There are possible variables which will be kept constant in the experiment as they might affect the readings if not kept constant.
The solar cell will always be 90° to the table as the angle affects how much light is taken in. If the angle is not constant then the readings will be irregular as the light intensity will vary. Each time the solar cell is moved, a set square will be used to check that it is still 90° to the table.
The voltage used will be the same. If the voltage is not the same then the light intensity will change as well and this will give inconsistent readings. The setting on the power pack will be kept the same to ensure this.
The same light source and solar cell will be kept constant as changing them could lead to a variation in the results. If a solar cell or light source breaks, the experiment must be done all over again.
Sensitivity of measuring techniques
The measuring techniques I will use are:
- Metre ruler: +/- .1cm
- Voltmeter: +/- 0.01v
- Vernier Callipers: +/- 0.001mm
I predict that the percentage uncertainty of readings will vary according to the distance. When the distance between the solar cell and the light source is relatively far away, the voltage reading will be low and that would make the uncertainty very high. When the voltage reading is high then the percentage uncertainty will not be that much of a problem. The same goes for the distance, with a high uncertainty for small distance and acceptable uncertainty for larger lengths. This should be taken into consideration in the evaluation, as well as a percentage uncertainty of as high as 50% when measuring the background lighting.
Accuracy of Measuring Techniques
Apart from sensitivity of instruments, other factors may affect the accuracy of my results.
Background light, the sun coming in from the windows will affect my results. The curtains in the room will be drawn to prevent too much affect on the solar cell. It is inevitable for there to be background reading as the curtains cannot fully block the light from the windows and other experiments around the room will definitely affect the readings. However I will try eliminating the background reading by taking a background reading before turning on the ray box and subtracting the two values.
Light could also be reflected from the shiny table surface. Black paper will also be used to prevent reflection of light.
Movement of fixed apparatus could cause a problem as it may affect the results. To prevent the apparatus will be clamped down to ensure that they do not move.
I will also repeat the experiment a few times to make sure that my findings are consistent.
Although this experiment is not dangerous, some safety precautions must be considered while carrying out the experiment. Since the room will be dark, safety should be observed while walking and not interfere with other’s experiment. The light source will be hot after a while and should be handled with caution to prevent burns. Electrical safety should also be observed, wires and plugs should be inserted in the sockets properly to prevent electrocution of any sort.
Observing and Recording
The trial run was very successful and when a graph V against 1/d2 is plotted, the line of best fit shows that the voltage is inversely proportional to d2(Figure 2.1), which satisfies my hypothesis.
There are a few uncertainties to the experiment though. The percentage uncertainties are:
Vmin – 0.01/0.48 x 100 = 2%
Dmax – 0.1/100 x 100 = 0.1%
Vmax – 0.01/1.94 x 100 = 0.5%
Dmin – 0.1/10 x 100 = 1%
As you can see the percentage uncertainties are acceptable and will not significantly affect the final outcome of the experiment.
Modifications to the Planned Method
My plan worked satisfactory, however to further improve it, I decided to make the following changes to increase my accuracy.
- There are two ways to increase the distance between the ray box and the solar cell; either moving the ray box or solar cell. During the trial run, I tried both methods and have come to the conclusion that moving the solar cell is much more difficult compared to moving the ray box as it is harder to position the solar cell at the same angle and spot each time the distance changes. So I shall be moving the ray box to change the distance between the cell and the light source
- To increase accuracy in my results, I will record the background reading before and after I record the reading when the ray box is switched on. I can then get an average of the two readings and subtract it from the actual reading.
- Instead of ending my experiment at 60cm, I have decided to continue to 100cm to obtain more results and a better graph that is suitable for proper analysis
- Gather all the equipment
- Measure the radius of the light bulb using vernier callipers and add it to the distance between the light bulb and the end of the ray box.
- Plug in the power pack and connect it to the ray box
- Tape the black paper on the table, with an inch going underneath the ray box
- Subtract the length measured from step 2 from 10cm and mark that length away from the solar cell on the black paper.
- Continue marking 5cm increments from the base(10cm) mark until you get to 100cm on the black paper
- Attach the solar cell to the clamp to a stand. Use a set square to make sure that the solar cell is 90° to the table.
- Turn on the voltmeter and note down the voltage output without the ray box on. This is the background reading
- Switch on the power pack and take the reading from the voltmeter.
- Turn off the power pack and note down the voltage output without the ray box. This is the second background reading. Find an average with the first background reading and subtract it from the actual reading that was obtained from step 9.
- Turn off the power pack and repeat steps 8 - 10 two more times to get an average
- Do steps 7-10 increasing the distance by 5cm (next marking), by moving the ray box from the solar cell until the distance is 100cm. Make sure that position of the ray box is constant every time it is moved by using a set square.
Interpreting and Evaluating
Table of results
As you can see from the table of results and the graphs drawn, my prediction was right and that the further the distance, the smaller the output potential distance. Further more, the graph (V against d-2) shows that Pin is proportional to distance-2. However the constant gradient starts to decline when d is 30. The line of the graph starts to level off as the distance between the solar cell and the light source is smallest. The reason for this could be that because the efficiency of the solar cell is only 23% and can only generate a maximum potential difference (of 1.44v, from results table), as it is saturated with photons from the light source. A better scientific explanation is that there is only so much electrons in the solar cell that can be lost from the atom to flow in the circuit to produce electricity and when the flux is too big, the electron flow will be saturated; all the electrons would be flowing through the circuit already.
From the Graph (V against d) you can see that the line of best fit was very close to all the data plots, which shows that there was no obvious rogues. However, observing the table below at the calculated percentage uncertainty, the percentages may be unacceptable, especially when the distance between the light source and the solar cell is very far away. Therefore, an error box has been drawn to demonstrate what the range of values could be. The error box is drawn at distance 85cm as that is when the voltages start to level off.
As you can see from the table and the graph, the spread of results and error box is very small and insignificant. This shows that the results were fairly constant and that they were accurately measured; taking into account that there was no full control over background lighting, which explains spreads more than 0.01V. The use of repeats increased the precision in my results and has shown that there were no rogue results, due to the low spread. This shows that my method was good. Another way to calculate my percentage uncertainty is by adding the percentage uncertainties of Vmin with Dmax, and Vmax with Dmin:
Vmin: 0.01/0.10 x 100% = 10%
Dmax: 0.10/100 x 100% = 0.1%
Vmax: 0.01/1.45 x 100% = 0.7%
Dmin: 0.10/10 x 100% = 1%
Vmin + Dmax = 10% + 0.1% = 10.1%
Vmax + Dmin = 0.7% + 1% = 1.7%
As you can see the overall uncertainty is relatively low. This additionally proves that the uncertainties do not affect my results noticeably, and that the instruments were the limiting factor of this experiment.
Certain limitations in this experiment could have affected my results.
The graph starts to level off as the distances increases. This is caused by the limitation of the solar cell since it is saturated by the light, as discussed in the conclusion. The solar cell was not large enough to obtain a good set of results.
An obvious limitation to this experiment was background light. Although background readings were taken and subtracted from the actual reading, the background light could have increased while the reading of the solar cell and the light source was taking place, thus my results unreliable.
My measuring technique could also have made my results unreliable. There may have been mistakes when moving the light source away from the solar cell and although may not seem like a huge error, could have affected the graph and results in some way. Another problem I faced was shadows; other people’s shadows could have blocked the light source from properly reaching the solar cell and would have affected my results
To overcome the problems identified above, modifications to the investigation could be made.
A larger solar cell would mean a larger voltage output, which would give a very accurate reading when d is very small because the solar cell would not saturate.
To overcome background light, this investigation should be done in a pitch-black room, something similar to a darkroom used to process film. This would eliminate any background light and give a very reliable and accurate reading. The investigation should also be done so that no reflections or shadows would affect the readings; for example no other people in the room and cover the walls and table with black paper.
A machine could be used to clamp the solar cell and move it closer or further away from the light source very accurately and this would almost limit human error and overcome any measuring errors.
Here's what a star student thought of this essay
Quality of writing
Their quality of written communication is generally good, and they have made very few spelling or grammatical errors. The report appears to be very well presented, and they have made good use of graphs and tables to present their results clearly. However, the layout is not perfect - the author has first describes a suitable method, but then does a preliminary experiment which should have been used to come up with this method in the first place. They have also not stated that this is a preliminary experiment, but treated it as if it was the real experiment, except that it doesnÃ¢â‚¬â„¢t follow the proposed method. They then have suggested improvements to the experiment that they didnÃ¢â‚¬â„¢t actually do based upon the preliminary. I would have described roughly the plan of the actually experiment, without giving any precise lengths or other measurements, then done a preliminary, explaining that this is to work out the most suitable lengths and other measurements to be used. The preliminary should have tested more than three points Ã¢â‚¬â€œ I would have recorded the voltage every 10cm until there is no longer any significant change between readings, then plotted all these points on a graph, not just three, which would show the point where the change in distance no longer affects the readings. This would be more useful and make it clearer to the reader what you are doing. Only after these steps would I propose a clear method. This more chronological approach is more conventional and makes it clearer to the reader what you are doing. Another small issue is that they have not clearly labelled their graphs with the units, clear headings, and a description of what the graphs actually is. This makes it slightly confusing to the reader. I would also have used standard units throughout, which makes it much easier to do calculations, as well as being more conventional. Although they have given good headings throughout the report, one stands out as being Ã¢â‚¬Ëœa bit GCSEÃ¢â‚¬â„¢ Ã¢â‚¬â€œ Ã¢â‚¬Å“Fair testingÃ¢â‚¬Â Ã¢â‚¬â€œ at A level, we are advised to say Ã¢â‚¬Ëœreducing uncertaintiesÃ¢â‚¬â„¢ instead. They have also used the word Ã¢â‚¬ËœrogueÃ¢â‚¬â„¢ instead of Ã¢â‚¬ËœanomalousÃ¢â‚¬â„¢, which is rather odd, and they have not been able to calculate anomalies as they have not repeated their experiment enough Ã¢â‚¬â€œ this can only be done when it has been repeated at least 5 times. However, they have presented their work well, have good spelling and grammar, have used tables and graphs to make their results clearer to the reader, and have shown most of their workings, so the occasional slip in convention could be overlooked.
Level of analysis
The author has analysed their results well and compared then to his original hypothesis, using this to come to a well-justified conclusion. However, occasionally they have made minor errors in their analysis, for example they say: Ã¢â‚¬Å“Metre ruler [uncertainty]: +/- .1cmÃ¢â‚¬Â Ã¢â‚¬â€œ we were advised that the uncertainty in a measurement is +/- half the smallest measurable value, so in this case +/- .05cm, because the smallest measureable value is 1mm. However, itÃ¢â‚¬â„¢s always better to overestimate rather than underestimate uncertainties so they may not have lost too many marks. I would also have calculated the uncertainties at all the points and plotted uncertainty bars on the graph, which would clearly show how large or small the uncertainties are compared to the values measured (although they have calculated the percentage uncertainties, it is much easier to visualise when they are plotted on a graph with the original data). I would also have plotted one graph with all the recorded data on it, which would show the spread of the data clearly, rather than only using the mean values. I would also have plotted graphs using the original data, rather than only plotting 1/d2 against the voltage output, which would allow more exploration of the relationship between the two. I would also have plotted a line of best fit (excel/ autograph can calculate these and give you its equation, which is really useful, otherwise you could use a more mathematical approach using logarithms. Some exam boards require you to plot them yourself using a pencil, so check with your teacher beforehand!). However, they have used their graphs well to discuss the relationship between the distance and the voltage output, so should have gained good marks for their use of graphs. Another, slightly more important suggestion for improvement is in the scientific explanations given. Their reasoning of the original hypothesis is excellent, however they show at times a lack of understanding about the solar cell, for example, while explaining why Ã¢â‚¬Å“The graph starts to level off as the distances increasesÃ¢â‚¬Â the author claims that this is due to the solar cell being saturated with light, when less light would be reaching the solar cell as the distance increases, so in fact the opposite is true - as the distance increases, the number of photons reaching the solar cell decreases, so less electrons are forced around the circuit. The graph levels off as the distance reaches a limit when the radiation emitted by the light source is not significantly different from the level of background radiation Ã¢â‚¬â€œ the Ã¢â‚¬ËœnoiseÃ¢â‚¬â„¢ is hiding the signal. To be able to continue the experiment beyond this point, a brighter bulb would have to be used throughout the experiment. Proof reading the report and really thinking about what is happening within the solar cell should help resolve these issues. Despite these mistakes, the author has analysed their results well, using them to come to a good conclusion. They have discussed the causes of uncertainties, and used this to discuss improvements, and explored the relationship between the distance and the voltage produced, showing a good level of analysis.
Response to question
The author has responded well to the task, by developing a suitable experiment to measure how the voltage output from a solar cell is affected by the distance away from a light source it is. They have made a well reasoned hypothesis and confirmed it by analysing their results, using this to come to a well justified conclusion. The question was given as Ã¢â‚¬Å“What affects the voltage output of a solar panel?Ã¢â‚¬Å“, so perhaps they could have also investigated, or at least mentioned, other things which may have an effect, such as the intensity of the light. However, their experiment was clearly carried out very well, and overall their response to the question is very good.