A student is investigating how the resistance of a light dependent resistor (LDR) changes. The student places the LDR in a bright room and then moves it into a dark cupboard. Explain what happens to the resistance of the LDR and how this affects the current in the circuit when the LDR is moved into the dark cupboard, compared to when it was in the bright room.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An LDR is connected in series with a battery and an ammeter. The LDR is first placed in a brightly lit room, then moved into a dark cupboard.
Model answer (5 marks)
The resistance of the LDR rises when it is moved into the dark cupboard because the light intensity on it is lower.
The higher resistance means the LDR offers more opposition to the flow of charge.
With the battery voltage unchanged, the increased resistance reduces the current according to V = IR.
Thus the current measured by the ammeter falls when the LDR is placed in the dark compared with the bright room.
The higher resistance means the LDR offers more opposition to the flow of charge.
With the battery voltage unchanged, the increased resistance reduces the current according to V = IR.
Thus the current measured by the ammeter falls when the LDR is placed in the dark compared with the bright room.
Examiner tips
- Use the term "resistance rises" and link to lower light intensity. Show the V = IR relationship verbally. Explain that the battery voltage is constant. Mention the ammeter reading decreases.
Common mistakes
- Saying the resistance decreases in the dark. Forgetting to state that the battery voltage remains constant. Using vague phrases like "current changes" without explaining why.
Mark scheme (5 marks)
- The resistance of the LDR increases when moved into the dark cupboard
- Because the LDR has less light falling on it / lower light intensity
- The greater resistance makes it harder for current to flow / opposes the flow of charge
- The current in the circuit decreases
- Correct linked reasoning — as resistance increases, current decreases because voltage (potential difference) of the battery stays the same / using V = IR logic verbally
Key terms in this question
light dependent resistor (LDR) · resistance · current
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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