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What Affects The Resistance Of A Piece Of Wire?

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Dan Moffat                                                                         03/10/2002

What Affects The Resistance Of A Piece Of Wire?


        This coursework assignment is to investigate resistance. To investigate it, we must first understand it. What is it? Where does it come from? The most fundamental basis to understanding resistance is to know about current.

Electric current is a flow of electric charges. Like water in a heating system, the charged particles are already in the conductors. Most electrons in a conductor (e.g. copper) are held tightly to their atoms, but each atom in a conductor has a couple of electrons that are loosely held. Since the electrons are negatively charged, an atom that loses an electron is left with a positive charge (since the protons remain), and is called an ion. This means that copper (and all similar conductors) consist of a lattice of ions surrounded by ‘free electrons’. The ions can only vibrate in their current state, but the electrons can move randomly throughout the lattice. All metals (conductors) are made this way. When a battery is attached to a metal, the free electrons are repelled by the negative terminal and attracted by the positive one. They still move randomly, but they all move slowly in the same direction with a steady drift velocity. This is a flow of charge, an electric current. Current is measured in Amps (I).

A simple circuit through a conductor looks like this:

image00.png         The greater the resistance of a component, the harder it is for charge to flow through it. In a conductor with a higher resistance, the electrons have more collisions with the ions than if they were flowing through a conductor with lower resistance.

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I will set up the equipment as it is shown:


        Firstly, stretch a length of the selected wire (28swg) out along a metre rule, ensuring that it is straight. Attach it at both ends with tape and connect these ends to the circuit shown. Get a voltmeter ready by attaching the probes to it. Set the power pack to 3V and measure the reading from the ammeter. Ensure that the ammeter reads O.5A before continuing. Alter the voltage on the power pack to change the ampage. Using the voltmeter, first, set one probe on the wire at the 5cm point (a little way in to be accurate). Then, making sure that you measure from the inside edges of the probe, place the second probe 5cm further along the wire (at the 10cm point). Measure the reading on the voltmeter and record in a results table. If the voltmeter reads a negative number, swap the probes around.

        After this, I shall repeat the experiment, but this time measure a 10cm length of wire (i.e. Put the probes at the 15cm point). I will use a wire that is made out of the same metal as the previous one (constantan), and the same diameter (0.367mm). I shall keep all other variables of the experiment (i.e. the 0.5A) current the same in order to keep the experiment fair.

        Repeat the experiment over and over, measuring different lengths of the wire. Go up in 5cm increments until you reach 100cm. Record all the results and any applicable observations.

        To ensure good results, I shall eliminate any anomalous ones by repeating the experiment again, so that I get two results for each length.

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So, from these results we can see that:

  • Resistivity = 0.00000423137972 Ωm.
  • Gradient = 0.04.
  • Cross-sectional area = 0.000105784493m2.
  • Voltage is directly proportional to the length of the wire.
  • Resistance is directly proportional to the length of the wire.
  • As the length of the wire doubles, so does the voltage and so does the resistance.

        From the consistency of the results I have gained, and the fact that proving the results using the gradient was successful, I can conclude that these results are reliable. The values that I noted were also very similar to the catalogued book values, which further supports the reliability of the experiment. The drawn line of best fit is accurate as the gradient drawn from it is identical to the one I figured out using the formula.

If I were to repeat this experiment, I would change it only slightly. I would allow more class time for the analysis and possibly allow a shorter time for the practical as it is very quickly finished (in fact, to do all the results in one lesson minimises the chances of getting irregular results, because you know that the conditions are the same). The whole thing worked very well. The experiment went smoothly, and the inclusion of a preliminary experiment helped a great deal. The actual physics of this assignment were challenging, but understandable. It was a very well rounded task.

        Extension work that could be included may be to test the resistance of other wires and compare them, or to go up to 200cm and see if there’s any difference. Also, by using another wire, other than constantan, we could see how temperature affects resistance, although, one would need to have done a constantan experiment in order to understand the dynamics of the other factors, otherwise the experiment would be to complicated.

Daniel Moffat     H11NF

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