954 words - 4 pages

Resistance in a wirePlanning -The resistance of a wire can be affected by certain factors:Length of wireCross sectional arearesistivity (type of metal)Type of wireTemperatureI chose to investigate how length affects the resistance in a wire. This means that the other factors (variables) must be controlled. This means I must keep the same type of wire, the same material of wire (a preliminary experiment will help determine this), and the same cross sectional area and maintain a constant temperature of the wire as any increase in temperature increases resistance. These factors must me kept controlled to ensure a fair test.Preliminary ExperimentI conducted a preliminary experiment to determine the best wire to use for my experiment. I tested two wires that are frequently used in electronic circuits, copper and in-home. We tested an equal length of each wire to determine a range of voltages that could be used for the experiment before the wire got hot (a rise in temperature would increase resistance). Copper started to warm up at around 0.50 volts and nichrome at about 2.00 volts. I will use nichrome in my experiment as it does not warm up as quickly and a wider range of results can be obtained, making the experiment more fair and precise. If more results are taken I will be able to plot a better graph with clear shape and form.Scientific theoryElectric current is the movement of electrons through a conductor. In this experiment a metal wire will be the conductor. When resistance is high, conductivity is low. Metals conduct electricity well because the atoms in them do not hold on to their electrons very well. Free electrons are created, which carry a negative charge, to 'jump´ along the lines of atoms in a wire which are in a lattice structure. Resistance is when these electrons which flow towards the positive collide with other atoms; they transfer some of their kinetic energy. This transfer on collision is what causes resistance. So, if we double the length of a wire, the number of atoms in the wire doubles. This increases the number of collisions and energy transferred twice, so twice the amount of energy is required. This means the resistance is doubled.PredictionI predict that that length of the wire will be directly proportional to its resistance. When the length is increased by e.g. Double, I think that the resistance will also double. This relates back to the theory that if you double the length of the wire, the number of electron 'jumps´ and so energy required doubles as well. The electrons will have double the distance to travel so the resistance will double.MethodA length of wire is taped at both ends to a metre rule. It is then attached to a circuit like this one with crocodile clips.One of the crocodile clips is then moved around the metre rule from 100cm, to 80, to 60 to 40 to 20. I will then take 6 readings from the voltmeter and the ammeter at each length. This will give a range of readings to determine resistance using the formula R=V/I. The readings will be plotted on a graph, and then the gradient of the graph will be measured to get the resistance for each length of wire.ResultsAnalysisThe graph was is a straight line through the origin, which means resistance is directly proportional to length. This means that if the length is 20cm, and resistance is 1 Ohm, then if length were to be doubled to 40cm, resistance also doubles to 2 Ohms. I discovered this using the formulaR=Pl/A. As variable P and A were kept constant, the formula can be simplified to R L as predicted.This is because of the idea, discussed in the prediction that if you double length, you double the number of atoms in it. As the charges move through the conductor they collide with other atoms. They transfer some of their kinetic energy. This transfer on collision is what causes resistance. So doubling length doubles the amount of atoms, and so collisions and resistance.EvaluationThe results of this experiment would be difficult to improve on because they are reasonably accurate, and there were no anomalous results. If I were to do this experiment again I would take a much wider range of readings and more readings so that a more accurate average can be taken. I would also take the reading from the ammeter more than once so I could get an average. This would make the experiment more accurate. I would use a longer length of wire to see if the graph of resistance changes from a straight line with a much longer length of wire.I would also investigate another factor, such as diameter of the wire. I predict that if I did investigate this, when I increased the diameter the resistance would decrease. I would also do the experiments under different conditions such as temperature to see the difference this makes to resistance.

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