Of course, this is only an initial method. The preliminary practical will give me a good feel of the equipment, and where things could go wrong. I will be able to change things for the final practical.
Preliminary Work
I will need to undertake a preliminary practical for many reasons. A preliminary is very good for helping me get a feel for the equipment. I will do it to find out how much resistance there is without the lamp being turned on, to see what effect other light has on the LDR. I will also see if this changes when the LDR is out in the open. From this, I can decide where I will put the LDR in the final experiment. To start with, I will have the LDR directly under the lamp, so as little light as possible gets to the LDR.
A second thing I will probably do, is slowly increasing the voltage until a noticeable difference is shown on the LDR. I will also get a good feeling of what I expect to happen in the actual main practical, and where I will be getting anomalous results. I may also be able to predict trends, and alter my graphs.
I will only take readings at a few voltages. I may take readings at 4 volts (the lowest reading), 8 (a middle reading), and 12 (the highest reading. These are all of some use, as they will give me an estimate of the trends, and by finding the highest and lowest readings, the range.
I will also need, in my preliminary practical, to take down the conditions that the practical is taken in. I will need to make the experiment as fair as possible, so I will need to see where all the extra light is coming from, so I can regulate that in my main experiment, and also things like the vertical distance from the LDR to the lamp.
Results from the preliminary
These are the brief results I eventually took from the preliminary practical. They differ slightly from above as some tests were unnecessary
0 volts- 170’000 ohms 4 volts- 1100 volts.
This was to see how the first light effected the readings.
14 volts 74 ohms
14 volts w/cover 58 ohm
This showed the difference when a cover intensified the rays of light
Amendments to plan following preliminary work
I found out from the preliminary that the resistance changes greatly from 2.0 to 4.0 volts, as the light bulb then switches on. This will be important in my evaluation later. I have made no changes to the method, as all the equipment worked correctly, and the readings seemed fair enough.
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
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 4.0 volts, so the resistance dropped greatly, as it actually had it’s own light source directly above it, so the effect on the LDR freeing up electrons was enormous. A part of my hypothesis said that the more light the LDR senses the less resistance there is. As there was no light at 0 and 2 volts, then there is no surprise that at the first reading where the bulb shone, the resistance plummeted.
As the voltage reached ten volts, the changes in resistance were a lot smaller. The resistance did not halve, probably because there were very few electrons to free up, and the changes in the light intensity became more gradual, as opposed to the massive leap at four volts. It is impossible for the resistance to stop falling, but perhaps once the resistance reached around 10 ohms, then the falls would be very small. Overall, the resistance fell as voltage increased, but decreased less each time voltage increased
Does this pattern support the hypothesis?
My hypothesis said that as voltage doubles, current doubles, the light intensity doubles and resistance halves, based on rules and the formula Resistance equals voltage over current. The experiment proves that the hypothesis is not entirely correct, as the drop in resistance varies greatly during the experiment. The gap between the 2.0 and 4.0 is massive, the 4.0-volt reading being only one 35th of the 2.0-volt reading. This is because the bulb only starts to show light at 4.0 volts, so this is the reason behind the massive drop in resistance. The next reading from 4.0 to 6.0 volts shows the reading drops by between a half and a third, a lot nearer to my hypothesis prediction. In the last two readings voltage drops only a small amount, around a fifth, so at each end of the scale the hypothesis is proved, whilst in the middle area, the results seem to back up the hypothesis. The middle two values 6.0 and 8.0 zero almost exactly back up the hypothesis.
Summing up my first hypothesis, it was correct in predicting that resistance decreased when voltage increased, but the maths in the hypothesis do not quite make sense, and to modify it I would need to add extra details, such as how the resistance decreases by smaller amounts each time, and would write something like,; as voltage increases resistance decreases, rapidly at first, halving at 8.0 volts, and then falling slightly on higher voltages. I could also explain when this happened and the fractions of the drops, such as the resistance at 4.0 volts is one 35th of that at 2.0 volts. I will then need to explain that this is because at 4.0 volts the bulb starts to light up, and that later on the resistance decreases to a very small amount, but will probably never reach zero, as there will always be a small amount of resistance in the conductors. To further the experiment I could try different conductors, as the conductor I used (LDR) may have had a low resistance, resisting current less, therefore altering the resistance readings.
Evaluation
In this section, I will evaluate my results and my graph. First of all the graph; I drew a best fit line to see if I got a straight line graph that would support my hypothesis exactly, or a curved graph, showing that my prediction that the resistance halved as voltage doubled was not entirely correct. The line is also used to find out if even after averages, I had glaring errors, or anomalous results, and overall I think that my results were good, and the graph tends to reflect that they are indeed accurate results. I think that I handled the equipment accurately, always taking the readings when I had the voltage on the dot, and changing the dial on the Multimeter appropriately, to measure the resistance to appropriate degrees of accuracy. The fact that there are no anomalies, and the graph tends to show that there are almost no errors, then I think that my readings were very accurate.
I knew that my results should have been reliable, because I took three sets of readings, so any bad results should hopefully have been ironed out. The smooth curves on the graph back this up, and there are no anomalies. I did not have to reject any results. The individual results are slightly different at the start, went the resistance was so high it fluctuated a lot. However, these results were not a long way off, and when averaged and put into a graph look correct, and so I believe that my results were by and large accurate.
To make absolutely sure of this I drew two graphs, one excluding the first two readings, so it easier to pick up the smaller readings and to see that they were too, reliable.
I think that my method worked very well, and I achieved my goal of getting three sets of result. I think that by following my method, I got three good sets of results, and comments I made in my original method helped to keep the test fair. The method definitely got enough good results to draw a conclusion, and as I concluded earlier, the higher the voltage, the less the resistance, but the drop in resistance is smaller each time voltage drops.
There is no way that the results were perfect, so I need to explain where the inaccuracies came from. For the first two readings the values fluctuated a great deal, because there was no light coming from my bulb, so any extra light from other bulbs would have made a great difference to these readings. All the other readings were susceptible to changes due to influence from the other light bulbs. This would have caused the resistance to be slightly lower than in perfect conditions.
I think that the resistance went down in my second and third sets of readings because all the wires and the bulbs had heated up more, so the resistance may have fell slightly.
To remove the last inaccuracy I should do the experiment with a couple of hour’s gap in between each set of results. This will ensure that the whole system has cooled down. To eliminate other light I could do it in a pitch-black room, when no one else was doing their experiment at the same time. I would also put a cover over the bulb to make absolutely sure that any excess light would not be able to influence my experiment. I could also try to make the test fairer by using exactly the same practical equipment each time. It is possible that the bulb may be damaged in some way, or that the LDR is slightly faulty. This would not make a large difference, but would make me feel that the test is that bit fairer.
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.
