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Resistance of a Wire Investigation Wire Conclusion 1. What is the pattern shown in your graph? 2. Choose 2 points from your best fit line and describe how they support your answer to Q.1 3. Why does the length of wire used affect the resistance like this? The resistance of a conductor is a measure of how easy or hard it is for a current to flow through it. A wire will warm up when a current flows through it. An increase in temperature will increase the resistance of the wire. So measurements have to be taken quickly before the wire has a chance to warm up. In this investigation you measured the current in a circuit containing a length of wire. You also measured the voltage across the wire when a current was flowing through it. From these measurements you calculated the resistance of the wire. The formula for calculating resistance is Resistance ( Ω) = Voltage (V) Current (A) Here are some sample results from a similar experiment Length of Test Test 2 Test 3 Mean wire /cm 1 / Ω / Ω /Ω Resistance /Ω 20 3.1 3.4 3.2 3.2 40 5.9 6.0 6.2 6.0 60 9.3 9.2 9.0 9.2 80 12.4 12.2 12.4 12.3 100 15.7 15.4 19.2 15.6 What was the Independent variable in this investigation (The variable that we deliberately changed)? The length of the wire. Here are the results again. Length of Test Test 2 Test 3 Mean wire /cm 1 / Ω / Ω /Ω Resistance /Ω 20 3.1 3.4 3.2 3.2 40 5.9 6.0 6.2 6.0 60 9.3 9.2 9.0 9.2 80 12.4 12.2 12.4 12.3 100 15.7 15.4 19.2 15.6 What was the range of this variable? 20cm to 100cm Was this a suitable range to choose? Yes Why was this? This spread was big enough for us to see if there was a pattern to the results. Suggest one thing that could affect the accuracy of your results? Allowing the wire to warm up? How could you prevent this happening? Work quickly before the wire gets hot. In this experiment we did not change the output from the power supply. We also kept all the same wires and connectors during the experiment. Why do you think we did this? If we changed anything except the independent variable it could have an effect on our results and so it would not be a fair test. Here are the results again. Look at the result in red Length of Test Test 2 Test 3 Mean wire /cm 1 / Ω / Ω /Ω Resistance /Ω 20 3.1 3.4 3.2 3.2 40 5.9 6.0 6.2 6.0 60 9.3 9.2 9.0 9.2 80 12.4 12.2 12.4 12.3 100 15.7 15.4 19.2 15.6 What is wrong with the result in red? It is Anomalous (it does not fit in with the other two results). What should you do if you get an anomalous result in an investigation? You can either ignore the result or you can repeat the test. Here is a graph of results from the investigation. Resistance of a wire resistance / Ohms 20 15 10 5 0 0 20 40 60 80 100 120 Length of wire / cm What is the link between the Independent and Dependent variable?. As the length of the wire increases the resistance also increases How do you know this?. The line of best fit shows a positive correlation between the length of the wire and its resistance. Why did we use a line graph to display our results? Resistance of a wire resistance / Ohms 20 15 10 5 0 0 20 40 60 80 100 Length of wire / cm Because the length of wire is a continuous variable. 120 Here are the results again. Length of Test Test 2 Test 3 Mean wire /cm 1 / Ω / Ω /Ω Resistance /Ω 20 3.1 3.4 3.2 3.2 40 5.9 6.0 6.2 6.0 60 9.3 9.2 9.0 9.2 80 12.4 12.2 12.4 12.3 100 15.7 15.4 19.2 15.6 Resistance of a wire In this graph we have plotted the results from the individual tests instead of the mean result. 25 Anomalous result Resistance /Ohms 20 15 The results for each length of wire show a spread or scatter 10 5 0 0 20 40 60 Length of wire /cm 80 100 120 Can you spot the anomalous result on this sort of graph? What can cause errors in our investigations? 1. Systematic errors. These are caused because either the equipment has been set up incorrectly or is not being used properly. This means the error will be the same each time 2. Zero Errors. These are caused because a piece of measuring equipment is giving a reading that is too high or too low because it has not been set at zero properly. This means the error will be the same each time 3. Random errors. These are caused by things not under our control such as the temperature of the room changing while we carry out the experiment. These errors can change each time and can make our results slightly different each time. To draw a line of best fit on a graph like this you try to take the line through the centre of each scatter of results. You can ignore anomalous results. Resistance of a wire 25 Resistance /Ohms 20 15 10 5 0 0 20 40 60 Length of wire /cm 80 100 120 Instead of length of wire imagine you wanted to find out if the thickness of a wire affects the resistance. What would be the Independent variable in this investigation? The thickness of the wires. What would need to stay the same about the wires you use? The length would have to be the same each time. The wires should be made from the same metal What else would you need to keep the same in the investigation? The output of the power supply Apart from the thickness of the wire what other measurements would you have to take and what equipment would you use to make them? The current in the circuit using an ammeter and the voltage across the wires using a voltmeter. How many times would you test each thickness of wire and why? We should test each wire at least three times so that we can spot any anomalous results. How many different thicknesses of wire should you use and why? We should use at least 5 different thicknesses. This is the minimum number of values needed to spot any pattern in the results