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1 markTransducers

CAPE Physics Unit 2 · 2019 · Paper 2 · Question 2(c)(iii)

The graph in Figure 4 shows the relationship between the resistance R_L of a particular LDR and the light intensity, I (lux), on its surface. The equation of the line on this graph is log₁₀ R_L = log₁₀ R₀ - b log₁₀ I, where R₀ is a constant.

R_dark is defined as the resistance of the LDR when the light intensity, I ≤ 0.01 lux. Determine the threshold value of R_dark using the equation derived in (c)(ii).

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Other parts of this question

  1. 2(a)Briefly explain what is a light dependent resistor (LDR).[3 marks]
  2. 2(b)Show that the bridge is balanced when R_L = R₁R₃ / R₂.[5 marks]
  3. 2(c)(i)Use the graph to determine numerical values for b and R₀.[7 marks]
  4. 2(c)(ii)Use the equation given in (c) to determine an expression for R_L in terms of R₀ and I.[1 mark]
  5. 2(c)(iv)Using the graph in Figure 4, determine the threshold value for R_dark.[3 marks]
  6. 2(d)Determine the value for I, the light intensity being measured.[4 marks]
  7. 2(e)Explain why it is desirable to make V_B (the supply to the Wheatstone bridge) as small as practicable.[4 marks]
  8. 2(f)State ONE example of a practical application of an LDR and briefly describe its operation.[2 marks]

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