Light-Dependent Resistor: Voltage Divider Method

INVESTIGATION 5.3 Light-dependent resistor: voltage divider method

The aim of this investigation is to gather information to produce a graph of how the resistance of a light-dependent resistor varies with light intensity.

You will need:  9 V battery  LDR  2.2 k resistor  voltmeter  light meter or data logger (if available)  a point light source, for example a 12 V, 36 W car headlight globe in a holder  connecting wires.

For a point light source, the intensity varies inversely with the distance from the light source: I  1 r2 . 1 If the distance is multiplied by a factor of x, then the light intensity will be x2 of the original value.

1. Set up the apparatus as shown.

2. Turn on the point light source and place it 5.0 cm from the LDR. Ensure that the LDR is perpendicular to the incident light from the source. 3. Darken the room and measure the voltage across the LDR and the resistor. 4. Repeat step 3 with the LDR at distances of 10 cm, 20 cm, 40 cm, 50 cm and 100 cm from the light source. 5. Calculate the resistance of the LDR in each case, using the potential divider formula.

Jacaranda Physics 2 2E CD-ROM Investigation 5.3 Page 1 of 2 6. Copy and complete the table below.

RELATIVE DISTANCE OF LIGHT LDR FROM INTENSITY IL SOURCE (OR LIGHT r (cm) INTENSITY) VLDR (V) Vtotal (V) RLDR (k) LOG IL LOG RLDR 5.0 1.00 10.0 20.0 40.0 50.0 100.0

Analysis

7. Plot log RLDR against log IL on graph paper.

8. Determine the relationship between RLDR and IL, if possible.

Jacaranda Physics 2 2E CD-ROM Investigation 5.3 Page 2 of 2