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The actual experiment Essay Substances, which allow an electric current to flow through them, are called conductors because of their structure. Copper is a good conductor and is used for connecting wires. Nichrome has more resistance and is used in heating elements of electric fires. Some conductors are better than others. Copper is a better conductor than iron. When electrons are moving through the metal structure they bump into the metal ions and this causes resistance to the electron flow or current. In different conductors the ease of the flow of electrons is different and so the conductors have different resistance. Effect of cross sectional area: For a particular conductor the resistance will depend on its length and cross sectional area. The longer the conductor, the further the electrons have to travel, the more likely they are to have collisions with the metal ions and so the greater the resistance. Resistance is proportional to length. The greater the cross sectional area of the conductor, the more electrons available to carry the charge along the conductors length and so lower the resistance is. Resistance is also inversely proportional to the cross sectional area. As cross sectional area increases the resistance decreases. Two wires in series are one long wire, because the electrons have to travel twice as far. Two wires in a parallel are like one thick wire so the electrons have more routes to travel along in the same distance. The amount of current flowing through a circuit can be controlled by, changing the resistance of the circuit. This can be done with a variable resistor or rheostat. Adjusting the rheostat changes the length of the wire the current has to flow through. Variable resistors are commonly used in electrical equipment for example in the speed controls on radios and hi-fi systems. Symbol for a variable resistor (rheostat) Effect of temperature If the resistance of a conductor remains constant then a graph showing voltage plotted against current will give a straight line. The gradient of the line will be the resistance of the conductor. However, the resistance of most conductors changes if the temperature of the conductor changes. This can be explained using a simple model of a conductor. As the temperature increases the metal ions vibrate more therefore provide a greater resistance to the flow of the electrons. The resistance of a filament lamp becomes greater as the voltage is increased and the lamp gets hotter. Planning Aim: To investigate how the length of a wire affects the resistance. Equipment: The longer the wire, the higher the resistance. This is because the longer the wire, the more times the free electrons will collide with other free electrons, the particles making up the metal, and any impurities in the metal. Therefore, more energy is going to be lost in these collisions (as heat). Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions. Method: The following circuit will be constructed to carry out the investigation. Firstly I will set up the equipment as a shown in the diagram. I will then set the voltage on the power pack to 3v. Then the first crocodile clip is clipped to the wire at the 0cm position on the metre rule. The second crocodile clip is clipped to the relevant position depending on the required length of wire. This length for this experiment will be a 10cm increment each time along the metre ruler, which starts from 0cm to 100cm. We decided to use 10cm intervals then testing each increment to get a more reliable set of results and to have more results to compare. The power supply is turned on. The voltage is then read off the voltmeter, and recorded. The power supply is then turned off and the second crocodile clip is moved to the next position. The above steps will be repeated five times for each length for accuracy Safety: In order to perform a safe experiment, a low voltage of 3V was chosen so that overheating will be kept minimal as much as possible. Furthermore, lengths lower than 10cm were not tried, which also helped to avoid overheating, which will lead to anomalous results. Evaluation I think my investigation did not go as expected because, I discovered many anomalies when I drew up graph. These were at 70, 80, 90, 100cm. They were circled because they did not increase steadily as expected. The reasons these problems may have occurred are as follows: The power pack may have been left off over a long period of time which allows the Nichrome wire the to cool down causing the wire to have less resistance and therefore leading to anomaly results. Perhaps it could have been that the crocodile clips that were attached to the wire may have slipped and become lose therefore not increases the voltage as much as it should be. The length that was measured for that particular distance was incorrect. It is possible that the length was shorter, causing a lower resistance and at a majority of the lengths it is possible that it was causing a higher resistance also leading to anomalous results. We also encountered many difficulties whilst carrying out the experiment such as placing the wire in the 5v hole and then reading the voltage off the line that went up to 15v instead of 5v. So we had to repeat the experiment in order to keep it a fair test. To avoid these errors in the future I would make sure the power pack would stay on for the same amount off time for each length I would do this by timing it. An alternative way of doing the investigation would be to use a data logging system which involves digital voltmeter and ammeter and use wire with a larger diameter therefore the clips will not come off easily. I think this would have gave me a more reliable set of results and it would also be a more efficient way of doing the experiment because more time would be saved. Vivian Okrah Candidate: 9428 Resistance of a wire Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.