Resistance in a Wire Investigation
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Introduction : Resistance is a force which opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long. e.g. The filament across an electric bulb is quite thin as needs to resist the flow of particles for the bulb to glow. Resistance is measured in ohms. George Ohm discovered that the emf of a circuit is directly proportional to the current flowing through the circuit. This means that if you triple one, you triple the other. He also discovered that a circuit sometimes resists the flow of electricity. He called this resistance. He then came up with a rule for working out the resistance of a circuit : V/I = R V - Volts I - Current R - Resistance. Extra Background Information : The total resistance of resistors in series is the sum of the resistance of each one. Each cm of wire has a particular resistance, if you double the length of wire, it is like having two of the shorter wires in series. If the small resistor represents a short wire and the large resistor is a long wire of double the length of a short one. One short wire has a resistance of 1ohm, 2 short wires have a resistance of 2ohms when connected in series. The long wire is just like two short wires put together. Aim : To investigate how the length affects the resistance of a wire. Prediction : I predict that if the length increases then the resistance will also increase in proportion to the length. I think this because as I know from my scientific knowledge that : Electric current is the movement of electrons through a conductor. In this experiment a metal wire (Nichrome will be the conductor).
Another, more significant thing is that it the increase is constant. This is indicating by the fact that the line drawn is a straight one. One may also note that the gradient of the line drawn is (1.85/40) .04625. Conclusion: I think that from my results I can safely say that my prediction was right. The resistance did change in proportion to the length. This is because as the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire causing more collisions and therefore a higher resistance. We can work out what the resistivity of the wire should be from our results using the formula It is obvious from the formula that R/l is simply the gradient of the graph, therefore Evaluation I feel that overall our results were quite accurate. This is can be seen when we look at the graph, which shows a straight line with all of the points apart from one being very close to or on that line. The one point that was not that close to the line was a slight anomaly, but it was only slight and did not effect the final gradient of the graph. I have found out that for the wire I was using, the resistivity at 20(c)C is 4.9 X 10-7 ohm-meter. From this we can then work out the percentage error of our results: The accuracy for this experiment is, theoretically, ± 15.7%, but as one can see this does not seem to be the case from looking at the graph. The reason for this could have been due to a number of different factors. Firstly the temperature of the wire was not necessarily 20(c)C when we conducted the experiment and the material of wire may not be as pure as it should have been. The main reason for this was probably due to the equipment that we used being inaccurate.
This table shows my results against some ideal results. This shows the margin of error in my results. Using 28SWG Constantine wire. Length cm My results Ideal results 100 4.62 5.0 90 4.16 4.5 80 3.70 4.0 70 3.21 3.5 60 2.78 3.0 50 2.35 2.5 40 1.89 2.0 30 1.43 1.5 20 0.97 1.0 10 0.51 0.5 As you can see from this table, my results were no more than 0.38 Ohms out from the ideal results. For 30SWG, the results were slightly less accurate but nevertheless were accurate enough to produce a good directly proportional straight line graph. I could have made more accurate results and a more varied set of results by doing the experiment at more than just 28SWG and 30SWG. This would have given more varied results at different thicknesses. I could have made my results more accurate by doing the experiment multiple times (perhaps 5 times) and taken the average of each set of results. This would have given me more accurate results. Inaccuracy in the results may have been due to the fact that the equipment used may not have been working properly. Also, when measuring the length of wire to be tested, it is possible that I may have slightly misread the length on the ruler by a couple of millimetres because of kinks and twists in the wire making it nearly impossible to get a perfectly straight piece of wire. This may have caused slightly inaccurate results. Thickness: My results for testing the thickness were, on the whole, quite accurate. There is only one result that seems to cause a "bump" in the curve on the graph: 30SWG. This could have been due to an inaccurate length of wire, a temperature change or an inaccurate reading. However, the other results produced a good smooth inversely proportional curve. If I were to conduct this experiment again I would have used more accurate equipment and tested more lengths at more SWGs than I did to give a more varied set of results.
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