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How does the length of a wire affect its resistance?

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Introduction

Physics Coursework

How does the length of a wire affect its resistance?

Plan

In a circuit, there are two main factors – voltage and current. The combination of these two is described as resistance.

Voltage could be described as how hard the electricity is being pushed around the circuit, and is measured as the amount of energy transferred between charges as they pass between two points – it is measured in volts, using a voltmeter in series.

Current could be seen as how much electricity is moving around the circuit at a time, and is measured in amps using an ammeter in series.

Resistance is an inverse quality – how difficult it is for the electricity to move through the circuit, or how hard the resistor is working to stop the electricity. It is measured in ohms (Ω), either using an ohmmeter, or calculated using this formula:

R = V/I      (V=voltage, I=current)

Many factors can affect resistance, including thickness, length and type of wire, as well as number of components in the circuit.

In this experiment, I am trying to find out how the length of a resisting wire affects the resistance.

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Middle

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  1. Clip the first 100mm length of 32 SWG nichrome wire between the crocodile clips, on top of the heatproof mat.
  2. Set the power pack to 4.5 volts. I chose this voltage because it gave a good range of results in my preliminary test and there was no reason to change it.
  3. Turn on the power pack and wait for the ammeter and voltmeter to settle.
  4. Take voltage and current reading, then calculate the resistance using R=v/I.
  5. Swap the wire for 200mm, then repeat, and then again with 300mm, 400mm and 500mm. Then change the wire and do all the repeats twice more, to finish with 3 results for each of the 5 lengths.

I will keep my experiment a fair test in the following ways:

  • Do not change nay parts of the circuit (wires, ammeter, voltmeter or battery pack) in between the measurements – this will prevent small discrepancies between circuit components from affecting the results.
  • When taking the measurements from the voltmeter and ammeter, wait for the reading to settle completely. If it doesn’t settle, make an estimate of the middle value being shown. Waiting for the reading to settle will prevent accidentally taking a reading while the value is fluctuating randomly when the power is first turned on. Unfortunately, the equipment available to me is not the most accurate, so in some cases I had to make an estimate of the average.
  • Use the same type and thickness of wire – 32 SWG nichrome – from the same reel for all the lengths and repeats, to ensure there are no slight variations between reels of wire.
  • Use 3 different samples of wire for the repeats of each length, instead of the same sample 3 times. This will avoid the wires increasing in temperature with use. This is also the reason I chose to use separate pieces of wire rather than a long wire with the crocodile clips attached at different lengths. Temperature is a difficult variable to control, but it can also be helped by doing all the tests in the same place, in the same room.
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Conclusion

Another possible explanation for the large error bars could be human error. As I neared the end of the experiment, me and my team may have rushed the setup and measurements more, meaning less accurate results. To improve this, I would take the measurements spread out, ie 100, 200, 300, 400, 500 rather than 100, 100, 100, 200, 200, 200 and so on. This would mean any human error was spread more over different results, and would affect overall accuracy less.

Overall, I am fairly confident in my conclusion, as my graph has a very straight line of best fit, and the smaller error bars still support the line, as do the means of the larger error bars. Most of the possible improvements to my method and equipment are unlikely to be available in my school environment, and so I believe my conclusion is as accurate as I could reasonably make it.

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This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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