Deciding the number of cells
Deciding the thickness of the wire
I chose a thin wire because the more thick that the wire is, the more collisions occur and therefore an increase in resistance as a result of the circuit heating up too quickly.
We additionally decided to use constantan wire instead of copper wire as the constantan wire heated up less, because of a lower current passing through it, and therefore a lower number of collisions. This was ideal because we wanted the temperature to remain constant, as explained in the ‘scientific theory’ section.
Deciding the type of wire
The number of repeats for each individual measurement of resistance over a range of answers was decided to be as high as possible without being to time consuming. In the end we decided to use 3 repeats per each individual measurement, because it was just about enough to avoid anonymous results and to get accurate results.
Digital voltmeters and ammeters was the decision I made regarding apparatus. This was because multimeters required continuous changes and we tended to receive over - high results. Similarly, analogue ammeters and voltmeters proved to be difficult to read off (because of fluttering results) and inaccurate.
Procedure
Method
Prepare apparatus for experiment, including digital ammeters and voltmeters, one cell, a thin constantan wire, crocodile clips (to secure the wire), sticky tape(to hold down the wire so it can be stretched from one end of the metre ruler to the other so that the wire is straight and ‘kinks’ are not present) and a meter ruler(with measurements of centimetres and millimetres, so that the results are recorded accurately). Then place the thin constantan wire between two crocodile clips, of which one is linked to the voltmeter and one is linked to the ammeter. Following this, it is necessary to check whether a complete circuit has been set up. Then, at intervals of 10 centremetres, measure the ammeter and voltmeter readings, changing the lengths between 0 and 100 centremetres. Do this three times, resulting in three results from the voltage and three results from the ammeter. Then draw up a table presenting these results, followed by a graph, plot these results and with the line of best fit drawn on.
Diagram
Fair Test
The concept of keeping the investigation as a fair test is important because it accounts for the accuracy of the results, and can explain anonymous results. The only factor that should be varied is the length of the wire, obviously because that is what the main aim of the investigation is: to investigate how the length of a wire affects its resistance. We need to measure the resistance at different lengths of the wire to see if there is a relationship between resistance and length, and so we need to keep changing the length of the wire to do so.
The amount of voltage supplied to the circuit must also be constant, because a change in the voltage would eventually lead to a change in the current flowing around in the circuit, which would consequently result in a temperature change within the circuit.
Furthermore the type of wire must be kept as constantan, because each different wire has its own specific ion arrangement and structure, and therefore there may be a different amount of ions and electrons, which could affect the number of collisions, and therefore the resistance. Similarly this is why the thickness of the constantan wire needs to remain constant - because the thicker the wire, the more ions and electrons present, also meaning the more chance of collisions, resulting in a higher resistance. The number of cells must be constant, because if this is changed each time so will the temperature, and therefore the resistance. Additionally over a long period of time too many cells could be proclaimed as a safety hazard, because they circuit could be over – heated.
Temperature additionally needs to be kept constant, and is the most important factor of the fair test concept. This is because temperature affects the activity of the ions and electrons inside the metal wire, and the higher the temperature, the more chances of collisions there are, (because the electrons are moving faster and the ions are vibrating more) and therefore the higher the resistance.
Accuracy And Precision
Metre sticks and rulers with measurements of both centimetres and millimetres will be used for the final experiment, so that anonymous results can be avoided. Additionally the digital ammeters and voltmeters are used as these give the voltage and currents figures correct to 2 decimal places, assuring us that the results are fairly accurate. For each variable length, the experiment is going to done 3 times, and then an average is going to be worked out from these results, so that if one result is wrong, the other two are most probable to bring the average to a near – accurate number.
Safety Measures
For safety, we will make sure that the wire is not touched by out fingers during the experiment, because it is going to get hot due to the higher number of collisions as the length increases. Additionally we need to make sure that there are no technical difficulties with the power pack, so that the electricity coming into the circuit is flowing safely. All liquids should be kept well away from the experiment, because if it comes in contacts with the electrical supplies, sparks will be given off at the minimum, and at the extreme it could cause a fire. Similarly food and drink should not be present near the circuit. Furthermore bags should be tucked under the tables and any other obstacles should be dealt with and move out the way, so no careless accidents occur during the experiment.
Results
*N.B. All Values are given to 2 decimal places and therefore are rounded off.
Conclusion
I feel that with evidence from my graph, there is weak positive correlation, evident through the numerous anomalous points on the graph. However I have not said that there is no correlation because these anomalous points are not too far away from the best line of fit in the whole. Despite these results, it is still evident that there is a relationship between the length of the wire and the resistance – as the length increased so did the resistance. Therefore we can assume from my results that the longer the wire, the more ions and electrons there are present, and therefore the more chance of collisions, (because the ions vibrate faster and the electrons move faster, and also because of the increase in the numbers of electrons and ions). The more collisions there are, the higher the temperature gets, (therefore we can also assume the longer the wire, the higher the temperature) resulting with a higher resistance.
I think that my results support my prediction in the way that there is a definite relationship between length and resistance - as length increases, so does resistance. Similarly to my prediction, my line of best fit goes through the origin of the graph (0,0). However with the idea of the length of the wire being proportional to the resistance, my results do not show sufficient evidence to support this. For example, when the length of the wire is 40, and we double it to 80, 2 is the constant. If we followed that the length of the wire is directly proportional to the resistance, then the resistance should double too. However it doesn’t, and instead goes from 5.52 ohms to 14.32 ohms; so instead of the constant being 2, it is 2.60 (2 decimal places). However this is evidence is strong enough to prove the original relationship.
Constantan was a good metal to choose because the temperature changed little as the resistance increased, and therefore a fair test was maintained in that aspect. The manufacturers of the ‘Griffin’ constantan wire at thickness of 36 SWG claimed there to be 16 ohms per metre (as seen in the ‘Griffin Education Catalogue’. However from my results I received 18 ohms per metre. This may be due to my human error, but additionally the wire that we were provided with for this investigation may not have been purchased from the ‘Griffin’ company, and therefore may vary because of the ionic configuration.
Evaluation
I feel that with evidence from my graph, there is weak positive correlation, evident through the numerous anomalous points on the graph. However I have not said that there is no correlation because these anomalous points are not too far away from the best line of fit in the whole. Despite these results, it is still evident that there is a relationship between the length of the wire and the resistance – as the length increased so did the resistance.
The main anomaly point (as labelled on the graph) is for the length of 40cm, where the result is too low in comparison with the line of best fit. The experiment was easy to set up, but the problems can be seen in the results, where there are too many anomalous results, and the points in comparison with the line of best fit is reasonably weak.
From the results table, we can see that most of the individual results were close to their average, and so there must have been something wrong in the way I conducted the experiment to receive such anomalous results. My results do however show some accuracy in that they follow more of a straight line then a curve, however only 3 out of the 9 points lie precisely on the line of best fit. The main anomaly was at the length of 40 cm, and in general the results do differ from my line of best fit in that the majority of the values are either too high or too low.
The investigation was suitable in that it was a safe and all the measures were followed, so there were no hazardous conditions. The main weakness of the investigation was inevitably the results, which were not really following my prediction where I claimed that the length of the wire was proportional to the resistance; and also there are too many anomalous results. To make my results more reliable, I will have to disregards the anomalous results, which leaves me with 5 points, but however these 5 points to support my prediction and are directly proportional and do show a clear relationship between the length of the wire and the resistance.
If I could improve this experiment or do it again, I would do at least 5 trials for each measurement, so that the average would be more accurate. Also I would need to purchase a voltmeter or ammeter that was not giving the values to 2 decimal places, and maybe if there were 5 decimal places my results could be precise to its maximum. I used the sticky tape to stretch the wire against the ruler so that there were no ‘kinks’, but maybe this was done incorrectly and the wire may have been out of shape slightly at a couple of points. To improve this I could get the wire fresh from the wheel, so that it is straight and ‘unkinky’ from the original production. I could iron the ‘kinks’, except this would affect the thickness of the wire if it got stretched too much, and so that would not lead my experiment to be a fair test because the only variable is meant to e the length, and the thickness of the wire is meant to remain constant. Similarly if I put a couple of 100-gram masses of books on the wire to straighten it out, the probability is that the wire would become thinner, and then my results would be inaccurate. Additionally I could have used thinner croc clips, because the end of the clip would be more precise and almost to the correct millimetre.
Additionally if I chose a different metal, for example copper, then I would have obtained different results because each metal has a different ionic arrangement; and the number of ions and electrons affects the temperature and therefore the collisions, resulting in a change in resistance. Furthermore, I could have chosen to do the experiment with a non – metal, so see what kind of results I would obtain then.
As we go across the periodic table there are more electrons, therefore meaning that each of the metals are denser, because the nucleuses get larger. Therefore the metals would have a high atomic number, and resistance would become higher, because if the electrons and ions were larger there is more chance of collisions and the collisions become larger.
