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To Investigate How the Resistance of the Light Dependent Resistor Depends On the Current In the Light Bulb?

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In the experiment, I am going to investigate how the current in the light bulb affects the resistance of the Light Dependent Resistor (LDR).

In this experiment, I am going to vary the resistance in the light bulb circuit in order to vary the current in the light bulb. At the same time, I am going to find out how the resistance of the LDR is affected by the brightness of the light bulb.

In the following stages, I will need this scientific knowledge:

The current in a series circuit is the same everywhere, but in a parallel circuit, the sum of the current in the branches is equal to the current of whole circuit.

The voltage in a parallel circuit is the same everywhere, but in a series circuit, the voltage of each component added together is equal to voltage of the supply.


I predict that:

  1. When the light bulb becomes brighter, the resistance of the LDR decreases.

This is because the Light Dependent Resistor (LDR) is made of Cadmium Sulphide. This semiconductor (a very poor electrical conductor) has a very high resistance in the dark, but when light is shone on it, its resistance falls. This means that when the light bulb is bright, the resistance of the LDR is low. Otherwise, the resistance of the LDR is high.

When the resistance of the variable resistor R1 decrease in circuit 1, the reading I1 will increase.

...read more.


The circuit was set up as shown above. A table was drawn out and the results were recorded.

To improve the accuracy, compared to my preliminary results for my main
Experiment, I have decided to use a 20MA ammeter in the 1
st circuit and use a 200MA ammeter in the 2nd circuit.

To collect the data for my graph I have chosen to take a range 2.5MA of the current of the light bulb. I have chosen a range of 20 as plot an accurate graph; I will need at about 20 points to mark on the graph if I want to make precise and reliable results, to see if there are any patterns and trends.

...read more.


One of the difficulties in this experiment is to control the current of the light bulb, when I carried the variable resistor R1, the current of the light bulb changed rapidly, so it is hardly to control it exactly what I want.

During my experiment, I have noticed several mortifications I could make to improve on the Investigation if I was to repeat it. 
The first of these modifications would be I could use a new or higher quality digital voltmeter and ammeter.

The graph shows that my results are reliable as there are only two anomalous points, to improve the reliability of my results, I could also have repeated the same lengths of wire more times.

As well as making these modifications, I could also expand on my investigation by measure the voltage of the light bulb in order to find out the power of the light bulb.

This is because the brightness of the light bulb is depended on the power of the light, in order to find out he power of the light bulb, I will need to know the voltage of the light bulb and then use power=current* voltage to find out eh power.

I will use the circuit and method used, as it was quite suitable although I would make the modifications above to improve my results. If I did this experiment again I would defiantly use top quality equipment, I would control the entire confounding variable, which includes the brightness of the room.

My alternative conclusion is that: the resistance of the LDR decreases as the current of the lamb decreases.

...read more.

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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