A student investigates how the current in a light-dependent resistor (LDR) changes as she varies the amount of light reaching it by placing different numbers of semi-transparent plastic filters over the LDR.
She connects the LDR in series with a constant DC voltage source and a digital milliammeter. A lamp is positioned at a fixed distance directly above the LDR.
State why the student carries out this investigation in a dark room.
Identify the control, dependent, and independent variables in this investigation.
The student's results are shown in the table below:
| Number of filter layers | 1st reading (mA) | 2nd reading (mA) | 3rd reading (mA) | Average (mean) (mA) |
|---|---|---|---|---|
| 0 | 82.4 | 83.1 | 81.9 | 82.5 |
| 1 | 45.2 | 44.8 | 45.6 | 45.2 |
| 2 | 24.8 | 25.4 | 25.1 | 25.1 |
| 3 | 13.5 | 13.9 | 14.0 | 13.8 |
| 4 | 7.6 | 21.2 | 7.8 | 12.2 |
| 5 | 4.2 | 4.1 | 4.3 | 4.2 |
| 6 | 2.3 | 2.2 | 2.4 | 2.3 |
Identify the anomalous reading in the table by stating the number of layers and the current value.
Calculate the correct average (mean) current (in mA) for the number of filter layers that has the anomalous reading.
Describe the relationship shown by the results between the number of filter layers and the current.
State how the resistance of an LDR changes when the intensity of light falling on it increases.
The student repeats the entire experiment but uses a brighter lamp at the same fixed distance. Explain how the curve of best fit on a graph of current against number of filter layers would change.
25 exam-style questions on Edexcel IGCSE Physics Energy and voltage in circuits. Each one has a worked solution and a mark scheme showing where the marks go.