Temperature: As the temperature in the wire increases, then gradually the resistance will also increase. This is because the atoms will be moving around a lot more and much faster in the wire and this will interfere with the collisions of the electrons and as a result it will also be harder for the electrons to pass the atoms.
Preliminary Method
In the preliminary experiment we carried out several different experiments with wires and black putty. We used these and with an ammeter measured the current and a voltmeter to measure the voltage, then using the formulae R=V/I we were asked to find the resistance. The main idea of doing the preliminary experiment was to understand and familiarise myself further with the method and if I had any problems I could correct them there and then. This would mean I would obtain precise and reliable results in my main experiment when investigating the connection between the length and diameter and the resistance of the wire. Furthermore, when I obtain the results for my preliminary experiment I can get a rough idea of what my final readings will be so I will have an idea if my readings are correct or incorrect.
Black putty experiment
This was the main preliminary test. It consisted of a 9volt power supply, six separate wires, a variable resistor, ammeter and a voltmeter in parallel with the resistor. The resistor consisted of 100grams of ordinary black putty with carbon mixed in with it so that it would conduct electricity. We were asked to roll out the black putty to 20cm long then half and half again. Then we tried to roll it to two 5cm lengths together until we got twice the diameter. We recorded the results and found the resistance. I believe this preliminary method to be very inaccurate as my results did not show an accurate trend and it is difficult to roll the black putty to an accurate size and diameter.
Main experiment
First, the circuit was set up as shown above. I had to be careful in connecting circuit, because the Voltmeter had to be placed in parallel and the ammeter, which had to be placed in series. Draw up a rough table onto a piece of paper with the appropriate headings. The wire was set on to a metre rule so the crocodile clips could attach onto the wire, making the results more accurate.
Basic method
To carry out this experiment, the power supply was switched on and adjusted to the appropriate Amp. To do this the slider on the variable resistor was moved up or down to increase or decrease the current. When the appropriate Amp was set, the voltage was read on the voltmeter. I could then use these two figures to work out the resistance.
Prediction
Length: I predict that if the length increases then the resistance will also increase in proportion to the length. I think this because the longer the wire the more atoms and so the more likely the electrons are going to collide with the atoms. Therefore, if the length is doubled the resistance should also double. This is because if the length is doubled the number of atoms will also double resulting in twice the number of collisions slowing the electrons down and increasing the resistance. My graph should show that the Length is directly proportional to the resistance. If the wire is twice as long, there should be twice the number of atoms, resulting in twice as many collisions and a predicted doubling of the resistance.
Diameter: I predict that if the diameter is increases then the resistance will decrease. Therefore, if the diameter is doubled the resistance should be halved. This is because if it is doubled the number of channels for which the electrons can pass through will be doubled resulting in twice the number of electrons passing through without colliding with the atoms of the wire. If my prediction is correct, my graph should show that the diameter is inversely proportional to the resistance. This will be shown with a curved line sloping down.
Safety
Handle the power supply carefully.
I am going to only use a voltage of 0.50 Amps so the wire will not burn out instantly.
Be careful when touching the wire, as it may be hot.
Start on the lowest voltage so the wire then will not melt or burn out instantly.
Be careful when the wire is connected, as it will get hot.
Apparatus
Power pack
Ammeter
Voltmeter
100cm of Wire & Meter Rule
Two Crocodile Clips
Connecting Wires.
Factors that must stay constant to keep the experiment a fair test
The power supply must go up to a maximum of 5A,
The wire must be measured at the same length,
The surrounding temperature must be constant (room temp.),
The equipment should be kept the same,
The edge of the crocodile clips should be at the edges measured length.
The Variable factors
The factors that I am going to vary are the length and diameter.
Analysis
In my prediction, I stated that:
If the length increases than the resistance will also increase in direct proportion to the length. From my graph, I can see that the resistance of the wire is directly proportional to the length of the wire. I know this because the Line of Best Fit is a straight line passing through the origin showing that if the length of the wire is increased then the resistance of the wire will also increase in proportion to each other. The line of best fit is a straight and it goes though (0,0) if there is no length, there is no resistance proving that the resistance of the wire is directly proportional to the length of the wire. This proves my prediction right. The length of the wire affects the resistance because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion. The resistance of a wire depends on the number of collisions the electrons have with the atoms of the material, so if there is a larger number of atoms there will be a larger number of collisions that will increase the resistance of the wire. If a length of a wire contains a certain number of atoms when that length is increased, the number of atoms will also increase.
When I observed the results of the diameter of the wire, the line of best fit was a clear curve, which did not pass through the origin. This suggests that resistance is inversely proportional to the diameter of the wire but it does however still follow Ohms law because as the voltage doubles the current also doubled. This proves my prediction correct as I stated, “If the diameter of the wire is increased, then the resistance will decrease.” I also only found one aluminous result on my graph, this proves that during my experiment I was quite accurate and that aluminous result could be put down to a rise in temperature or the wire was not measured accurately.
The most common trend in both of the experiments of the length and diameter of the wire I found that as the voltage doubles the current also doubled this suggests that both follow Ohms law. I also found that as the length increased the resistance also increased in proportion and as the diameter of the wire was increased the resistance decreased.
Evaluation
In the Analysis and the graph I have shown a few main anomalous points, this means that there must have been a slight error in my experiment. Although the graph is overall accurate and the results precise, it is easy to see the anomalous averages plotted because they do not all lie along the same best-fit line. The graph shows that my results are reliable as there are only a few main anomalous points. To improve the reliability of my results, I could do more repeats in doing this my average would be more reliable.
As I increased the wire length, the wire became hotter and gave off heat. This could explain why the anomalous results are at the top of my graph, 80cm and 90cm. I think one of the reasons why my experiment is quite accurate is because the wire was already secured to the metre rule making it easier to measure it more accurately and the crocodile clips could be securely fitted on the appropriate point.
Accuracy of experiment
During my experiment, I have noticed several modifications I could make to improve on the Investigation if I was to repeat it.
The first of these modifications would be the circuit that I would use. Instead of connecting the voltmeter to the main circuit, I could connect it to the wire that is being tested. I would do this so that the voltmeter is measuring the voltage of just the wire being tested and not the wires of the main circuit as well.
To also improve on my results I could use a digital device that would directly measure the overall resistance of the wire thus getting rid of the voltmeter and ammeter and also the arithmetic required to work out the resistance (r=v/i). The graphs shows that my results are quite reliable as there are only a few anomalous points, to improve the reliability of my results, I could also have repeated the same lengths of wire more times. Although the wires diameter measured 0.28, the thickness of it may vary by a small amount and maybe helping to cause the anomalous results. Sometimes the voltmeter flicked between a decimal point, I maybe could have thought it was the wrong number and therefore would have ended up with the wrong average resistance, to eliminate this particular problem; I could have used a voltmeter with at least three significant figures after the decimal point. In the experiment, I did not control the room temperature but instead just assumed it was keep constant throughout my experiment; this could have made the wire get hotter and therefore making my experiment not as accurate. In future experiments I would control this variable factor and make it constant.
