The change of resistance of a metal Essay Example for Free

The change of resistance of a metal Essay The power I supply to the circuit will be very important. There must be enough current to measure but not too much which would cause the wire to generate a significant amount of heat, which would affect the result. The current must also remain fairly constant throughout the experiment so any affect it has on the resistance has the same effect on the result. A battery has a relatively low current but in a closed circuit it will drain very fast so it would not be suitable. A power pack would overcome this problem but I will have to keep the output very low. From this equation, Heat Energy = V2t/R it can be seen that p.d, time and resistance determine the amount of heat given out by the wire and thus its effect on resistance. Due to this I want to keep the p. d and time as low as possible and the resistance a value that is not too small. There are also several other problems I will have to overcome if I want to make the experiment as reliable as possible. If I use tap water in my experiment there will be a possible current running through the water due to the slight impurities in the form of ions within the water. This could cause inaccuracies in my results. To overcome this I have decided to use distilled water so my results are as reliable as possible. Another problem I may have is the event of parts of the wire touching in the water, especially if I have a fairly compact coil. To overcome this I have decided to use varnished wire that will guard the wire against unintended conducting. A final consideration is the period of time to supply the current to the wire, if the time is too great then it will cause the wire to considerably heat up which will effect my result, thus I have decided to keep the current running for a minimal amount of time but enough so that the readings on the voltmeter and ammeter are settled and constant. After a preliminary experiment I found I could comfortably fit 5m of varnished, coiled wire in a beaker. I also found that the thinnest available wire I could use was 0. 021 cm in diameter. In conclusion I have decided to use a varnished copper wire of length 5m, with a diameter of about 0. 021cm, which was the smallest available. This should give adequate resistance to measure. I have also decided to use a power pack on a low output setting. Basic Theory The theory of resistance can be very complex to calculate on a molecular scale and it is mainly done by observation. However I can explain in simple terms the theory of how temperature changes the resistance of a material. Electrons colliding with impurities within the vibrating lattice arrangement of metals cause resistance. The energy lost is given out in the form of heat. See diagram bellow. The amount of collisions is dependant on the amount the atoms in the lattice are vibrating. The faster the atoms vibrate the higher the probability of a collision. The temperature of the metal is what affects the vibrating atoms. More heat energy means the atoms vibrate more thus creating higher resistance. Prediction Due to the theory above I predict that the resistance will increase as the temperature of the water rises. From the formulas mentioned earlier and the information from the table I obtained from the Internet, I can predict more accurately what my results may look like. Temperature Resistivity of Copper at 20 Temperature coefficient Resistivity at Temperature Length of Wire Area Expected resistance (Degrees C) Degrees C (ohm m) ? (Ohm m) (m) (Cm2) (Ohms) Plan. I am going to use a beaker of distilled water to vary the temperature of a length of wire, knowing that resistance varies with temperature. To measure the resistance I am going to pass a current through the wire and take readings of the current and potential difference across the wire, which will allow me to calculate the resistance. To do this I will assemble my circuit so the wire is connected to a power pack, I will put a voltmeter in parallel with the wire and the ammeter in series. I will hold the wire in the water my making an MDF support as shown in the diagram with two crocodile clips. I will then heat the water to different temperatures using a Bunsen burner and take a reading every 10? C from 20 to 100? C. I will then turn the power pack on for a period of about 5 seconds to take readings from the ammeter and voltmeter. I will take every reading three times to ensure of no discrepancies between results. I will then average the results. I have decided to use a varnished copper wire of length 5m and diameter 0. 21mm made into a coil. This will give suitable resistance and will change sufficiently to measure the temperature of water. Apparatus needed: MDF piece 5 meters 0. 21mm wire 2 crocodile clips Connecting wires Voltmeter Ammeter Large beaker Bunsen burner, tripod, gauze, mat Power pack Distilled water Thermometer Safety During my experiment I must consider the following:   Caution when using the power pack alongside water   Caution when using boiling water   Use of appropriate equipment when moving boiling water Results Trial 1 Temperature i C Potential Difference (V) Current (A) Resistance (ohms) Trial 2 Temperature i C Potential Difference (V) Current (A) Resistance (ohms) Trial 3 Temperature i C Potential Difference (V) Current (A) Resistance (ohms). Average resistance results Temperature i C Resistance (ohms)   Analysis From the graph you can see that there is positive correlation between temperature and resistance and in this set of results it appears to be linear. There are anonymous results at 60? C and at 100? C. This could have been caused by several things, which I will mention later. My graph also satisfies my hypothesis that as the temperature rises; the resistance of the wire increases also. Firstly I would like to compare my theoretical results with my actual results to see the discrepancies between them and if they are at all related. From the graph you can see that the two sets of data are similar. My actual results are in general slightly higher than the predicted ones. Contact resistance between the crocodile clips and the copper wire could have caused this, especially since the wire was varnished and it was difficult to remove all of the coating. The wire may also have been stretched slightly when I was coiling it, leading to thinner areas of wire, which would have increased the resistance. There may also have been kinks in the wire that would have affected the results. I heated the water to the different temperatures after which I removed the heat. During the time between when I removed the heat and when I took the reading it is possible that the water cooled slightly thus giving lower resistance. Another big influence would have been the heating of the wire by the energy given out by its resistance; this would have created higher resistance. Finally the varnish on the wire may have been inconsistent leading to parts touching in the water thus making the circuit shorter and lowering the resistance. The anonymous result at 60? C could have been made by a general loose connection when I took that reading, caused by the experiment being knocked, as it was consistent through my three sets of readings. A more likely explanation is that two parts of the varnished wire were touching where there may have been a scratch leading to a shorter circuit and thus reducing the resistance. When I took my reading at 100? C I had to keep the Bunsen burner underneath to keep it at this temperature. This could have heated up the wire more than the water did, thus causing a greater resistance as shown in the graph. Evaluation My experiment was successful as I managed to construct and use a system to take measurements. The results were reasonably accurate and allowed me to confirm my prediction. I was pleased that my results were strongly correlated. There are several things I could have done to improve my experiment:   Use of more accurate multimeters would have increased the accuracy. I could have used data logging equipment with a thermometer connected to a computer to monitor my results more carefully and accurately   I could have done the entire experiment several times and taken different readings each time, instead of taking all three readings immediately after one another I could have used different lengths and thickness of wires to further prove my hypothesis   I could have tried a greater range of temperatures of water by raising its boiling point by adding a salt or similar, I could also have lowered the temperature using ice and a salt. I could have also used a lower current or put in a resistor before the wire to reduce the heating effect   I could have used solder connections and thicker wires to reduce any error in faulty connections If I were to repeat the experiment I would consider the above and in general take more readings to remove any anonymous results. 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