Hypothesis
When the length of wire is increased the resistance also increases. When the length of the wire is decreased the resistance of the wire in decreased. I think this because the longer the wire is the more nuclei there is for the electrons to bump into, therefore making their path more difficult.
I
Preliminary Test
I need to find a safe voltage, which also gives good, usable results.
In the lab we couldn’t reasonably use lengths more than 100cm, as the desks we are working on are just over a meter in length. I am going to use 10cm as my shortest length because any shorter the wire might heat up making the investigation unfair. So, I have chosen to measure the voltage and current (so I can work out the resistance) in wires at the lengths of: 10cm, 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm.
Method
- Set up a series circuit making sure the ammeter is in series and the voltmeter is in parallel. (See below)
- Cut 100cm of chosen wire
- Place the wire across the middle and attach with two crocodile clips.
- Take readings of the voltage and current every 10cm.
Fair test
I am making the investigation fair by just changing the variable, which is the length of the wire. I am going to use the same copper wire throughout the investigation. I also will keep the voltage the same and keep it at a constant temperature.
Equipment
Power pack
Voltmeter
Ammeter
100cm of copper wire
Crocodile clips
Wires
Safety
In order to perform a safe experiment, a low voltage of 3V was chosen so that overheating was avoided. Furthermore, lengths lower than 10cm were not tried, which also helped to avoid overheating
Results
Results table
I took three sets of readings so my results were as accurate as possible. I then found the average of all three results (add all three up and divide by three) and used Ohms law (R= V/I) to calculate the resistance in the wire.
Conclusion
As I predicted, an increase in length resulted in an increased resistance. The wire shows 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 also doubles.
The increase in resistance is due to the longer length of wire and the electrons, which make up the current, have to travel past more of the fixed particles in the wire. This causes an increased number of collisions and therefore a higher resistance.
A Short wire
A Longer wire
From this diagram you can see that the longer wire has more nucleus for the electrons to bump into therefore increasing resistance.
I predicted that my graph would curve, however it did not. My graph was a straight line, with only a few odd points on (40cm, 70cm, 80cm and 90cm), these odd points may be due to human error. I may not have measured the wire correctly, I may have taken a wrong reading from the ammeter or voltmeter because they tended to fluctuate between a few points making it hard to read or I may have not let the wire cool down completely before doing my repeat test. This would give me an odd result because the vibrating nuclei take up more room and therefore will let fewer electrons through.
Evaluation
Accuracy of results
Length of wire- It is difficult to cut exactly 10cm off the wire each time. It is possible that some measurement may have been slightly out.
Readings- The ammeters and voltmeters tended to fluctuate between a few points making it hard to take a reading.
Temperature-At some points (more likely in the lower lengths), the wire might not have been left long enough to allow it to cool completely so that the temperature may have been higher for the next measurement. Whilst unlikely (due to the three sets of results), this would cause a higher resistance because of vibrating nuclei.
There were no significant problems or difficulties that were encountered when carrying out this investigation. The accuracy and reliability of the results and conclusions are very good. Within the accuracy of the method used, and for the range of values investigated, it is clear that the length of a wire is proportional to amount of resistance in the wire.
Improvements
The procedure used was simple and straightforward and no difficulties were encountered. A small improvement that could be made would be to measure the length of the wire using a rule with millimetres. This may have made the measurements slightly more accurate. I could have also made sure the wire had completely cooled down after each reading.
More repeats could be taken but I don't think this would add much to the accuracy of the conclusions.
Longer lengths of wire could easily be tried, up to whatever maximum could be obtained. With a suitable location a length of several metres could be obtained. If the wire gets too long then a higher voltage might be needed in order to give good results. It would be interesting to try shorter and shorter lengths although a limit would be reached when the wire begins to heat up. It may be possible to use a lower voltage although readings for longer pieces of wire would not be so good.
My graph was a straight line, but it did have a few odd points on it, these odd points may have been because I measured the wire incorrectly, I may have taken a wrong reading from the ammeter or voltmeter because they tended to fluctuate between a few points making it hard to read or I may have not let the wire cool completely before doing my repeat test. This would give me a wrong result because the vibrating nuclei take up more room and therefore will let fewer electrons through.
Extending the investigation
To extend the investigation I could extend the range of lengths investigated and to see if the observed trend continued.
I could also test alternative wires such as constantan or nickel chrome. I would do this to see if when the length of these wires is increased if the resistance also increased as it did with copper. I think that other types of wire would have different amounts of resistance because different wires have different densities. A denser wire would have more resistance because its nuclei are more tightly packed together, so the electrons are more likely to bump into the nuclei, thus causing more resistance. However I think other wires would still display the same trend as copper did (Resistance is proportional to length) I think this because an increase in resistance is due to the longer length of wire. The longer the wire the more fixed particles the electrons, which make up the current, have to travel past. This causes an increased number of collisions and therefore a higher resistance.
If I did this experiment again, I would measure the wire using a millimetre ruler to keep any wrong lengths to a minimum therefore my results would be more accurate. I would also make sure I had let the wire cook completely before starting the next experiment.