Here is a mark scheme from the website: www.cie.org.uk
Nov 08 Paper 5 (9702)
1 A student wishes to investigate how the resistance R of a light-dependent resistor varies with the distance d from an intense light source.
It is believed that the relationship between R and d is
R = kdn where k and n are constants.
Design a laboratory experiment to test the above relationship. The light-dependent resistor has a resistance of 100 Ω when it is in bright light and a resistance of 500 kΩ when no light falls on it.
You should draw a diagram showing the arrangement of your equipment. In your account you
should pay particular attention to
(a) the procedure to be followed,
(b) the measurements that would be taken,
(c) the control of variables,
(d) how the data would be analysed,
(e) any safety precautions that you would take.
[15]
Mark Scheme
Planning (15 marks)
Defining the problem (3 marks)
1. d is the independent variable or vary d (allow in table if numbers given) [1]
2. R is the dependent variable or measure R as d varied (allow in table) [1]
3. Keep output of light source constant (allow constant current / e.m.f. / voltage / power) [1]
Methods of data collection (5 marks)
4. Diagram showing an LDR in a circuit and an independent lamp. [1]
5. Diagram showing ruler measuring appropriate distance or d labelled correctly. [1]
6. Correct circuit diagram for LDR using conventional symbols; allow labelled diagram [1]
• Ammeter and voltmeter with power supply,
• or potential divider methods
• ohmmeter without power supply,
• or bridge methods.
7. Method of determining R. [1]
• Ohmmeter.
• R = V/I justified.
• Potential divider equation
• Description of balancing bridge with correct equation.
8. Perform experiment in a dark room/tube [1]
Method of analysis (2 marks)
9. Plot a graph of log R against log d [1]
10. Relationship is correct if log R against log d graph is a straight line [1]
Safety considerations (1 mark)
11. Do not look directly at bright light source / do not touch hot light source. [1]
Allow safety glasses with reference to light source.
Additional detail (4 marks)
• Detail on measuring the distance
• Keep orientation of LDR with respect to the light source constant
• Reasoned method for keeping light and LDR in correct orientation. (E.g. use of set square,fix to rule, optical bench or equivalent)
• Determination of a typical current
• Range of ammeter / ohmmeter
• Control (or monitoring) of an additional variable e.g. temperature
• Reason for performing experiment in a dark room related to the LDR
• Method for checking the output of the light source is constant.
• Identifies gradient = n and/or y-intercept = log k for log R against log d graph
• Do not allow parallax when reading ruler, or reflectors.
Mark scheme published in website : http://www.cie.org.uk/qualifications/academic/uppersec/alevel/subject?assdef_id=758
[Total: 15]
