The temperature of the wire: as the temperature of the wire increases, so does the resistance. This is because as the temperature increases, the ions in the metal conductor gain more energy and vibrate more vigorously. This obstructs the flow of electrons even more. Therefore as the rate of the flow of electrons decreases, the resistance increases.
The length of the wire: as the length of the wire increases, the resistance also increases. If the length of the wire doubles, then the resistance will also double as twice the length of wire will be equivalent to two equal resistances in series. This means that there will be more ions to obstruct the flow of electrons, which increases the resistance.
Preliminary Investigation
In this investigation, I am going to investigate which type of wire is the best to use in the experiment to find how length affects the resistance of a wire. For this, I will find out which type of wire has the highest range of resistance. The materials, which I will test, will be Nichrome, Constantan and Manganin. For the materials, the highest swg number and the lowest swg number will be used. This stands for standard wire gauge. The swg of a wire gives an indication of its cross sectional area, measured in mm2. The higher the swg number, the smaller the cross sectional area of a wire. The table below shows the diameter and cross-sectional areas represented by the swg numbers:
Apparatus
D.C. power supply
Nichrome wire
Constantan wire
Manganin wire
Analogue voltmeter
Digital ammeter
Crocodile clips
Wire cutter
Wires
Ruler
Method
To keep the experiment a fair test, the current will be kept at 0.4A. This is so that the wire does not get too hot (temperature affects the resistance of a wire) and the results do not become inaccurate. Therefore the current is kept low and constant. The circuit was set up as shown in the diagram on the next page. The highest swg and the lowest swg of each wire were cut at exactly 10cm and 30cm to find the range. Each wire in turn was placed into the circuit. The power pack was turned on and the reading on the voltmeter was noted down. The wires were held together with crocodile clips and were stretched out to make sure there were no bends in the wire. The results of each were recorded and the resistance of each of the wires was found using the equation:
Resistance = Voltage (V)
(Ω) Current (A)
Results (the current was kept constant at 0.4A)
Conclusion
As the results show, Nichrome 32swg had the highest range of resistance from 30cm to 10cm. For the main investigation, I will investigate how the length of a wire (Nichrome 32swg) affects its resistance. I am going to use 32swg as this has the largest resistance range. As 32swg is the narrowest wire, we can also learn that the narrower or thinner the wire, the larger the resistance.
Fair test
- The current was kept constant throughout the investigation to ensure that the results were not affected by a change in the amount of current supplied, and by the temperature of the wire.
- Two lengths of wire were used, at 10cm and 30 cm, so a range of lengths could be used to make the results more accurate.
- The temperature was kept the same, at room temperature, to ensure the final value for resistance was accurate and not affected by temperature.
Safety precautions
- The power pack will be turned on and off as quickly as possible to prevent the wire from over heating.
- The circuit will be connected up securely and will be checked before being used, as any lose flow of electrons could be dangerous.
- When cutting the wire with the wire cutters, care will be taken to prevent being cut by the wire cutters.
Prediction for main investigation
I predict that as the length increases, the resistance will also increase. However, I also think that the length and resistance will increase in proportion i.e. as the length of the wire doubles, the resistance will also double. This is because twice the length of wire will be equivalent to two equal resistances in the series. This means that there will be more ions present to obstruct the flow of electrons, which will increase the resistance. As the length increases, the number of collisions the current carrying charged particles make with fixed particles increases and this increases the resistance.
Main Investigation
I am going to investigate how the length of a wire (Nichrome 32swg) affects its resistance.
Apparatus
D.C. power supply
Nichrome 32swg wire
Analogue voltmeter
Digital ammeter
Crocodile clips
Wire cutter
Wires
Metre ruler
Method
To keep the experiment fair, the current will be kept at 0.4A. This current is not too high and will not over heat the wire so the temperature will not affect the resistance. The circuit will be set up as shown in the diagram. The lengths that will be investigated will be: 10cm, 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100 cm. These will be used so that a wide range of results can be obtained. Each wire will be placed in the circuit. The power pack will be turned on and the reading on the voltmeter will be recorded. The wires will be connected to the circuit using crocodile clips. The wires will be stretched out to remove any bumps or bends. The whole experiment will be repeated once, as an average result of the readings is more accurate than a single reading.
Results (Nichrome 32swg wire)
Analysis
As the graph shows, as the length of the wire increases, the resistance also increases. As the graph shows they also rise in proportion. This can be measured at 20cm, 40cm and 80cm, for example. At 20cm, the resistance was 3.6Ω. At 40cm, the resistance was 7.3Ω. At 80cm, the resistance of the wire was 14.9Ω. From these results, we can see that as the length doubles from 20cm to 40cm and then again to 80cm, the resistance is also doubling.
The results support the prediction. I predicted that as the length of the wire is increases, the resistance would also increase. I also predicted that they would rise in proportion i.e. as the length doubles, the resistance would also double. The accuracy of the results can be seen on the line of best fit on the graph.
From the line of best fit, we can also see that only two anomalous results were recorded. However, these results are only slightly anomalous and do not affect the overall line of best fit. The results still support the prediction despite these results.
The length and resistance of the wire rose in proportion because, twice the length of wire will be equivalent to two equal resistances in the series. This means that there will be more ions present to obstruct the flow of electrons, which will increase the resistance. As the length increases, the number of collisions the current carrying charged particles make with fixed particles increases and this increases the resistance. As the material, temperature and cross sectional area of the wire were kept constant throughout the experiment, it is clear that the resistance should be directly proportional to the length. Everything was kept constant so the results would not be affected.
Evaluation
In this experiment, I was investigating the factors that affect the resistance of a wire. For this investigation, however, all the factors were kept constant except the length. The factors that were kept constant were temperature, cross-sectional area of the wire and the type of wire being used.
From the graph, it could be seen that most of the results were accurate with the exception of two results, which were slightly inaccurate. This could have been because the length of the wire was not cut to the exact measurement.
There were a few limitations to the experiment. An exact length was slightly hard to obtain as the wire had many bends in it. Most of these were removed but tiny bends were harder to get rid off. Also, because the investigation was carried out over a period of time, the temperature on different days could have been slightly different. Another limitation was that in the experiment, a 0-10V voltmeter was used. This was harder to read as the scale went up in every 0.2V. this would have made the results more inaccurate as it was harder to tell the exact reading.
There are different ways in which the method could be improved to make the whole experiment more accurate. In this investigation, only one factor was investigated however, other factors could have been investigated. Also, different types of material could have been tested to see if the theory: Resistance ∝ Length remains true. Also, a more accurate or sensitive voltmeter could have been used to make the voltage readings more accurate. An ohmmeter could have been used as well as the method which we used, so that the two sets of readings could be compared. From this we could have seen how accurate the sets of results were. The resistance of the wire could have also been measured using the equation:
Resistance = L
A
The results for that could have also been compared with the results obtained from the original investigation.
Overall, I think that the investigation was carried out fairly and accurately and this can be seen by the results, which support the prediction and the theory that length is proportional to resistance.
BIBLIOGRAPHY
Microsoft Encarta 96 Encyclopaedia
GCSE Science Assessment Sc4
