The range of lengths I will use for the experiment are 100-600mm of wire. The lengths I will use are 100, 200, 300, 400, 500, 600mm. I will use a 2-volt setting.
For reliability I will repeat each test, at each length, three times making at least 18 tests. If I find, as I am plotting my results, there it a reading that does not fit the expected pattern I will repeat that test for a fourth time and discount the wrong result in my conclusion.
Prediction: I predict that the longer the wire the higher the potential across it and the lower the current. This therefore leads to an increased resistance. This could be written as: the length of the wire is directly proportional to the resistance. If the length of the wire doubles so does the resistance it provides. This may be true up to the point at which the resistance is greater than the current and the flow of electrons stops.
The reason for this is the longer the wire the more static positive nuclei are present in the wire. It is the non-tightly held, free, electrons in the outer shell of the atoms of metal, which flow and move around the circuit creating a current. If there are more nuclei in the way the moving electrons have a greater chance of colliding with them. When an electron hits a nucleus some kinetic energy is transferred in the form of heat, which slows down the electron. It is this transfer of energy and speed, which decelerates the flow. Therefore the longer the wire the greater the chance of a collision between an electron and a nucleus. Also with more nuclei in the way, which cannot move, the distance that the electrons have to travel, to avoid or go around the nuclei, is increased. This again means the flow is slowed down. I predict if I double the length of wire the resistance will be doubled.
I found this information in: The G.C.S.E Revision Guide pages 336-339, Useborne Essential Science, Nelson Science Physics by Ken Dobson pages 180-181, , , www.bitesize.com.
To make the experiment safe, I will:
- Only use low voltages,
- Execute the experiment sensibly and efficiently,
- Make sure everything is ready before the experiment begins,
- Plan ahead so I know exactly what I am doing to avoid confusion,
- Stop the test if the wire begins to burn the board or glow.
Method: I will first set up the circuit as shown in the diagram. To make it safe I will keep everything tidy and then only use a low voltage. Then I will:
- Take the board with the wire on it and mark out 100mm divisions on it.
- Prepare a table of results with graph.
- Turn on the power
- Press the two contacts down onto the wire with equal force and take the reading of the volt and amp meters.
- I will not hold the contacts down for more than 5 seconds to prevent the wire becoming hot and affecting the result.
- Repeat the tests using the same amount of force so my results are accurate.
- As I complete each test I will add the information to a sketch graph so I can spot any anomalous results quickly allowing me to repeat that test.
Apparatus:
Power pack
Connecting Leads
Amp meter
Voltmeter
Wire to be tested
This apparatus will be set
up as shown in the
second diagram.
Results:
I found the resistance by using Ohm’s Law of: Resistance = Voltage
-----------------------------
Current
Analysis:
To display the information in a way I could analyse the results I put them into a graph.
A graph to show the relationship between the length of wire and resistance of the wire
= Line of best fit
This graph clearly shows a straight line, my prediction was therefore correct. As the length increases the resistance increases. This means the length of the wire is directly proportional to the resistance.
The resistance increases as the length of the wire increases because with a longer wire there are more static nuclei in the way of the moving electrons. With more in the way the electrons collided with the nuclei more often. Each time they collided some kinetic energy was transferred into heat energy and so the electrons travelled more slowly. This is electrical resistance. Also with a longer wire the electrons will have to travel further to avoid all the nuclei and so will take longer and be slower.
The potential difference across the wire is different to the input voltage (2 volts) because the connecting wire can provide some resistance. At 100mm there are more connecting wires than the actual test wire. Since the connecting wires have some resistance, 0.9 volts are dissipated in them leaving a 1.10 volt potential difference across the wire. At 600mm there is about the same amount of test wire as connecting wire so there is less dissipating happening in the connecting leads and a higher potential difference across the test wire. Since the input power was constant, as the voltage changed the current also changed. To show this I plotted a graph.
The straight line shows the power is constant.
The straight line shows the power is constant.
The electrons in vacuum would just accelerate at a constant rate. It is the resistance of the wire, which is the collisions, which keeps the electrons flowing at a constant rate. Each collision stops the electron dead until the EMF starts accelerating it again. If you were to plot a graph with velocity against time it would look like this.
In the wire with the less resistance, the electron collides less often and so the electron can accelerate for longer in-between collisions picking up greater speed. If the average speed is higher there is more kinetic energy on the electron and so there is less lost.
The results I got were accurate and reflected what I wrote in my prediction. At 300mm the resistance was 0.912. At 600mm the resistance was1.802 and so doubling the length doubles the resistance with a small degree of error.
There were no anomalous results in my tests with only a few variations of 0.01 or 0.02 ohms off the line of best fit. If my results had revealed some big differences they could have been due to the heating effect (explained before). They could have also been due to the amp or voltmeter not settling so the result would be an approximate. The wires provided were not new and fresh and so kinks in the wire could have made the length of wire longer than expected. The wire could also have been damaged if the wire has been heated a lot before in previous experiments. To over come this new wire should have been used.
To get accurate results I repeated the tests 3 times. The results were slightly different each time but the lines of best fit are almost the same for all three tests.
I made a graph to show the difference
A graph show the relationship between my first set
of results and repeats
* = Anomalous Result
= Line of best fit
A large range was used to provide a greater width of graph so there is less degree of error.
My results could have been better if the results were all the same in each test because I would have had more evidence to support a pattern. However they were quite accurate and did support my prediction almost perfectly with a straight line. However they are only accurate to the degree of accuracy I could use. For instance the smallest measurements were 0.01 of an amp and volt. However even with this defect I read the results accurately and I could use them to support a conclusion as I used an average of my results in my explanation. My results also support Ohms law.
The method I used could have been improved by doing many things. First I could eliminate the variable of how hard I pushed down on the wire by wising crocodile clips, to maintain a constant pressure, and have a switch to turn the circuit on and off. I could also remove the heating effect by adding a variable resistor to the circuit.
By adjusting the variable resistor for each length of wire, the current could remain constant and so the heating effect is exactly the same for each test, which makes it fair.
The range of reading I took could be improved by making the range more concise. This would allow more points on a graph and so a better conclusion.
For additional evidence the experiment could be extended to include the testing of much longer pieces of wire (1-6 meters) or to investigate the cross sectional area of a wire. Also we know how one metal reacts and what its resistance is but different metals may react differently.
My experiment supports Georg Simon Ohm’s theory of resistance.
Science AT1
Resistance of a Wire
By Kieran Wood