.56 (A = Πr² = 2.46x10-7 m²)
So to work out the resistance of each thickness of wire/length, I have to use the equation: 4.9 x 10-8 x 0.1 = Resistivity of constantan
6.16 x 10-8 = surface area (A)
Certain numbers will have to change for each thickness of wire and the length.
I.e. the 0.1 on the top line is the wire length, and the whole button line is the surface area for each of the different wires we will be using.
I already know that certain factors affect the resistance of my wire that I will be using. These being the material it is made out of, the temperature, the width and length. I will make sure all these factors are the variables (and more) in my test.
Prediction
I predict that the longer the wire the higher the resistance will be, as the electrons will have further to travel. If the wire is longer then the electrons will take longer to push through all of the atoms. By using my background knowledge on this subject, I can predict what the resistances will be, by using the equation in my background knowledge.
I also predict that the thicker wire will have less resistance, and that the resistance will increase slower than the thin wire when the length of the wire is lengthened.
Lastly I predict that the voltage will only start to read up to 5v when we reach the longer lengths of wire, this being due to the wires length. I also predict that the graph will have 3 lines looking something like this;
Predicted results
Here are the predicted results that we got from using the equation in my background knowledge. The ‘/’ sign stands for divide, so no confusion can take place.
These are our predicted results; I expect the real ones to be slightly higher as the heat will affect the results slightly even showing how much we try to keep the wire cool.
Fair Testing
To make sure the test is a fair one I will have to know all the variables and know which ones are fixed and which are not.
The temperature will be fixed (apart from when the wire is heating up from the charge), the voltage will always stay the same (5v), and the thickness of each wire will always stay the same. The Length of the wire and the resistance will both be independent variables. The length I will change and the resistance will change with it.
This is an equipment list of the parts I will use for my test;
Mains cell – the power supply for my experiment with a constant supple of 5 volts
Connecting wires – to connect all the different parts of my experiment together
Ammeter – to read the current flowing through the circuit
Voltmeter – to read the voltage flowing across the wire
Crocodile clips (2) – to connect the circuit to the wire (Testing wire)
3 Different wires (made of constantan) – with thicknesses of .28 .40 and .56 mm
When we actually did the experiment, we had to make sure that all the variables were kept the same (if they were fixed) and all the independent variables changed (if needed to). So we had to make sure at all times that the voltage going through the circuit was the same (5v) this was done by using a 5v power pack that could only send 5v no more and no less. The temperature must have been kept the same on the wire (otherwise Ohms law is not true). So even if a wire did heat up it was given appropriate time to cool down for the next experiment.
Reliability
We do not need to take more than one measurement for each length on each wire, as if the experiment has been set up properly, only one reading will be needed. And any other readings taken will read the exact same.
Range of Evidence
We will be using 3 different thicknesses of wires to gain a series of results.
Obtaining Evidence
At this point we will start to obtain the evidence we need. Then show it in graphs and charts, to show if we were correct with our predicted results, also to show if any anomalies or anomalous results happened during our experiment.
Safe Procedure
When we started to obtain the evidence we needed to prove our prediction, we had to make sure our procedure of doing so would be a safe one. Making sure all the equipment was dry (for obvious reasons). Making sure all our equipment is safe before usage (a quick eye check to make sure everything is working ok) and to check the outlet voltage of our power pack to make sure it is giving a steady 5 volts. The last part to make sure we were being safe was to make sure that we used the equipment in a safe manor. Making sure no parts from one wire touched or the electric charge was not left going through the wire for too long.
Table of results
These are the actual results we gained through the experiment, the resistance was used by using Ohms law (V = I x R) or in this case V divided by I = R (V/I = R).
The resistance has been calculated to 2 decimal places.
Analysis
As you can see the increase in length and resistance is proportionate (mostly) in all the graphs, the line of best fit verges up to the top right hand corner of the graphs in a steady slope in all graphs in the same general direction. (It is the red circled result in the medium graph)
There were no obvious anomalies, apart from the reading on the medium wire at the .7m length of 2.84. This reading was slightly lower than the other results but still acceptable. I still feel that this anomaly needs pointing out as it is still out of proportion to the rest of my results.
As the larger all information giving graph shows, there are clearly 3 separate sets of results. A high going line, a middle going line and a low going line, there are 2 sets of results on each of these parts, the lower being for the actual results and the higher being for the predicted.
Conclusion
My prediction was correct, the longer the wire the higher the resistance. This happened because when the wire was shorter the electrons had less to travel and therefore didn’t take as long to give a reading of volts and amps. Also when the wire is shorter the electrons move quicker because they don’t have as many atoms to push themselves through. I gave myself a good range of results by giving 3 thicknesses of wire in my tests, this will greatly help prove my prediction, which was (as said above) true.
I had one slightly anomalous result which I will said was there and circled in both graphs it appears circled by a red pen.
The prediction I made about the graphs were also correct, they all went up in the proportions that I predicted.
Evaluation
I found the experiment quite easy to set up, with no real difficulties of using the procedure I said I would use. All the tests went to plan according to what I had written. The quality of the procedure I used was overall, satisfactory with few changes needed to improve it. There was only one slight anomaly, which may not have been one but if looking at the rest of the results it looks out of place. If I were to do this again I would make sure that all the wires were pulled straight so to make sure all the lengths were as accurate as possible. I would also use different wires for each single test, so to make sure a previous test did not damage the wire in anyway. Also to cut the wires to the length the test would be done on, not just moving the crocodile clips to there relevant place. My evidence is reliable as I know as long as I kept the temperature at the same level for each test (around room temperature) my results would be reliable ( I must consider that it is hard to keep the temperature the same at all times but making sure it is would help greatly), and would support my conclusion. Other possibilities to improve my work would be to, as said before use the right amount of wire for each test, make sure it is stretched to the right length if not using different wires for each test, to make sure all the wire is not on a conducting base (i.e. iron etc) make sure it is one wood and not connected with metals either, make sure the crocodile clips are always connected properly, and lastly I must try and make sure room temperature is kept the same throughout all of the experiments.
