2. The first crocodile clip is clipped to the wire at the 0cm position on the metre rule.
3. The second crocodile clip is clipped to the relevant position depending on the required length of wire.
4. The power supply is turned on. The voltage and current are then read off the ammeter and voltmeter, and recorded.
5. The power supply is then turned off and the second crocodile clip is moved to the next position.
- Collect apparatus: a voltmeter, an ammeter, 5x wires, 2 crocodile clips, 10, 20 and 40 cm of both nichrome and copper wires and a power pack.
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Set apparatus up as shown:
- Set the power pack on as low a voltage as possible. (So that there is not too high a current passing through the circuit.)
- Place the 10 cm of nichrome between the two crocodile clips to complete the circuit.
- Turn on the power pack and record what the ammeter and voltmeter read.
- Replace the 10 cm of wire with the 20 cm of nichrome remembering to keep the voltage the same. Turn on your power pack and record what the ammeter and voltmeter say.
- Change the wire to the 40 cm of nichrome wire and repeat the experiment.
- Do the same for the copper wires.
- Work out the resistance for all the results using Ohm's law. V = I*R
- Record your results in a graph.
Equipment
Diagram
Fair Test
required lengths, because the cuttings might not be accurate and affect the results. Instead, I would use one 1 metre wire, and use a crocodile to slide on the wire to change the lengths of the wire that is connected to the circuit.
· Use the same wire throughout the experiment, to keep SWG exactly the same.
· Use an ammeter and a voltmeter to measure the figures accurately and to 2 decimal places.
· I would do the experiment in the same environment every time, so the temperature would be the same, so the resistance of the wire is not changed.
· Use different voltages (2V, 4V, 6V and 8V) as repeats to avoid anomalous results.
The factors, which must stay constant to keep the experiment a fair test, are:
· The power supply must stay on 2V
· The wire must be the same thickness
· The surrounding temperature must be constant
· The equipment should be kept the same
To keep this experiment as accurate as possible we need to make sure, firstly, that the length of the wire is measured precisely from the inside edge of the crocodile clips, making sure that the wire is straight when we do this. We must also make sure that the wire is straight when we conduct the experiment. If it is not, short circuits may occur and bends and kinks in the wire may affect the resistance. The reading that we take of the voltage should be done fairly promptly after the circuit is connected. This is because as soon as a current is put through the wire it will get hotter and we want to test it when heat is affecting it the least, i.e. at the beginning.
I will make sure that my test is fair by using the same apparatus each time, by repeating the test for each length three times and then taking an average and by making sure I only change one variable each time. This variable will be the length, I will keep the temperature, thickness of wire and the circuit constant each time.
I have decided that I am going to use a total of 6 different lengths of wire when I collect my results. I have chosen a range of 6 as this will allow me to plot an accurate graph. I will repeat each length 3 times and then take an average.
Measurements
To collect the data for my graph I have chosen to take a range of 5 lengths. I have chosen a range of 5, because to plot an accurate graph I will need at least 5 points to mark on the graph . I have also chosen to take 3 repeats at each length and then take an average. I am going to do this as this will be more accurate and should get rid of any anomalous results.
When I have taken all my readings, I will calculate the resistance of the wire.
The resistance of a length of wire is calculated by measuring the current present in the circuit (in series) and the voltage across the wire (in parallel). These measurements are then applied to this formula:
V = I ´ R where V = Voltage, I = Current and R = Resistance
This can be rearranged to:
R = V/I or
Safety
This is a experiment with very low risks, but precautions must still be taken:
· The lengths of the wire in the circuit must not be lower than 40 cm to prevent over heating.
· Make sure the surrounding environment is free of liquids, to prevent leak of electricity from circuit and forming a short circuit.
To ensure a safe experiment the volts must be kept to a safe level to stop the wire getting too hot and burning which is very dangerous.
· I will handle the power supply carefully.
· I am going to only use a voltage of 2 volts.
· I will be careful when handling live wires.
In order to be as safe as possible I will make sure that my experiment is set up correctly and I will be careful when handling my power supply. Although there is no need for safety goggles in this particular experiment, as I will not be using any chemicals or naked flames, I will take the usual precautions regarding my uniform, ie. Tucking my tie in, just to minimise chance of any accidents happening. I will also, obviously, be careful that I do not conduct my experiment anywhere near water as water is a good conductor of electricity
Range of Readings
Prediction
I predict that as the length of the wire increases the resistance will also increase.
- The longer the wire, the higher the resistance. This is because the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions (as heat).
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Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions.
An atom consists of a nucleus and orbiting electrons. These electrons can create a flow of current, so the more free electrons there are, the more conducting capability that material has; thus copper is more conductive that iron. Alloys tend to have less free electrons so they will be less conductive. The order of resistance is consequentially copper, iron, constantan then nichrome increasingly.
Wires with wider diameter have more free electrons because the cross-section surface area is larger in proportion to the length, so the wider the wires are, the less reactive they would be. Resistance is proportional to the cross-section are of the wire given that the length and the material should be the same.
Longer wires will cause an increase in resistance because the electrons have to travel past more atoms and collisions between the electrons and the atoms are more likely then in shorter wires. Resistance should also be proportional to the length of the wires.
Results table
Graph
Evaluation
The inaccuracy could have been because of the wire coming from a different manufacturer to the predicted results. The ammeters and voltmeters could have been damaged and reading falsely on both the meters used. But if I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings, and more readings so that a more accurate average can be taken. But if I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings, and more readings so that a more accurate average can be taken.
- As mentioned previously, the biggest downfall of the investigation was the apparent mistakes when choosing the wire, in that they would appear to be of differing diameters. This did not, in this case, cause a big problem as the same wire was used for each set of results so it is known that the results for each wire are correct.
- Generally speaking, wire 1 would appear to contain the most accurate results due to the fact that all of its points bar one sit on the line of best fit for that wire. The only one that does not is the point at 90cm, which was exactly at the point that the black mark (mentioned previously) was found to be.
- Wire 2, on the other hand, had three main anomalous results: at 50, 80 and 90cm. They are by no means that far off but in an experiment such as this, which is generally a very accurate one anyway, such anomalous results should not be quite so common. Possible explanations for these anomalies are as follows:
- The length of wire for that particular measurement was not correct. At 50 and 80cm it is possible that the length was shorter, causing a lower resistance, and at 90cm it is possible that it was longer, causing a higher resistance. The solution to this is to measure the lengths more carefully and ensure that the wire is pulled tight against the metre rule.
- For a particular result, one or more of the connections could have been faulty, causing extra resistance at the connections. A solution to this would be to, before each experiment, connect the connections together without the wire in place and measure the resistance then. If it is higher than it should be then the connections could be cleaned.
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Whilst extremely unlikely, it is conceivable that the power supply was providing a different voltage for some of the results. This is unlikely to be a problem in this investigation but it might have been an issue had we used batteries instead.
NB: If one were to assume that Ohm’s Law applies, then another possible explanation could be that at some points (more likely in the lower lengths), the wire was not allowed to cool completely so that the temperature was higher for that measurement. Whilst unlikely (due to the two sets of results), this would cause a higher resistance as explained previously. However, it is now known, after researching the metal alloy “constantan,” that the resistivity (the electrical resistance of a conductor of particular area and length) of this alloy is not affected by temperature. Therefore, in these experiments Ohm’s Law does not apply.
Evaluation Experiment one: This experiment was quite accurate, as when it is compared to the manufactures line which is on the same graph, we can see that this line is at most only 0.4? different form the manufactures line. This is a percentage difference of approximately 8%, using the formula: Difference ? original X 100 This shows that the results were good, as 8% is a very small margin of error. The error bars on the graph show that the most inaccurate result was the 60cm result. This could be down to an error in the measurement of the wire or a temperature rise. The two results for 100cm are exactly the same, and it is near to the manufacture’s line, so this is the most accurate point.
The other three readings have almost the same inaccuracy, an average of 10%, which again, is fairly accurate. The inaccuracy could have been because of the wire coming from a different manufacturer to the predicted results, as there is some discrepancy between the amount of copper and nickel in different brand’s wire. The ammeters and voltmeters could have been damaged and reading falsely on both the meters used.
Measuring the lengths of the wire is also a inaccuracy as the rulers used are not exact, and it is difficult to get an accurate reading of length by eye, as the wire might not be completely straight, it may be of different thicknesses throughout the length. These would have contributed as well to the error. These results would be difficult to improve on as they are reasonably accurate, and there were no anomalous results. But if I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings, and more readings so that a more accurate average can be taken.
I would also investigate other factors, such as temperature, voltage and current, and see how these effect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance. As these results had a range of only 5 readings, from 0-100cm, and were only repeated twice, and that the results are not 100%, accurate due to the errors discussed earlier, then I would say that these results are not strong enough to base a firm conclusion on because there are so many sources of error, which are explained earlier.
Experiment two - These results were not as accurate as experiment one. I had predicted that the resistance should halve as area doubles, which it does, however not to the predicted curve. When the resistance is 24ohms, the % inaccuracy is 6%, and when the resistance is 6 ohms, the inaccuracy is 8%. These inaccuracies are fairly large. The error bars, however, are too small to be drawn accurately on the graph. They are at most 3% inaccurate, using the same formula as before. This suggests that the inaccuracies were not experimental, but permanent errors due to problems with the measuring equipment.
These results were this inaccurate as the tool used for measuring the diameter of the wire were very inaccurate due to a zero error on the screw reading, i.e. the mark given for zero mm was not the real mark, hence throwing all the results off by the same amount. The ammeters and voltmeters could have been damaged and reading falsely on both the meters used. Measuring the lengths of the wire is also a inaccuracy as the rulers used are not exact, and it is difficult to get an accurate reading of length by eye, as the wire might not be completely straight, it may be of different thicknesses throughout the length. These would have contributed as well to the error.
There was one slightly anomalous result, at 0.25mm2. This could have been due to a unique error in the measuring and or reading of the meters, or a temperature change. These results could be done better. If I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings, and more readings so that a more accurate average can be taken. I would also investigate other factors, such as temperature, voltage and current, and see how these effect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance.
As these results had a range of only 7 readings, from 0.1mm2, and were only repeated twice, and that the results are not 100% accurate, due to the errors discussed earlier, then I would say that these results are not strong enough to base a firm conclusion on because there are so many sources of error, which have been explained earlier.
Conclusion
From the results I can conclude that if the length of the wire is increased the resistance will also increase. The longer the wire the more atoms there are in the wire so the greater the chance of collisions between the atoms in the wire and the electrons in the current meaning the resistance has increased. If a 10-cm length of wire is doubled to 20 cm there will be double the amount of atoms and so double the resistance. The line of best Fit is a straight line proving that the resistance of the wire is proportional to the length of the wire. The length of the wire affects the resistance of the wire because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion.
Having performed the investigation, the following conclusions were drawn:
- As predicted, an increase in length resulted in an increased resistance. This can be clearly said for both wires tested.
- Both wires show a strong trend of a straight line, i.e. the length of the wire is shown to be directly proportional to the resistance – double the length and the resistance doubles.
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The overall resistance of the two wires seems to differ considerably. Due to the strong correlation of the results, the explanation of this is unlikely to be the method used to obtain the results. The more likely explanation would be that the first wire was actually of a larger diameter than the second one. Obviously this is a rather important oversight and this will be discussed more in the Evaluation section. The reason why this is the likely explanation is because resistance is known to be inversely proportional to the cross-sectional area, i.e. if you increase the cross-sectional area (by increasing the diameter) then you decrease the resistance. This is because a wider wire means less likelihood of the free electrons having collisions and losing energy.
It is important to realise, however, that despite the fact that it would appear that the resistance of wire 2 is double that of wire 1, that does not mean that the diameter is half that of the wire 1. That is because if you halve the diameter then you decrease the area by a factor of about 3 (A = πr2)
The graph of experiment 1 is a straight line through the origin, which means R is directly proportional to L. This means that if the length is 40cm, and resistance is 2?, then if length is doubled to 80cm, resistance also doubles to 4.
This is because of the scientific idea, stated in the planning that if you double length, you double the number of atoms in it, so doubling the number of electron ‘jumps’, which causes resistance: The results support my predictions well, the results turned out the way I had expected, they match the predicted line well. I had predicted a straight line through the origin, which means R is directly proportional to L.
The graph of experiment 2 is an inversely proportional curve. This is because R is directly proportional 1/A, this means when A doubles, R halves. for example when the Area is 0.025mm2 the resistance is 4.8. When A doubles to 0.05, R halves to 2.4?. When A doubles again, R halves again to 1.2. This is because, as stated earlier: We see that if the area of the wire doubles, so does the number of possible routes for the current to flow down, therefore the energy is twice as spread out, so resistance might halve, i.e. Resistance is directly proportional 1/Area.