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Find out how the length and width affect the resistance of a graphite track.

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To find out how the length and width affect the resistance of a graphite track.


In this experiment I will be investigating the electrical resistance of carbon in the form of graphite. Carbon is widely considered to be a conductor; in fact it is the only non-metal conductor. However, physicists think of carbon as a semi conductor. It is a poor conductor at low temperatures, but improves to become reasonable conductor, though not as good as for example copper, silver and gold at room temperature. If one refers to appendix B, the resistively[1] chart shows how much more of an efficient conductor gold is in comparison to carbon. This means it is easier for electrons to carry charge through gold than carbon. Carbon has a unique structure, which allows electrical energy to be conducted. There are several types of carbon: carbon 60, graphite and diamond. Carbon exists in these different structures because it has allotropes. Due to its covalent structure carbon can form different allotropes, because of the way the electron orbits join. This applies to carbon in the form of graphite. Carbon in the form of diamond is an electrical insulator. This is because it does not have any loosely held electrons. The structures of carbon look like:

However, graphite is bonded in a different way. The electrons orbit in a figure of 8. Electrons spend only half their time on each side of the atom. There is a “loose” bond between layers thus allowing electrons freedom to move. By consulting the full Periodic Table (Appendix A) you can see that carbon is very close to the semi metals, meaning it possesses some of their unique properties.

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I could have use a current and a volt meter, as well as a battery cell instead of the ohmmeter. However the ohmmeter automatically shows the resistance without having to work it out using the R= V/I equation. Furthermore the ohmmeter provides energy for the circuit, so a cell is not required.

I have decided to take readings of resistance for lengths up to 27 cm. The readings for the width will go up to 15 cm. I think by using these values I will be able to obtain a wide range of values, thus good results. I will make a reading every 2cm interval. To make sure the readings are reliable I will repeat each reading at least twice. If the results have a big difference then the test for this particular value will be repeated. To make sure my readings are as precise as possible I will measure the length of the carbon track with a ruler with millimeter values on it. The resistance in ohms will also be measured to 2 d.p. by the ohmmeter. Having obtained these results they will be entered into a matrix. Graphs can then be drawn from this table.image00.png


For this experiment I decided to use card with grid lines photocopied onto it. Card is stronger than paper, so when being coloured upon I will not rip the card. The grid lined will make it easier to colour in exact amounts of the card. I will also use a low HB pencil as they have more graphite in them and less clay, making my carbon track more pure. When coloring in the card I must make sure that the spread of the graphite is uniformly thick (for a fair test).

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If I were to repeat or expand this experiment I would make a number of changes. Firstly I would try to find a way of making the carbon track more uniformly coloured in. Carbon pencils, sold by art shops could be used to make the track more uniform.

When conducting this experiment I took two sets of readings for the same length of carbon track. At the time I thought this to be repeating the experiment. However, in Hindsight this was not a repeat. I was merely redoing the same experiment again with the same errors.  If I were to conduct this experiment again I would turn the carbon track around, with the fixed connection point at the opposite of the original end. This would have been a correct repeat, giving me a different set of values to compare, thus a % error could have worked out. An even better way of repeating results would be to have drawn a completely new carbon track.

This experiment could be furthered in a number of ways. Different conductors could be tested, in order to find out their the resistivities. Constantine, copper and lead could all be experimented upon to further my understanding of the principles of resistivity. Having found out that Resistance is inversely proportional to width, and the length is proportional to length for a carbon track, one could investigate whether this rule is the same in other metals.

[1]An allotrope is a different form of an element due to its physical properties.

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