Preliminary Test
Before I start my real test I will first undertake a preliminary test in order to test that the circuit works and also to get used to using it and recording data. I will record some brief results in order to use as a benchmark for when I undertake my real test in order to check that I am doing it right. My results from my preliminary test are as follows:
My table from my preliminary test will give me a rough and brief, scaled down guide to what my real results table should look like and provides a benchmark for my results. It also illustrates that resistance increased with length. I believe that my data is accurate because I have repeated the test three times in order to get a well-rounded average – this is what I will do in my real test.
Prediction Aim: To investigate how the length affects the resistance of a wire.
I predict that as the length increases then the resistance will also increase in proportion to the length. This is due to the idea of the free moving electrons being resisted by the atoms in the wire. In a longer piece of wire, there would be more atoms for the electrons to collide with and so the resistance would be greater. The relationship between the wire length and the resistance should be approximately proportional. This is because in a wire twice the length of another wire there would be double the amount of atoms causing the resistance. In theory, if the wire is doubled, then so will the resistance. If the length is twice as much, then there will be twice as much collisions, which would increase the resistance.
An example of this would be in a 20cm wire. The electrons would have to travel double to distance they would have to go through a 10cm wire. This would in turn double the amount of atoms that the electrons would collide with and then resistance would double. Furthermore, my preliminary test backs up my prediction because it illustrates that resistance increased with length.
I can further support my hypothesis because I know from my scientific knowledge and research that:
Electric current is the movement of electrons through a conductor. In this experiment a metal wire (Nichrome will be the conductor). So when resistance is high, conductivity is low. Metals such as Nichrome conduct electricity well because the atoms in them do not hold on to their electrons very well. Free electrons are created, which carry a negative charge, to jump along the lines of atoms in a wire, which are in a lattice structure. Resistance is when these electrons, which flow towards the positive, collide with other atoms; they transfer some of their kinetic energy. This transfer on collision is what causes resistance. So for example, if we double the length of a wire, the number of atoms in the wire doubles. This therefore, increases the number of collisions and energy transferred twice, so twice the amount of energy is required. This means the resistance is doubled.Based on my hypothesis, I predict that my graph should show that the length is proportional to the resistance.
Fair Test:
In order to receive some reliable and accurate results I will have to make sure I conduct a reasonably fair test. When measuring resistance in the length of a wire the independent variable is the length of the wire and the dependant variable is the resistance. To carry out a fair test the factors that I will try to keep the same during this experiment are: the temperature, the thickness of the wire, the length of the wire and the type of wire. I will also use the same piece of wire if I did not finish the test on the same day in which I started it and had to carry on with the test another day.
Firstly, in terms of temperature I shall be using a low voltage so that the wire does not get too hot and create much resistance, which would thus affect my results. This heat energy is transferred from the kinetic energy of the free electrons when they collide with atoms in the metal. The kinetic energy is dissipated through the metal in vibrations, which generate heat due to friction. This low voltage should reduce any feedback resistance generated by heat of the wire generated by the resistance created by the wire itself. I will try to follow Ohm’s Law: the resistance of a metal wire or a piece of carbon will stay the same, as conditions are not changed e.g. temperature. I will use the same voltage, wire width and size and I will repeat experiments to find an average in case one piece of wire has impurities in it. I will use the same voltmeter and ammeter and the same wires to connect the components as some may have more resistance than others. I will also try to conduct my experiment within one hour so that day-to-day climatic changes will not affect my results as temperature can affect the resistance of a conductor. I know this to be the case because in my research I have previously observed experimentation with thermisters. When a substance increases in temperature, its atoms vibrate more. This means that the free electrons, which are free to move throughout the crystalline structure of a metal, are more likely to hit a fixed vibrating atom, thus slowing it down and reducing the current of the electricity. I have found from prior research and experimentation that if I were testing different widths of wire, the thinner ones would have more resistance. Therefore, I will use the same thickness of Nichrome wire for my whole test. Furthermore, if I experience any anomalous results I will not repeat them, as I would not be conducting a fair test, as I would have repeated these results twice and all of the others only once. Therefore, if I do receive any anomalous results I will leave them alone because when I calculate the average current for the three tests I will be able to observe any serious anomalous results on my graph.
Wire Information
The label on the Nichrome wire in which I used stated that the wire’s diameter was 32swg constantan. From my research I have found out that the wire should give approximately 4-7Ω of resistance for every 1m of wire. To see if I have conducted a reasonably fair test I will see if this is shown in my results table later on in my experiment, I will explain the result in my analysis.
Safety Precautions:
There are not many safety precautions that need to be taken into consideration, in this experiment. The main precautions I can think of are stated below:
Do not carry out the experiment in wet areas, as water is a very good conductor, and thus could be very dangerous. Do not touch the wire when the Power Pack is switched on, because the current would heat up the wire. Do not hold the crocodile clip on the 10cm measurement for too long, as it would burn out the wire and possibly cause a hazard. Use a low voltage for the experiment (4 Volts) to reduce the risk of any hazards such as the ones stated above.
- I am now ready to undertake my experiment. I will record my results from my experiment onto a results table. In this results table I will record the data from my chosen points (50 points ranging from 10cm to 500cm). I will record the voltage and the current passing through each section of wire. I will repeat the test three times to get three different readings of current and I will then find the average current. I will then, using Ohms Law calculate the resistance of each chosen point. I will then be able to illustrate my results in my results table by plotting them on my length and resistance graph. The results table and my length and resistance graph are on separate pages.
Analysis and conclusion
My length and resistance graph shows that resistance is very roughly, approximately proportional to length. On my graph there is a reasonably strong positive correlation, which is illustrated by my line of best fit. This means that the results here are closely related, so when the length of the wire increases, the resistance also increases. The results are also approximately proportional, with the line of best fit traveling roughly through the origin throughout some of the graph, meaning that when length doubles here, so does resistance (roughly). An example would be at the wire length of 150cm, the resistance is roughly 5Ω and at 300cm it is roughly 10Ω. This is approximately double the size (on a rough scale). I have also to an extent proved my hypothesis, which predicted that as the length increases then the resistance would also increase in proportion to the length. This is because of the scientific idea, stated in my hypothesis that if you double length, you double the number of atoms for the electrons to collide with and so the resistance doubles: The results support my prediction quite well, however they are not exactly what I predicted. I had predicted a roughly straight line through the origin, which means Resistance, is directly proportional to Length. I did not get a direct straight line through the origin, however, I have identified that this is because I was not controlling the voltage properly in my test, this has resulted in a slightly low gradient throughout as I was not feeding enough voltage into the circuit (I was leaving the voltmeter as it as instead of adjusting it back to 4v each time I moved down the wire). I will go into this in further detail in my evaluation. The line of best fit shows that the results followed the expected pattern quite well to an extent, but the gradient was lower than what I predicted which meant that in most places length and resistance were not proportional. However, on the line of best fit, which travels throughout most of the graph, the points are quite close, touching the line in some cases. Even though they were not quite what I predicted this shows how my results were fairly constant, as the gradient remained the same throughout most of the graph.
Earlier on in my method I stated that from my research, the Nichrome Wire in which I was using (the wire’s diameter was 32swg constantan) should give approximately 4-7 ohms of resistance for every 1 meter.
I have taken these results off my results table and this is what they show: 0-100cm = 2Ω 100-200cm= 4Ω (Approximate Figures) 300-400cm= 4Ω 400-500cm= 7 Ω As you can see from my figures I have gathered from my results table, the wire did on average give 4-7Ω of resistance for every 1m (or 100cm). However, from 0-100cm the wire only gave 2Ω of resistance, a set of slightly anomalous results in terms of the predicted resistance. This may be due to a number of things such as manufacturing errors in the wire or even in the natural material, which the wire is made of. However, I think it may be because I have not conducted a fair test because I was not controlling the voltage right in the experiment, as I stated before I was therefore not supplying enough input voltage. This may have resulted in a decrease in current and therefore a decrease in resistance than what there should have been, as illustrated at the start of my graph. As illustrated towards the end of my graph and results table the resistance is not too much higher than the maximum of 7Ω, but is still enough to confirm that there is something in the experiment, which is not perfect. However, this may also be due to the voltage problem previously mentioned. Also, the facilities available to me, thou adequate to do the experiment, were not the same as is available to people from which I found my information. Therefore, I do not control all the variables, which the wire manufacturer could. For example, the exact temperature of the wire, the temperature of the room etc. Therefore, my results will vary slightly from those that were predicted for me.
Therefore, after studying all the evidence I have gained from my experiment, I think that, roughly speaking the length of a piece of wire does affect the resistance. As the length of the wire increases so does the resistance. I did with some of my results find that length and resistance were directly proportional. However, from my experiment I have not proved that resistance increases in proportion to length because of the error in which I made, by not controlling the voltage properly. As shown by my length and resistance graph the gradient is lower than what it should be and therefore, I cannot prove that length and resistance are directly proportional.
Evaluation
From my graph I can see that my results that I collected are quite reliable. I know this because my graph does not show any individual anomalous results. I can see on the graph that none of the results plotted are anomalous because all the points lie roughly along the same straight line, except for at the start of my experiment and towards the end; here my results stray slightly from the line of best fit.
As illustrated by my graph, my results at the start of the experiment were slightly higher than the pattern set and therefore, did not follow the line of best fit. However, this may have been due to the fact that when the Power Pack has not been switched on for long there is a lot of resistance in the wire and this may therefore account for the results not following the pattern. In addition, my results towards the end of my experiment also do not follow the line of best fit. However, I think that these results are correct and the results that follow my line of best fit are slightly wrong. I think this may be because I was not conducting a fair test. For example as I stated in my conclusion I was not conducting the test properly because I was not controlling the voltage properly as I was not feeding enough voltage into the circuit, I was not adjusting the voltmeter back to 4v each time I moved to a new point on the wire. As illustrated by my length and resistance graph, this has resulted in a slightly low gradient throughout, as I was not feeding enough voltage into the circuit to push the current. I think my results towards the end of my experiment are correct as they follow the proportional line of the origin. However, the line of best fit is not as steep as it should be if it were to travel straight through the origin and I am sure that this is because I was not keeping the voltage constant and in doing this I was not supplying enough voltage to transfer the current and therefore create the true amounts of resistance. Without the sufficient current and therefore resistance this has resulted in the slightly lower gradient than what it would have been if I had conducted a fair test, then the line of best fit would travel straight through the origin as length and resistance would be directly proportional.
Furthermore, when I was measuring the lengths of the Nichrome wire, my measurements might have been slightly inaccurate as the rulers used might not have been exact, and it was difficult to get an accurate reading of length by eye, as the wire was not completely straight, so it may have been of different thicknesses throughout the length. This would have contributed as a slight error in my results. Also, my readings may not be precise, as I could not get accurate readings off the ammeter and voltmeter because they were analogue devices. Therefore, without truly accurate readings this also may have contributed to a slight error in my results and therefore would not have made the test totally trustworthy. However, as this factor probably only made a slight difference, it would not have affected my results to a great extent.
Moreover, the crocodile clips and the connecting leads could have affected the fairness of the experiment. They are a different type of metal from the Nichrome wire and may have different properties and therefore different resistance. Therefore, the resistance of the Nichrome wire which showed up in my results was slightly more than it actually was. To solve this problem, I would have found out the resistance of the connecting leads and crocodile clips before each experiment and minus it from the overall resistance of the Nichrome wire plus the connecting leads plus the crocodile clips.
c = a - b
a = overall resistance of Nichrome wire, crocodile clips and connecting leads
b = overall resistance of crocodile clips and connecting leads
c = resistance of Nichrome wire
I believe the method of experimentation in which I used by finding three sets of results for the current and finding the average current, though time consuming (150 different results) accounted for more accurate results and because of this I believe it was a suitable procedure. However, I believe the overall accuracy of my results is not that good because as I said before I was not adjusting the voltage properly. However, I believe that the reliability of my experiment far from perfect but is more or less sufficient to support my conclusion. As I have mentioned before I have probably not conducted such a fair test because of various problems such as: not adjusting the voltage to the required input each time I moved up the wire, the inaccuracy of the ruler, the voltmeter and the ammeter and the varying resistances of the components: the crocodile clips and the connecting leads. However, as I have stated in my conclusion, length is very roughly or approximately proportional to resistance but my gradient is lower than it should be. This is illustrated by my length and resistance graph. I know that by not controlling the voltage properly this was the main downfall of my experiment and it puts the reliability of the test into question. However, as this was the main problem I believe that the reliability of my results support the conclusion because this problem of not supplying enough voltage or “push” for the current resulted in a gradient which was not as steep as it should have been and I stated that this was the case and the outcome of my results in my conclusion. Also, in line with my results I did not as such have any anomalous results, therefore, there are not really any other factors that play a big part in the reliability of my experiment and as I have explained the voltage problem and how it shares a correlation with the outcome I stated in my conclusion I believe that the reliability of my results do support the conclusion because I have based my conclusion on the trend shown in the graph which has been strongly influenced by the voltage scenario in which I experienced.
I believe the overall procedure and the way in which I undertook the experiment (calculating averages and using long lengths of wire to get more accurate results) could not really be improved on because I did not experience any anomalous results and it accounted for reliable results (not accounting for the trend set by the voltage problem). I also found that the experiment was quite easy to set up, as it was simple and uncomplicated. However, if I had to do the experiment again I would preferably use digital voltmeters and ammeters to get more precise readings. Also, I could definitely improve my experiment, preferably by controlling the voltage right! As shown in my length and resistance graph this error affected all my results and meant I was not able to prove that resistance increases in proportion to length.
Further experiments I could do which are related to the resistance in a wire, would be to see whether the following factors would make a difference in the resistance of a wire: (I have made a prediction for each factor from my own scientific knowledge on how I think the resistance would change in a wire for that particular factor):
" Wire width:
I think that if the wire width is increased the resistance will decrease. This is because of the increase in the space for the electrons to travel through. Due to this increased space between the atoms there should be less collisions.
" Temperature:
I think that if the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance.
"Material:
I think that the type of material of the wire will affect the amount of free electrons, which are able to flow through that wire. This is because the number of electrons depends on the amount of electrons in the outer energy shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high numbers of electrons causing a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase.
Resources I used to help me with this coursework are:
Science For GCSE – Graham Hill
Physics In Action – Hal Leonard (US)