Question 2
AIM: To investigate the operation of a Light Dependent Resistor (LDR).
- Connect the circuit as shown in Fig. 4 with the LDR in horizontal position.
- Ask the supervisor to check the circuit connection.
- Close switch S.
- Place the flashlight at distance, d = 50 cm away from the LDR and switch it ON.
- Read and record in Table 1, the current, I, flowing through the circuit.
Table 1
S/N |
d (cm) |
I (mA) |
1 |
|
|
2 |
|
|
3 |
|
|
4 |
|
|
5 |
|
|
- Repeat (d) and (e) for the other values of distance, d (cm) in Table 1.
- Plot a graph of current, I (mA) on the vertical axis and distance, d (cm) on the horizontal
axis.
- Using the graph, state the relationship between the distance of light source and the current
flowing through the circuit..
- State two applications of the LDR in a television receiver.
- State one difference between the LDR and a:
(i) fisxed resistor;
(ii) thermometer.
- State two precautions taken to ensure the accuracy of your readings.
Observation
The expected responses were:
Table 1
S/N |
d (cm) |
I (mA) |
1 |
50 |
2.50 |
2 |
40 |
3.00 |
3 |
30 |
4.00 |
4 |
20 |
6.00 |
5 |
10 |
10.00 |
(g) A non-linear graph showing increase in I (mA) as d (cm) decreases. Graph is expected to have features such as: title of graph; suitable scale; labelled axes, correct point plotting and line/curve of best fit.
(h) Relationship between d (cm) and I (mA)
As the distance between LDR and lamp decreases, the intensity of light incident on LDR increases. Hence, the amount of current flowing in the circuit increases.
(i) Applications of LDR in television receiver
- Used in remote control
- Used in automatic brightness control
- Used in automatic contrast control
(j) (i) Difference between LDR and fixed resistor
- The resistance provided by LDR is a function of the intensity of light incident on it while the fixed resistor is not sensitive to light.
- LDR is light sensitive device while fixed resistor is not.
(ii) Difference between LDR and thermistor
- The LDR is sensitive to light while the thermistor is sensitive to temperature.
- LDR is light sensitive device while thermistor is a temperature sensitive device.
(k) Precautions that should be taken to ensure accuracy of readings
- Ensure tight connections
- Ensure the flashlight is switched off after taking each reading
- Ensure the circuit is switched off after taking readings
- Ensure that the room/environment is dark enough for better performance of the LDR
- Use a sensitive ammeter for readings
- Avoid error due to parallax
- Ensure direct focusing of light on the LDR
Candidates were expected to connect a simple circuit and vary the distance dependent variable and thereafter, use the readings obtained for evaluation and deductions. The Chief Examiner reported that majority of the candidates attempted question 2 and that a larger percentage of those who attempted it responded well while others seemed not to have taken the necessary precautions that will enhance the accuracy of their readings.