A student is investigating the resistance of a light-dependent resistor (LDR) separated from a source of light by different depths of water as shown in Fig. 1.1. light LDR water [Figure 1.1] It is suggested that the relationship between the resistance R of the LDR and the depth d of the LDR in the water is R = 4πd² K where K is a constant. Design a laboratory experiment to test the relationship between R and d. Explain how your results could be used to determine a value for K. You should draw a diagram, on page 3, showing the arrangement of your equipment. In your account you should pay particular attention to • the procedure to be followed, • the measurements to be taken, • the control of variables, • the analysis of the data, • any safety precautions to be taken.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

1. [1] d is the independent variable and R is the dependent variable or vary d and measure R
2. [1] keep intensity/power of light source constant
3. [1] labelled diagram showing a light source fixed above container of water with the labelled LDR positioned in the beaker
4. [1] correct circuit diagram to measure R, e.g. V and I methods or ohmmeter
5. [1] method to determine R, e.g. R = p.d. across LDR / current or read off ohmmeter
6. [1] method to determine d, e.g. use a ruler or drawn labelled vertical ruler adjacent to container with d indicated
7. [1] plots a graph of R against d²
8. [1] relationship valid if a straight line produced passing through the origin
9. [1] K = 4π / gradient
10. [1] dark glasses to prevent damage to eyes due to light source or do not look directly at light source or do not touch hot lamp/use gloves to position hot lamp/heat-proof gloves to position lamp
11. [1] dark room or shielding LDR (so as to avoid light from other sources)
12. [1] use high intensity lamp or collimated beam or laser
13. [1] method described to check that current in light source is constant, e.g. use an ammeter and variable resistor / variable power supply
14. [1] keep position of light source constant or distance between light source and LDR constant
15. [1] light source is placed close to water surface to increase intensity/reduce reflections or light source is placed further away to make it more directional
16. [1] use tall container to give a wide range of d or R or to reduce uncertainties or use a wide container to reduce reflections
17. [1] method to position ruler vertically to measure d described e.g. use a set square/spirit level
18. [1] use of horizontal fiducial mark from ruler to meniscus or middle of LDR, e.g. pin or d = reading on rule at surface - reading at top of LDR
19. [1] ensure that the electrical connections/wire to the LDR are waterproof