Aim: To find out which factor or factors determine the resistance of a length of carbon in the form of a pencil. Introduction: The reason we are investigating carbon is because it has very interesting properties and because it is such an important element in electronics. What makes carbon so interesting is that it is a semi-conductor, this means that it behaves like an insulator when cold but when warm it becomes poor conductors. Also carbon in the form of graphite, which we are using in this investigation is very unusual because it is a non-metal which conducts.
In fact it is the only non-metal pure element that conducts electricity and this is the reason why it is so useful in electronic circuits. Theory: Resistance in terms of this experiment is the slowing of the flow of current through the carbon track by anything in the carbon track. To explain this definition fully we first have to explain conduction, which is the flow of current through a conductor. The current flows by means of free electrons through the conductor and so the more free electrons or the larger the “sea” of free electrons the better conductor the material is. EG.
Iron has less free electrons than Gold and so gold is a better conductor. The electricity put into the material then gives the free electrons a charge and so they begin to move randomly and crash into other free electrons. This happens all along the length of the conductor and so the electrons begin to move from the negative connection to the positive connection (this happens because electrons are negatively charged). This is how the current flows. The resistance in the carbon track is caused by these charged free electrons crashing into fixed particles (nuclei and protons), other free electrons and impurities.
The free electrons lose some of their energy, which is wasted as heat energy as a result of these collisions. This loss of energy causes them to travel slower and so the flow decreases. This means that the longer the length of the carbon track the more resistance there will be. This is because the more wire the electrons have to pass through the more collisions they make and so the higher the resistance. I expect the resistance to be proportional to the length because if you double the length their will be double the amount of collisions.
Having a wider piece of carbon track or metal wire is like having several pieces of wire in parallel. This usually results in the resistance being reduced. The reason for this is that more current can flow with the same battery voltage because there are several current pathways which all add. Having more current with the same battery voltage means the resistance has decreased. Ohms law: a steady current flowing through a metallic conductor is directly proportional to the resistance between the ends of the conductor, provided that the temperature and other physical conditions are kept constant.
I am also going take ohms law into consideration with this experiment even though it mentions only metallic elements. I think because carbon is a very unusual non-metal in that it conducts, the law may apply in this experiment. This is just to highlight that it is very important that the current flowing through the carbon track remains constant. This is because as more current is put through the carbon track the hotter it gets and temperature change affects the resistance of a conducting material quite dramatically.