A student sets up a circuit to investigate how the resistance of a light dependent resistor (LDR) changes. The circuit uses a constant voltage supply and an ammeter connected in series with the LDR. The student gradually moves a lamp further away from the LDR, taking current readings at each distance. Explain what happens to the resistance of the LDR as the lamp is moved further away, and explain why the LDR behaves in this way.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student sets up a circuit using a constant voltage supply, an ammeter in series, and an LDR. The student moves a lamp progressively further from the LDR and records the current at each position.
Model answer (5 marks)
As the lamp is moved away, the light intensity incident on the LDR falls.
1. Light intensity ↓ → resistance ↑.
2. Because the supply voltage is constant, Ohm’s law (I=V/R) gives I ↓ as R ↑.
3. In complete darkness the LDR has its maximum resistance.
4. This inverse light‑resistance relationship allows LDRs to act as light‑sensing elements in automatic circuits, e.g. night‑lights.
1. Light intensity ↓ → resistance ↑.
2. Because the supply voltage is constant, Ohm’s law (I=V/R) gives I ↓ as R ↑.
3. In complete darkness the LDR has its maximum resistance.
4. This inverse light‑resistance relationship allows LDRs to act as light‑sensing elements in automatic circuits, e.g. night‑lights.
Examiner tips
- Use the inverse relationship between light intensity and resistance; mention Ohm’s law for current change; state the maximum resistance in darkness; link to practical application.
Common mistakes
- Confusing resistance increase with current increase; forgetting that voltage is constant; not mentioning the maximum resistance in darkness; omitting the practical use of LDRs.
Mark scheme (5 marks)
- As the lamp moves further away, the light intensity reaching the LDR decreases.
- The resistance of the LDR increases as light intensity decreases.
- Because the voltage is constant, the current in the circuit decreases as resistance increases.
- The resistance of an LDR is greatest in darkness / when light intensity is at its lowest.
- This property makes LDRs useful in automatic circuits that respond to light levels, such as automatic night lights.
Key terms in this question
light dependent resistor (LDR) · resistance · current
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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- Decode the mark scheme abbreviations →
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