Resistance (? ) = resistance of the metal(? cm) x length(cm)cross sectional area (cm2) The cross sectional area of the wire is constant, and so is the material I am using. It is only the length that will be changing, so as you can see from the formula, the length and resistance must be directly proportional. Evaluation On the whole, I was very pleased with the way my experiment went: I got the results I wanted and proved that my prediction was correct.
The variables of the experiment were as follows: temperature diameter type of wire length of wire Because of the nature of these variables, they were very easy to control. Temperature: the temperature of the room that the experiment was conducted in was always constant so that was not a problem. The other factor which had the ability to cause a change in temperature was the amperage: if this started above my chosen limit of 1 amp then the experiment would have got too hot, resulting in an apparent increase in resistance which was not related to any change in the length of the wire. I avoided this problem using a rheostat, which kept the amperage low.
Diameter: this variable did could not have changed throughout the experiment, and so was not a problem. Type of wire: again, this was not a variable that could have altered during the experiment. Length of wire: This was the variable that was to be changed. It was definitely the most difficult to accurately control. Problems arose with the fact that the wire was not entirely straight, and so was very difficult to measure. Also, the wire was attached to a wooden rod, which made it impossible to measure the 35cm point on the wire because it was hidden by a notch in the wood.
This made it impossible to accurately measure it. I had to miss this reading out. Evaluating the accuracy of the Measurements In general, I was very pleased with the accuracy of this experiment. Having said that, it certainly wasn’t as accurate as it could have been. I would predict that if the experiment were done to a more meticulous standard, the result I have taken would have been exactly directly proportional. The biggest problem I have with the results is the fact that in the repeat test that was done, the results, in general, were quite significantly lower, than in the first.
When averaged out however, the results produced points on a graph that were remarkably close to the line of best fit. Many of the points were in fact on the line of best fit. Despite this, there were clear anomalies: The readings for 45cm and 50cm were 3. 15? and 3. 44?. These two results are within 15ohms of each other, whereas all the other results are about 30 apart. Another problem with these two results is the fact that the second, 50cm, gave a lower resistance than the reading at 45cm; with all my other results the resistance has been steadily and evenly rising.
There are many factors which could have contributed to these anomalous results: Accuracy of readings One major mistake I made when doing this experiment was to read from the wrong scale on the voltmeter. I realized this halfway through the experiment, and had to convert all previous readings back to the correct scale. This was difficult as the scales were difficult to read and compare, and it would have been very easy for me to have misread at least one of the results. The voltmeter itself had a scale that was very difficult to read. Unlike the ammeter, it was not a digital display: it was just a needle.
I am not convinced either that the voltmeter was in very good condition, as the needle did not rest in one exact place. This made it virtually impossible to get an exact reading. Condition of equipment The equipment I used for the experiment was not in the best of conditions. In particular, the first power supply we used stopped working halfway through the experiment and had to be changed. The crocodile clips that were used to connect the nichrome wire into the experiment were very rusty and therefore could have given inaccurate readings.
The nichrome wire itself was very bent and twisted. This made it difficult to measure each of the 5cm intervals with a very great degree of accuracy. There could have been a particularly twisted part of the wire around the 45cm and 50cm marks, which would have caused them to be less accurate. Carrying out of the experiment One problem with the actual execution of the experiment was the fact that it was don’t over a period of two lessons. The equipment had to be put away and set up at a later date half way through the experiment.
This meant the experiment was not set up with the same equipment as when it was started, and this meant that because none of the equipment available was in very good condition, it could have lead to different results, and a change not in the gradient, but the position of the graph half way through. The 45cm and 50cm results were either side of the two-lesson break. This is why they are so drastically different from the rest of my results. As you can see from the graph of Experimet 2, the gradient on either side of the lesson-break is the same, there is just a gap in between.
On the graph of Average Resistance, the anomalous results are not quite so obvious, but there is still a dent around the 45cm and 50cm points. Evaluating the Conclusion Despite all the inevitable errors in my results, I am very confident that my conclusion is correct: The length of wire and resistance of wire are directly proportional and increase together. I have been able to give credible reasons why some of my results have been erroneous, and why some are not as accurate as they might have been. I am very satisfied that my results are accurate enough to draw an acceptable conclusion from.
My conclusion not only agrees with my prediction, but also with my scientific knowledge: Resistance (? ) is also equal to the resistivity of the wire(? cm) multiplied by its length(cm), and then divided by its cross sectional area(cm2). resistance (? ) = resistance of the metal(? cm) x length(cm) cross sectional area (cm2) The cross sectional area of the wire is constant, and so is the material I am using. It is only the length that will be changing, so as you can see from the formula, the length and resistance must be directly proportional.
Improvements I think that it would have made major difference to my experiment if I had used a computer to monitor the variables. A computer could have given me exact results for voltage and amperage, and would have eliminated any need for reading tricky scales. Another thing that may have made a difference would have been if the equipment I used had been in good working order. I was not at all happy with the condition of the connecting wires, or with the reliability of the power supply. I would have liked the wire to be much straighter.
It was incredibly difficult to measure accurately in the condition it was in. I am confident that it would have made a big difference to the accuracy of the results if the wire had been properly measured. Another problem that should have been more carefully tackled was that of the two lesson break. If I had thought ahead, I would have recorded what equipment we used so that we could use the same again. If I had done so, it would have made it easier to prevent the anomalies at 45cm and 50cm. Improvements – additional evidence There are many ways in which I would have liked to provide more evidence.
It would have been a good idea to do the original experiment three times, because that way, if one set of results was wrong, it would be easier to tell as it would stick out as being different. I would have liked to repeat the experiment with different metals: to see how they vary, and to check that my prediction is correct for more metals than just nichrome. It cannot be taken for granted that all metals will behave the same as nichrome has done. I would probably use copper and constantan in the same circuit as I used for nichrome.
Because I would be changing the metal, I would have to keep all the other variables the same as they were before, and take the same fair-testing precautions. I would not use quite so many different lengths of wire as I did when testing nichrome because all I would want to discover from this experiment was that the trend I found in nichrome is true for all metals. I would expect it to be true, as there is strong scientific proof behind it, which I have explained fully in my prediction: All conductive metals share a similar structure, and their free electrons behave in the same way, so it follows that they should behave the same.