An investigation into the resistance of a wire - FREE A GRADE GCSE Physics Coursework -
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In this investigation I am going to investigate what affect the resistance of a wire.
Electricity flows in metals. Metal wires are made of millions of tiny metal crystals. Each crystal’s atoms are arranged in a regular pattern. The metal is full of ‘free’ electrons that do not stick to any particular atom. They fill the space between atoms in a metal. When these electrons move they create an electric current.
Conductors have resistance, but some are worse than others. The free electrons keep bumping into atoms. A wires resistance depends on four main factors which are:
- The length of the wire
- Cross sectional area
- The temperature of the wire
I am going to investigate how the length of the wire affects the resistance. I have done a preliminary experiment to help me decide the best way to do my investigation. The results should also help me make a prediction.
Below are my results from the preliminary experiment (see table 1). I have taken three readings each from the Volts and current to make sure it is as accurate as possible
From the results I can see that as the length of the wire increases, the resistance increases as well. Furthermore I have noticed that if you double the length of the wire, the resistance is roughly doubled. E.g. when the length of the wire is 20cm the resistance is 3.14 ohms, when the length of the wire is 40cm the resistance is 6.18 ohms which is roughly double 3.14 ohms. In my main investigation I will see if this theory applies to my results.
I found that the apparatus I used was suitable but I think that I could possibly increase the range of my results to obtain very reliable results, perhaps maybe increasing the length of the wire by 5cm each time instead of 10cm.
Investigating the resistance of a wire
I am going to investigate the resistance of a wire in relation to its length.
I predict that the longer the wire the larger the resistance. This is because the free electrons in the wire have to bump into more atoms and therefore, it is harder for electricity to flow. Similarly, the shorter the wire the smaller the resistance because there will be less atoms for the electrons to bump into and so, it will be easier for the electricity to flow. Furthermore, the resistance of a wire is directly proportional to the length and inversely proportional to the area, so doubling the length of a wire should increase the resistance by a factor of two. This is because if the length of the wire is doubled, the electrons bump into twice as many atoms so there will be twice as much resistance. If this is correct the graph will show a positive correlation which is what I expect.
The apparatus I will be using in this experiment is as follows:
- 1 ammeter (to measure current)
- 1 voltmeter (to measure voltage)
- 5 x wires
- 2 crocodile clips
- Power pack
- 100cm nichrome wire
Firstly I will collect the apparatus I need and set it up as shown in the diagram below. Next I will set the power pack on as low voltage as possible to ensure that there is not too high a current passing through the circuit (which could potentially affect the results because the wire will get too hot).
I will place one crocodile clip at 0cm on the wire and the other at 5cm to complete the circuit. I will then turn the power pack on and record what the voltmeter and ammeter read. I will switch off the power pack, move the crocodile clip that was at 5cm up to 10cm and switch on the power pack. Again I will record what the voltmeter and ammeter read and turn off the power pack. I will repeat this method every 5cm until I get up to 100cm, taking three readings from both the voltmeter and ammeter each time to ensure it is a fair test. Also after each reading I will switch the power pack off to ensure that the wire doesn’t get too hot and affect my results.
Diagram 1 (see below)
To ensure it is a fair test I will record the voltage and the current three times every 5cm and take the average reading. This will ensure that there are no false readings and will also cancel out any anomalous results. I will also ensure that the wire does not heat up too much by firstly making sure that I do not set the voltage too high on the power pack and keep it the same for every reading, and secondly, making sure that I turn the power pack off after each reading. I will try to make this investigation as accurate as possible.
There are different variables which can be changed in this experiment – the independent variable. I am however, due to my line of enquiry, only going to change the length of the wire. The variables I will control will be the type of wire (resistivity) and the cross sectional area of the wire. I will also control, using the power pack, how many volts pass through the wire in the first place. Below is a table illustrating the effect of changing the variables (see table 2):
Table to show Variables (Table 2)
I will ensure the experiment is safe by making sure that all the wires are connected properly and that none of the insulation on the wires is worn. I will also ensure that there is a clear indication that the power is isolated by means of a switch and a L.E.D. I will stand up during the investigation to ensure that I do not injure myself if something breaks.
Below is a table of my results (table 3). As can be seen I have taken three reading and have also worked out the average (shown in red).
Table of results (table 3)
From the table above I can see that as the wire increases in the length the resistance increases to. This shows that the first part of my prediction that, ‘the longer the wire the larger the resistance’ was correct.
Also my theory that doubling the length of the wire increases the resistance by a factor of two is correct:
The table (table 4) to the right illustrates this:
I will make a graph to illustrate my results and so I can further analyse my results.
(make a graph from the table of your results)
...if the results are similar to those above...your graph should follow the same pattern as the statement below...
"The graph is almost a straight line and has a positive strong correlation which shows that, as the length of the wire increases the resistance gets bigger. It also shows that my results were consistent and that my prediction, “the resistance of a wire is directly proportional to the length and inversely proportional to the area, so doubling the length of a wire should increase the resistance by a factor of two,” was correct."
I am going to take a sample from my results and draw a graph to show error bars. I cannot use all the results because I have to enlarge the scale in order to illustrate the error bars
Overall I think that my results are very consistent. Most of the points were on, if not close to the line of best fit. There are a few points which are further away from the line of best fit than the others, but they still fit in with the general trend. There are no anomalous results which I would consider to be far away from the line of best fit.
There are possible sources of error which might have lead the results to be inconsistent such as a kink in the wire. This would have prevented the area of the wire from remaining constant and would have affected my results. However I made sure that the wire remained straight throughout the experiment.
I think that the range of my results was sufficient enough for me to make a valid conclusion about how the length of the wire affected the resistance. This was because I could plot a graph and show the general trend.
I think that the pattern/general trend would continue beyond the range of values I used. However, I think that unless I had specialist equipment the results would be distorted because the wire would eventually get very hot. Also, the apparatus I had use of in the school would not be suitable if I were to keep increasing the length of the wire, e.g. in a classroom environment I could not increase the length to more than 150cm because of the safety factor and minimal space.
I think my method could have been improved to make my results even more consistent. I could perhaps consider using the same type of wire but a different piece of wire each time. This would prevent one wire getting hot and affecting the results. However this would have been impractical and would waste a lot of time which I did not have in a lesson. Overall I think my method was very good in obtaining reliable results.
To support my prediction and conclusion I could do further experiments such as using a different types of wire instead of just nichrome and observing to see if the same theory applied to the results. I could also consider using different cross-sectional areas of wires or even change the temperature of the wires deliberately and see how it affected the resistance of the wire.
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