Another variable that could affect the resistance of a wire is the material of that wire. This is because different metals have different amounts of electrons in their outer shell.
The more electrons that a metal has, the more electricity that can pass along the wire, therefore it would have less resistance. This is because the electric energy is passed from electron to electron through collisions, so the more electrons, the more energy can be carried along.
Another variable that will affect the resistance of a wire is the cross-sectional area of the wire. The thicker the wire, theoretically it should have less resistance. This is because with a thicker wire, there would be more atoms. Therefore more free electrons to carry the current, but with still the same distance to cover. As there are more electrons to carry the current along the wire, there would be less resistance.
The final variable that will affect the resistance of a wire is the length of a wire. This effects the resistance because as the length increases, there is a greater distance for the energy to travel, for example, if the length exactly doubled, there would be exactly double the distance to travel for energy. Therefore there would be twice as much energy loss through heat from collisions between atoms and electrons. The variable of the length of wire is the variable I will be investigating.
From doing preliminary work we can investigate what differences in length will be best/most appropriate for my experiment. Also what thickness of wire and what voltage would be most appropriate aswell. In our preliminary work, we will use two different wires, one 28 and one 26swg. We will use lengths of 10, 20, 30, 40 and 50cm and on two different voltages (2v and 4v). We can not use higher voltage as in a quick test carried out, using 20 cm 0f 26swg and a higher voltage, the wire heats up and melts. This was because the resistance was too high as the wire was to thin and short for that voltage. We can also not allow the temp of the wire to increase, as this would make the test unfair, as the temp affects the resistance as well.
From using the results gained from the preliminary work, I have worked out that a 10cm gap each measurement is too small. This is because this didn’t give a wide enough range off results. Therefore, in the experiment I will use lengths of 20, 40, 60, 80 and 100cm, as this will give a wider range of results. We will also do the experiment at 4volts as in the preliminary work this showed a more definite and almost equal increase in resistance each time the length was increased. For the thickness of the wire, I am going to use 20swg. This is because with 26 and 28, the shorter lengths began to increase in temperature too quickly making it an unfair test. Therefore I need a lot thicker wire as the temperature would affect the resistance.
The equipment was set up the same as in the preliminary work. A meter length of wire (20swg) was attached to a meter ruler and taped at each end. A circuit was then set up as in the diagram, using an analog. An analog was used because the readings ranged so I could judge where the middle was. In the circuit, the two crocodile clips were clipped onto the wire using the ruler to measure out the right distance between the clips each time. On the first length measured, the power pack was switched off and on to take the reading three times. This was to make sure the reading was precise as each time the exact same reading was given. We only did this on the first measurement as otherwise it might start to heat up the wire. This was done because the heat increase in the wire would affect the resistance, so this variable had to be kept constant to keep the test fair.
From looking at the preliminary work and using my own knowledge on collision theory, the structure of metal atoms and the transfer of electricity through electrons. My prediction is that as the length increases, the resistance will increase to the same proportion. I think this because I know that as the length of wire increases, there is further for the electrons to carry the electricity. This means that the resistance would be increased as more energy would be lost as heat from collisions between the electrons, and the impurities and atoms of the metal.
The results on the table and graph show a strong positive correlation. They show that as the length doubles, the resistance doubles as well. For example, the length is changed from 40 to 80. The resistance subsequently changes from 0.8 to what 1.47. This matches ohm’s law (states that the resistance and length of wire are linked directly, to the exact same proportion). Although ours has not matched up exactly, our experiment could have been improved and made more accurate. For instance, to make it more accurate, any tiny kinks in the wire would have to be measured as this altered the length to what we measured.
In conclusion, I think that the resistance of the wire will change in the same proportion as the length is changed. For example, if the length were doubled, the resistance would double as well. My evidence supports this as the line of best-fit shows this pattern. Although my results individually don’t match this exactly, I think they are still reliable enough to draw this conclusion. This is because they were only ever slightly off the amount this rule suggests, and this fault is down to insufficient equipment. This is because various factors could have slightly affected the resistance. Such as small kinks in the wire (length of wire measured could have been about 1mm out), wires quality altered from bending, crocodile clips too wide which could have lost accuracy. I think that as the results become more accurate, they would become closes to an exact match to ohm’s law.
