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# Light intensity

Extracts from this document...

Introduction

Year 11 assessed practical: light intensity

Hypothesis

• A light bulb gives out light when electricity passes through it
• The more volts you add, the more intense the light is. As you double the voltage, the light intensity doubles.
• This is because the rays are more concentrated. The rays are twice as concentrated as the intensity doubles, as more power is running through the circuit
• The LDR (Light dependent resistor) measures how intense the light is
• The LDR frees up electrons, decreasing resistance
• The more intense the light is, the lower the resistance in the LDR
• Ohms law says that current increase is proportional to potential difference
• This only occurs if all conditions are the same
• As Potential difference doubles, Current doubles.
• Resistance is voltage over current.
• If voltage rises proportionally with current, then resistance will halve as voltage doubles.
• So as I halve the voltage, the current halves, light intensity halves, and resistance doubles.

Method

This is how I will set up the apparatus. Below is a circuit diagram.

The set up of the circuit is simple as shown above. The 12-volt lamp is set up in a simple circuit. Then in a SEPARATE circuit, the light dependent resistor is connected to the Multimeter, which shows the resistance readings. If I did connect the LDR in the same circuit, I would almost certainly fry the LDR, and ruin the experiment! I want to keep the distance from the lamp to the LDR the same.

Middle

Fair Test
• Change only one variable (Voltage)
• Keep distance from LDR to bulb the same
• Make sure all readings are taken in darkest possible surroundings
• Try to use the same bulb each time
• Measure each voltage as accurately as the setting allows
• Keep my experiment as far away from others as possible, to get minimal interfering light
• Keep the height of the bulb above the LDR the same

Results table showing resistance of the LDR as potential difference (volts) are changed

 Potential difference(volts) Resistance  of LDR 1 (Ohms) Resistance  of LDR 2 (Ohms) Resistance  of LDR 3 (Ohms) Average Resistance of LDR (Ohms) 0.0 60,400 102000 107000 89800 2.0 50,200 35000 32000 39067 4.0 1005 1102 1168 1092 6.0 394 398 412 401 8.0 203 205 208 205 10.0 138.8 139.3 150.4 143 12.0 101.3 102.1 106.2 103 14.0 79 81.5 84.1 82

Distance of the LDR from the bulb kept constant (directly underneath the bulb, 0cm)

Analysis

On the previous page is the graph; now I will try to sum up the results.

As the potential difference increased, the resistance decreased, dramatically at first, indicated by the steep slope on the graph. As the Potential difference was increased further, the resistance continued to decrease, but more slowly.

At first (0.0 to 2.0, then 2.0 to 4.0) the resistance decreased three times, then thirty times as potential difference doubled. Later, as potential difference increased from 12 volts to 14 volts, then resistance didn’t even halve, but decreased by about 20%.

These are massive differences, and the reason for the large resistance readings at 0.0 and 2.0 volts is that the LDR sensed no light with which it frees electrons, therefore decreasing resistance. The bulb visibly gave out light at 4.

Conclusion

These could all be incorporated into the method, and if I did this experiment again, I could add these extra points to the method

• Keep a cover on the bulb to make sure all the light hits the LDR
• Use the same practical equipment
• Try to work as far away from other people as possible.

Other than that I think the method works well. I left out the points regarding letting the system cool down, as there is not enough time to do the experiment in, and I could certainly not use a whole day to take results. If I had the time, I would also try to take more sets of readings, up to 10 sets, to get the best graphs. However, the above points are a realistic way of improving my results next time.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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