OUTPUT VARIABLE: The resistance of the wire.
INPUT VARIABLES:
*Length of the wire* – affects the resistance because there are more electrons moving and the longer the wire, the more atoms they will hit. There is a greater chance of hitting atoms if there are more atoms, and this will affect the resistance.
(Source= physical processes, Nick England)
Temperature of room – affects the resistance because the atoms vibrate more the higher the temperature so they are harder to pass by for the electrons. If the atoms are harder to pass the electrons will bounce in the wire more and the resistance will be affected.
(Source= physical processes, Nick England)
Wire width/thickness – affects the resistance because the thicker the wire is, the faster the flow, this is because more pathways are provided for the electrons to flow through.
(Source= physical processes, Nick England)
Wire type (material) – Different materials will have different numbers of atoms and so the flow of electrons will depend on the type of material and the number of atoms in it.
(Source= physical processes, Nick England)
APPARTUS: Wire: Copper 26, 32 (standard wire gauge.)
Nichrone 26, 32, 36 (S.W.G.)
Constantan 26, 32 (S.W.G.)
The standard
Multimeter /OHMmeter Ω - to measure the resistance of the wire. This is the most accurate resistance meter I can use because if I use an ammeter and a voltmeter than there are two readings to measure and calculate and the resistance will not be as accurate as if an ohmmeter were used. The ohmmeter can measure the resistance to the nearest .1 Ω so it is very accurate.
Metre ruler – This is used to measure the length of wire, it is quite accurate as it can be used to measure to the nearest .5 mm but if the wire is not totally straight
it will not be efficient. The wire will not be straight
as it will have been be coiled, to keep this a fair test
I will try to straighten the wire as much as possible
but not stretch it, as this would change the resistance.
Electrical leads and crocodile clips – I will have to
take into account the resistance of these wires and
clips when I perform my experiment as they could
have a significant difference on my final results. I
will plug the leads into the ohmmeter and use the
clips, attached to the other end of the leads, to
clip onto the wire and measure the resistance.
Masking tape – I will use this to tape the wire
to the ruler to make it easier to measure.
Wire cutters – I will use these to cut the
wire I will use for my experiment.
PRELIMINARY WORK: The input variable I have decided to change is *Length of the wire*. This is because it is likely to have the biggest effect on the resistance. I will use the apparatus listed above to conduct the experiment.
PREDICTION: The input variable I am using for my experiment is *Length of the wire*. I think that this will change the resistance (output variable) because dependent on the length of wire will be the amount of atoms. If there are more atoms (in a longer length) then there will be more chance of the electrons colliding with the atoms and so the resistance will change. I predict that the longer the wire, the higher the resistance will be, this is because the longer the wire the more chance of electron/atom collisions there will be. (Source= physical processes, Nick England & class notes)
PRELIMINARY METHOD:
- Firstly I will gather my apparatus to conduct the experiment.
- I will then tape the wire to a metre ruler with masking tape.
- Now I will connect the crocodile clips at the appropriate places for the set lengths.
- I will use my Ohmmeter to measure the resistance and record my results in my result table.
- When I have finished with one length I will move the masking tape along and then clip the crocodile clips at the appropriate places.
- I will take three readings for each length. This is to make sure my results are reliable and that I do not get an anomaly once and ignore it.
- For my preliminary results I will measure up to 100cm in steps of 20cm to get a good spread of results to base my real experiment upon.
- I will change the length of the wire but keep all the other variables (temperature, type of wire and width of wire) the same to make sure I have a fair test. I will change the wire only to see which wire is best for my experiment, as this is the reason for conducting my preliminary results, otherwise the wire type/width will be the same.
- I will calculate an average for my results and record my findings in a table.
PRELIMINARY RESULTS:
“A table of results to show how the resistances of three wires change as they are changed in length.”
WHICH WIRE? : From my results I can see that the difference in the resistance of copper is so small that it is unreliable. To be 0.01 at 20cm and 0.04 will give me no difference in results and so I will not use copper. If I had a much longer length of copper, then it might be possible to try using copper in my experiment. As my results are, however, I will not be able to use copper for my experiment.
Nichrome has a steep line of resistance, it is nearly 5Ω for every 20cm. If I used Nichrome for my experiment then it would have to be in smaller steps and with a lot more readings. But this way is not ideal, as I want to calculate the resistance over a length of wire and so need at least 100cm.
Constantan seems to have relatively even steps of roughly 0.7Ω per 20cm. This means that it has an even step but is not too high and so can be measured in steps of 10cm, this means I can get a number of readings to give me an idea of resistance against length.
I can tell from the data above that constantan is the best wire to use for my experiment. If I use constantan 26, which is a thicker wire than some, e.g. constantan 32, then there will be fewer atoms in the way and so the resistance will be lower.
PREDICTION: My prediction after doing my preliminary results is the same. I predict that if there are more atoms (in a longer length) then there will be more chance of the electrons colliding with the atoms and so the resistance will change. I predict that the longer the wire, the higher the resistance will be, this is because the longer the wire the more chance of electron/atom collisions there will be. I predict that the resistance will change in equal steps according to Ohms law that when wire length is doubled then resistance is also doubled. (Source= physical processes, Nick England & class notes)
REVISED METHOD:
- Firstly I will gather my apparatus (see list above and diagram below) to conduct the experiment.
- Instead of taping the wire to the ruler I will twist it around so that I can get the measurements more easily and so that it is easier to move the crocodile clips to get the different lengths. When I did my preliminary work I found it difficult to tape the wire to the ruler.
- Now I will connect the crocodile clips at the appropriate places for the set lengths.
- I will use my Ohmmeter to measure the resistance and record my results in my result table.
- When I have finished with one length I will change the position of where I have wrapped the wire around the ruler and then clip the crocodile clips at the appropriate places.
- I will take three readings for each length. This is to make sure my results are reliable and that I do not get an anomaly once and ignore it.
- I will calculate an average for my results and record my findings in a table.
- For my results I will measure up to 100cm in steps of 10cm to get a good spread of results with which to draw my graph.
- I will change the length of the wire but keep all the other variables (temperature, type of wire and width of wire) the same to make sure I have a fair test.
- I will calculate an average for my results and record my findings in a table.
-
To ensure safety I will:
*Not carry out the experiment in wet areas, as water is a very good conductor, and this could cause danger.
*Not touch the wire when the multimeter is switched on, because the current would heat up the wire and burn somebody.
- My apparatus is the same as shown on page 3.
RESULTS: “A table of results to show how the resistance of constantan 32 changes as it is changed in length.”
ACCURACY: I used the ruler to the nearest +/- 2mm as the wire was not totally straight but was a bit coiled as it had been on a roll. Every measurement I straightened the wire but tried not to stretch it in any way. I used the ohmmeter to the nearest 1/10 of an Ω but the averages are to the nearest 1/100 of an Ω. When I placed the crocodile clips on the wire they may not have been in exactly the right place and so this also need to be taken into account. I am accounting for human error during the experiment. This was the fairest way to use the apparatus.
I can tell that my results were accurate because when looking for the actual resistance of constantan 32 in a book (source= science Data book, R. Tennent) the resistance was stated to be 8.26Ω per metre. I can tell my results were reliable because my end result (7.81Ω per metre) is not far from the actual resistance of constantan 32. When the final resistance is taken into account the fact that the wire may not have been exactly 100cm and also that the measuring may not have been totally accurate I can see that my accuracy was quite good but also that the results were not wrong.
GRAPH: For my graph showing the resistance of a wire as it changes in proportion to its length, see the next page.
ANALYSIS AND CONCLUSION: At the beginning of my graph the resistance is an average of 0.7Ω per 10cm. This changes at about 60cm where there is more resistance per 10cm. From here onwards the resistance is roughly 0.8Ω but it decreases at 90and 100cm to 0.7Ω Although my graph has a line of best fit, some of the points are not directly on the graph, this is because at these places the resistance is higher than the average all together and is also higher than the point before it. The reason for this is that the line has to go through as many points as possible to have a best fit and so although the steps of resistance at these points are higher, they are below the line of best fit.
The scientific idea that if you double the length of the wire, you double the number of atoms in it, so doubling the number of electron collisions is true in accordance with my experiment. The results support my predictions well and most of the results turned out the way I had expected. I had predicted that when length changed so would the resistance. I also predicted that if length doubled then so would the resistance as is stated in Ohms Law. (See intro. ‘What is resistance?’) This prediction was not quite true, as the resistance did not double when the wire length was doubled i.e.
10cm - 0.73Ω doubled =1.46Ω
20cm - 1.46Ω doubled =2.92Ω 40cm - 2.86Ω doubled =5.72Ω
80cm - 6.13Ω doubled =12.26Ω
Although these lengths appear to double at first, some have higher steps and some have lower steps and so in my experiment Ohms law was not true.
From my results I conclude that when the length of a wire is increased the resistance will increase proportionally but not always, as stated in Ohm’s Law in the same steps. I conclude that the steps will not always be accurate but will be quite near each other, that they will always be within the region of ‘double the length, double the resistance’.
I thus conclude that when a wire is lengthened,
the resistance of that wire increases.
