As the length of the wire is increased the number of collisions the current carrying charged particles make with fixed particles also increases and therefore the value for the resistance of the wire becomes higher. Resistance, in ohms (R) is also equal to the resistivity of the wire, in ohm-metres (ñ) multiplied by the length, in metres (l) divided by the cross sectional area, in square metres (A).
The material and cross sectional area of the wire is constant throughout the experiment. Therefore it is clear from the formula that the resistance should be directly proportional to the length.
Fair Test?
In order to make it a fair test, there are a few things I will need to take into account.
I will need to make sure that I use the same type of wire for every different length I test. If I use different materials, my results will be inaccurate as different metals have different amounts of resistance.
Although it is unlikely that the temperature would vary enough to make a real difference in my results, I should try and keep the room temperature the same throughout.
I would also need to keep the thickness of the wire the same, but if the type of wire is the same then the thickness will also be the same.
Apparatus:
Power Pack
Voltmeter
Ammeter
Wires
Crocodile Clips
1000mm nickel wire
Metre Rule
Method:
I will connect up the circuit as follows:-
The piece of wire is held in place between the two black dots using crocodile clips. The voltage across the wire is measured using the voltmeter, and the current is measured using the ammeter.
I will take the 1000mm piece of wire and connect one crocodile clip to one end of the wire, then, using the metre rule, measure certain lengths of wire and place the other clip at that particular length. I will then switch the power pack on and take the readings from the Voltmeter and Ammeter. After taking down the readings, I will then use the equation ‘R = V / I’ (where R = resistance, V = voltage and I = Current) to work out the resistance in the wire at different lengths.
Accuracy
To keep this experiment as accurate as possible we need to make sure, firstly, that the length of the wire is measured precisely from the inside edge of the crocodile clips, making sure that the wire is straight when we do this. We must also make sure that the wire is straight when we conduct the experiment. If it is not, short circuits may occur and bends and kinks in the wire may effect the resistance, also. The reading that we take of the voltage should be done fairly promptly after the circuit is connected. This is because as soon as a current is put through the wire it will get hotter and we want to test it when heat is effecting it the least, i.e. at the beginning
Results
I carried out the test 3 times, in order to make it a fair test. The following are the results I obtained:
Average
Conclusion
I think that from my results I can safely say that my prediction was right. The resistance did change in proportion to the length. This is because as the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire causing more collisions and therefore a higher resistance.
Evaluation
I feel that overall my results were quite accurate. This is can be seen when we look at the graph, which shows a straight line with all of the points apart from one being very close to or on that line.
Most errors in my experiment were encountered in the measuring of the wire. This is because it simply was not very practical to hold a piece of wire straight, whilst holding it next to a ruler and then trying to accurately fix crocodile clips to the right part on the wire. Also I do not feel that the crocodile clips were always fixed securely to the wire with a good connection. This also meant that they were easy to move about on the wire changing the length of it. Errors rarely occurred in the setting of the current and the reading of the voltage. It was just in the preparation area that they did occur. Another example of this is the wire was never totally straight when I started the experiment, which may also, as said earlier on, effect the resistance of it.
I do not think that doing any more results in my experiment would have made it any more accurate. I feel that the only way to make it more accurate would be to use a different method – perhaps if I used a metal bar that did not bend in place of the wire. I could even use a rheostat in place of the wire, because it is essentially a long coiled wire that is connected at different lengths to change the resistance of the circuit.