- The length
- The cross-sectional area
- The temperature of the wire
- The chemical composition and type of the wire.
Looking at these 4 variables I have decided to investigate the effect of the length of the wire on resistance by varying the length of wire while measuring the resistance. After choosing which variable I am going to alter I must make sure I keep all the other variables identical. It is important to control the experiment in this way so a true and accurate result is obtained and bias is avoided. I will keep the type of wire and the cross sectional area identical by using the same wire type and size of wire throughout my experiments. It will be necessary to do some preliminary work to determine at what voltage the resistance of the wire causes the temperature of the wire to increase, as I know that high voltage causes high current which causes heating and increasing the temperature of the wire causes the resistance to change what could make the experiment a unfair test, even though I am attempting to minimise the effect of this variable by using Constantan wire. I will conduct this experiment using a long and short length of wire. I will use a long piece as I think this will have the highest resistance as so be most likely to heat up, and I will use a short piece to ensue the voltage and current are not too much for the wire to carry. If I do this l know that if I do not exceed the critical voltage with this length, the temperature will not change with longer lengths of wire in the circuit. I will then make sure that I do not exceed this voltage in my subsequent experiments.
I intend to set up a simple circuit with pieces of wire of different lengths. I will also include an ammeter and a voltmeter in my circuit so I can measure the current and voltage of the circuit. I will use a crocodile clip to change the length of a single long piece of wire that is connected in the circuit. I expect that if I choose a circuit containing a power pack and use a voltage of 2 volts I will get a measurable current and voltage so I can work out the resistance.
Preliminary Experiment
Apparatus used
Voltmeter
Amp meter
Wires
Power pack
Crocodile clips
Constantan wire
Cellotape
Metre rule
Circuit Diagram
I set up a circuit as shown in the diagram above using a amp meter, voltmeter and a piece of Constantan wire 100 cm long. I started with a length of 10 centimetres and went up 10 CM each time. I recorded the voltage and current flowing in the circuit; I then worked out the resistance by using this formula:
Resistance is voltage divided by current (R = V/I).
I obtained the following results:
From my results I can see that the longer the length of wire the higher the resistance of the wire. This is because wire has a certain resistance per unit of length. Thus the longer the piece of wire, the more resistance the piece of wire will have.
For all future experiments I am going to use a voltage of 4 volts, double the voltage used in the preliminary experiment, in the circuit because this voltage will giver higher readings on the meters which will be easier to read and so more accurate and reproducible.
I think that the results in my main experiment will show the same relationship in that the longer the length of wire the higher will be the resistance proving that there is resistance in wires. In my main experiment I am planning on using a current of 0.11 amps. Whilst doing the preliminary experiment I found that I tripped over a few bags on the floor, so when I perform my main experiment I will make sure that all the bags are tucked tightly under the tables to make it a safer environment to work in. Also I think that I will clear my table before doing the experiment just for safety reasons and also because my desk was clustered when doing my preliminary experiment which was a hazard. I will change the procedure that I used in my preliminary experiment because I think that I need to do more lengths of wire, ranging from 10cm to 80 cm and record the results of each experiment three times and average the results, to make my results more accurate, so that I can see whether wires have resistance or not. Also I will check that the crocodile clips are on the exactly correct place to make it a fair test each time because in the preliminary work the experiment was rushed a bit meaning the results might not of been fully accurate, but the rest of the procedure was satisfactory and safe and I will probably keep most of it the same. I am not going to use the same results from my preliminary experiment for my main experiment, I am going to get new ones and repeat the experiment 3 times using the same equipment each time to get the most accurate results possible.
Prediction
I predict that as the length of wire tested in the circuit gets longer, so the calculated resistance will increase. I also predict that the relationship between length and resistance will be a straight line with a 45-degree angle, so that when the wire length increases by a factor of 2, so the resistance will double. This is because I think the unit resistance of Constantan wire is constant, as long as all other variables except length are kept the same.
Main experiment
Apparatus used:
Voltmeter
Amp meter
Wires
Power pack
Crocodile clips
Constantan wire
Cellotape
Metre rule
Scissors
Exercise book to record results in
Circuit Diagram
Method
I constructed a circuit as shown in the diagram above, using the apparatus listed above. The variable in the circuit was the length of the piece of wire, and I measured the length of the piece of wire used, the voltage and the current flowing through the circuit during each experiment. The piece of Constantan wire was laid along a ruler. A crocodile clip was placed at 0cm and the length of wire to be included in the circuit was determined by connecting a crocodile clip to the wire at exactly the correct place. The voltage was set at 4 volts. The voltmeter was connected in parallel to the wire and the ammeter in series. From the voltage and amps reading I calculated the resistance of the piece of wire. I performed each experiment three times, using 8 different lengths of measured wire. I averaged the results for each length of wire to increase the reliability of the results. I then plotted a graph of my results. I drew a line of best fit through all the points, and looked for any outlying points.
Obtaining Evidence
An experiment to determine the resistance of a length of wire.
Using the circuit described above I obtained the following results. I did each measurement 3 times.
Analysing the evidence
Having obtained the raw data I now need to process the data to work out the resistance for each wire length. To calculate the resistance I divided the voltage in volts by the current in amps to find the resistance in Ohms. The tables are shown below. I then averaged the resistance results for each of the three experiments done at each length of wire. Then I plotted a graph of my average results, plotting resistance against length, and finally fitted a line of best fit to the results.
Experiment 1
Experiment 2
Experiment 3
Average resistance
My results show that as the length of wire incorporated in the circuit increases, so the resistance also increases. The line of best fit is straight, so the relationship is linear and the slope of the graph is constant, indicating the unit resistance stays constant and the resistance doubles as the length doubles.
The final results graph has all the points directly on the straight line that is the line of best fit. There are no anomalous results. This is in contrast to the results graph from the provisional experiment, where the resistance for the 20cm wire is above the line and the resistance is higher than expected. I think the results in the final experiment fit better because the experiment was performed more carefully, it was performed 3 times and the results were averaged. I conclude that in the final experiment the action I took to make it a fair and accurate experiment have been successful. In addition the straight line indicates that the only variable that changed in the experiment was the length of wire and my attempts to keep other variables identical were successful.
In my prediction I predicted that resistance would increase, as wire length got longer, that there would be a straight line of best fit and that the slope of the line would be constant. All my predictions have turned out to be true from the graph of the final results plotting length against average resistance. This also indicates that the resistance of a piece of wire is proportional to the number of jumps that an electron has to make while flowing through the metallic bonding of the metal atoms of the piece of Constantan wire. My results are in keeping with the theory of electrical conduction as stated in the scientific theory and background at the beginning of this project.
Evaluation
I feel that the experiments outlined in this project went well. I thought that both the preliminary experiment and the final experiments were quite easy to perform and that the results I obtained were accurate and reliable. The experiment was safe and satisfactorily performed and the data recorded reliably and processed accuratly.
In the final experiment there were no apparent anomalies and the line of best fit agrees with the actual results very well. However in the preliminary experiment there is one point that is an anomaly, the resistance at 20cm being higher than would be expected from the other results and the line of best fit. The problem would appear to be, from looking at the raw data, to be that the current was recorded at too low a figure, 0.05 amps where at the other lengths the current was0.06 amps. If the 20 cm resistance is recalculated using 0.47/ 0.06 the resistance would be 7.8 ohms, which would put it almost exactly back on the line of best fit.
While it is possible the current reading is simply an error, it is also true that the current in the preliminary experiment was very low, and in the final experiment I used both a higher voltage and current, to make reading the results both easier and more accurate. In addition I used the average of three readings so that a single error would have a much lesser effect on the final reading, especially after the results have been processed into average resistance. The fact that the line off best fit joins all the average resistance data points on the graph indicates this method has worked.
I do not feel that I could have improved the method or the equipment for this experiment and that the results obtained were accurate and reliable. The experiment was fair and safe. The results obtained were completely in keeping with both my prediction and the outcome I expected from my reading of background information and scientific theory.
Although my experiment proves that there is resistance in wires there are also other experiments that could be done to learn about the resistance of wires, for example, a ‘Micrometer screw gauge’ this measures very small lengths such as the diameter of a wire. So the thickness of wire could be tested to learn about the resistance in wires.
Also I could test all the factors mentioned in the planning of my investigation such as the temperature and the Chemical composition of the wire. I could use several different types of wires and I could use different temperatures of wires to see if those factors affect the resistance of wire.
Even though I haven’t done these experiments for the different factors I would expect the same results and the same out come proving that Wires have resistance.
To gather my information I used ‘ Physics for today and tomorrow’ by Tom Duncan
