- Cross-sectional area
I think that the larger the cross-sectional area, the lower the resistance. Using my pervious example of the person and the tunnel we can see why: it would be a lot easier for the person to travel though a larger tunnel where they could stand up as opposed to a narrower one where they could only crawl. It is therefore, I think, easier for the electrons to travel though a wire with a bigger cross-sectional area.
- Type of wire
Some wires are much better conductors than others are. For example, copper is a much better conductor than nichrome. The better the conductor, the lower the resistance of the wire. Using another example now, it is like a river flowing down hill. When the river bed is smooth, water will flow fast but if it is rocky, the flow will be slower and energy released in the form of noise. We find the same kind of effect in a conductor with a high resistance. It cuts down the flow of charge and energy is released. The conductor gets hot.
Some wires have more useful properties than others do. An example of this is constantan wire which is an alloy of copper and nickel and whose resistance is unaffected by temperature.
- Temperature
Temperature will affect resistance I think (except in the case of the constantan wire – see ‘Type of Wire’ above) because as the wire heats up, the electrons will gain kinetic energy and start to vibrate. This will make it harder for them to ‘travel’ along the wire making the resistance higher. During my experiments I may find that I have to pause so that they wire can cool down so that it doesn’t affect the resistance too much.
For my investigation, I am going to investigate two of the above factors thoroughly so that I may come to a conclusion about what affects the resistance of a piece of wire. I have chosen to investigate the length of the wire and the cross sectional area of the wire.
In my investigation, I am going to use two different methods to investigate each factor. In order to investigate how the length of wire affects resistance, I will attach 100 cm of my chosen sample of wire to a meter rule. To make sure that it doesn’t ‘gape’, I will attach it at four or five points along the ruler with masking tape so that it remains as close to the desired length and as straight as possible. The measurements will therefore be more accurate. Then, I will set up the circuit as shown in the diagram below:
As mentioned above, I am going to use two methods to test my theory. Firstly, I will simply use an ohmmeter to measure the resistance. This circuit is shown above in diagram 1. I will then repeat the experiment using a voltmeter and ammeter so that I can work out the resistance myself as described below. This will need a slightly different circuit as shown above in diagram 2. This will ensure that my results will be as accurate as possible as any mistakes will become clear with two different sets of results.
For my first reading, I will attach one crocodile clip at 0 cm on the ruler and the other one at 10 cm. For each recording I will move the second crocodile clip up by 10 cm so the for the next reading it will be at 20 cm and then at 30 cm etc. I will do this from 10 cm to 100 cm. I will also measure the resistance each time of the connecting wires, as these will affect my final results.
I will repeat this three times and if there is any inconsistency in the readings I will take an average to use when drawing graphs and analysing my results. I will then repeat my entire experiment using another piece of the same type of wire to aid me in producing results which are as accurate as possible. I will take an averages of the two experiments to use in my analysis.
Once I have completed my experiments concerning the length of the wire, I will progress to investigate how the cross sectional area of a wire affects the resistance. I will set up a similar circuit as before, shown below in diagram 1. I will take a piece of wire of about 50cm with a cross sectional area of 0.28mm. I will then attach two crocodile clips, as before, 10cm apart on the wire. I will record the resistance using an ohmmeter. Then I will fold the wire over on itself and connect the crocodile clips to both ‘bits’.
This means that electricity can flow through a wire which has now doubled in cross sectional area. I will again record the resistance using an ohmmeter. I will continue folding the wire over, connecting the crocodile clips and recording the resistance five times. I will repeat the whole experiment twice and again I will use averages when there are fluctuations in the readings due to temperature changes or other conditions which change. Then I will repeat with a second method, as before, using an ammeter and voltmeter. This circuit is shown below in diagram 2. I am going to repeat so many times so that I may gain the most accurate results possible.
I will then carry out the complete experiment as detailed above, with all the repeats, using a wire with a cross sectional area of 0.40mm.
When I take readings using my second method of an ammeter and voltmeter I will need to calculate the resistance of the wire. To work it out I will need to use a rearranged form of the equation:
Current = Voltage _ or I = V
Resistance R
Rearranged it gives us:
Resistance = Voltage or R = V
Current I
Preliminary Experiments
Before I conduct my experiments, I needed to do some preliminary work to determine which wires I am going to use. I had six options to choose from:
Constantan or Nichrome wire in the any of the following thickness:
0.28 mm or
0.40 mm or
0.56 mm
I did a similar experiment for length as detailed above for all six wires but I only took five readings (at every 10 cm) because I do not need such detailed results. I did not repeat the experiment. I then chose the thickness that offers the biggest range of readings, as it will be clearer to analyse my results if they are not too similar. Overleaf are my results.
The graph clearly shows that the Nichrome wire with a 0.28mm cross sectional area has the biggest range (2.3 Ω to 9.9 Ω giving a range of 7.6 Ω). I will therefore use this wire in my length experiments. However, if time permits, I will test as many of the six wires as possible meaning that I will have more results to compare and analyse.