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Investigation into the Physics of a Light Dependent Resistor.

Extracts from this document...


Investigation into the Physics of a Light Dependent Resistor

Introduction and Explanation of how an LDR functions

In this essay, my aim is to examine the physics behind a Light dependent resistor by measuring the voltages across it when exposed to bulbs of various wattages.

As with all experiments, it is necessary to make an initial prediction. I believe that the voltage across the LDR will increase if a higher wattage of bulb is used.

However, we find ourselves asking the question, ‘Why should the voltage change across this component just because the light intensity around it varies?’

In order to answer this question we have to examine the physics behind an LDR.

The LDR:


‘Success in Electronics’ (Tom Duncan 1983) provides this symbol as the representation of an LDR and tells us that this component, sometimes called a Photoresistor, varies its resistance according to light levels.

The resistance of an LDR depends upon the amount of Charge Carriers inside the component.

Charge carriers are particles which are capable of carrying charge (!) and are free to move across electron levels.

According to Ohm’s law, the resistance falls in the LDR as the current throughout the circuit increases.

...read more.


This inaccurate theory taken into account let us then examine two of my results. I obtained an average voltage across the LDR for a 20 Watt bulb of 1.58. For a bulb of 100 Watts I obtained an average reading of 1.92.

Imagine my thinking at the beginning of this experiment. I thought that the voltage across the LDR was directly proportional to the intensity of light falling on it solely.

I performed the method you will read later and then plotted my results table.

Imagine then my confusion when I examined my results. If the voltage reading depended on light intensity solely, why wasn’t the voltage across the 100W roughly 5 times (5*20=100) that of the 20W bulb?

The non-quantum theory I mentioned above had to be discarded, as based upon this explanation, the voltage should double every time the light intensity doubles. The results did not bear this theory out so I realised that there must be another factor which was affecting the voltage across the component.

What was initially a simple and mundane experiment developed into great significance. Either there was a gigantic margin for error (a point I will discuss in my evaluation)

...read more.



Through careful examination of the results table, graph and of my explanations, I think we can safely say that the voltage across an LDR will increase when the intensity of light at a certain frequency increases.

This ties in with my initial prediction that the voltage would increase with the wattage of the bulbs.

Had I more time I would like to examine the voltage across the LDR when exposed to lights of different frequencies i.e. all the colours in the spectrum.

Since we know the frequencies of all the colours of light, it would be possible to prove a definitive link between light frequency and the amount of energy transferred.

If we wish to talk about the Quantum aspects of this experiment, this experiment goes a long way to prove that light has particle-like properties as the amount of energy needed to make an electron move from one level to another is fixed and yet the light seems to provide this exact amount every time it falls on the atom.

Essentially, the electrons are receiving a certain amount of energy each time. It could be that every single light ‘wave’ contains an identical amount of energy or it could be true that single photons are delivering an exact ‘packet’ of energy to allow the electrons to become Charge carriers.

        Page         4/30/2007

...read more.

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Here's what a teacher thought of this essay

4 star(s)

This is a well structured and written report.
1. Several subheadings are missing and some information is in the wrong section.
2. There is a running commentary that needs to be removed.
3. The conclusion needs to quote data from the investigation to back up the claims.
4. The evaluation needs to suggest further research opportunities.

Marked by teacher Luke Smithen 13/08/2013

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