducial marks, not just “use a ruler”.
Safety must fit the experiment: a hot lamp (let it cool, heat-proof gloves) or a bright filament (do not look straight at it). “Wear goggles” alone scores nothing.
For a power law, plot lg R against lg x. Valid if the graph is a straight line; n = gradient and c = 10y-intercept, with c as the subject.