An investigation into the factors affecting the resistance of a wire.

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Richard Tandy                                                                               September 2003

Year 11 Science Coursework

An investigation into the factors affecting the resistance of a wire

Introduction

        There are many factors that are known to affect the resistance of a wire. However, there are a few factors, which influence the resistance of a wire more than anything else. One factor, which determines the resistance of a wire, is the material in which the wire is made out of as the structure of metals influences resistance. The dimensions of the object and the temperature in particular also affect the materials resistance, as the temperature increases the molecules gain energy, which results in more collisions and therefore more resistance. Moreover, the length of the wire will also govern the amount of resistance created. Density too (how thick or thin the wire is), has a large affect on the amount of resistance. For example, a large surface area has less resistance because a small area has tightly packed atoms, which in turn rebound many of the electrons. The factors that can be investigated in Physics are those known as quantitative results – these are results that have continuous data (such as length).

For this piece of coursework the factor that I will be investigating will be how the length of a wire will affect the resistance in which acts upon it. I am not going to be investigating temperature, as it is too difficult to measure the temperature within a wire. Temperature affects the wire but as I am using the same piece of wire in the same classroom, I think that the wires temperature will be fairly equal throughout my experiment. Though it will not be 100% accurate.              

Background Knowledge

Voltage is the electrical force, or "pressure", that causes current to flow in a circuit. It is measured in VOLTS (V or E). Voltage is the force that pushes the electrons around the circuit.                                                                        Current is the movement of electrical charge - the flow of electrons through the electronic circuit.  Current is measured in AMPERES (AMPS, A or I).                                                                                                                       Resistance is a force, which opposes, resists or restricts the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. Resistance is anything that causes an opposition to the flow of electricity in a circuit. Resistance is caused when electrons in the current, which flow towards the positive, collide with the atoms in which the material consists of, in doing this they transfer some of their kinetic energy. This transfer on collision is what causes resistance. Resistance is used to control the amount of voltage and/or amperage in a circuit. Everything in the circuit causes a resistance, even the wire.

The knowledge that I already have is on Ohms law, also how to measure resistance using a voltmeter and an ammeter. Ohm's law states that the amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive force impressed on the circuit (meaning that if you triple one, you triple the other) and inversely proportional to the total resistance of the circuit. George Ohm discovered that a circuit sometimes resists the flow of electricity and called this resistance. The resistance of a circuit is measured in ohms, named after the Greek letter, Omega and is obtained using the rule he devised, ohms law.

I can now work out the resistance using the current and Voltage: R=V/I (ohms law)

R - resistance in ohms (Ω ohms)

V - voltage (volts)

I - current (amperes)  

I have discovered the circuit itself can resist the flow of particles if the wires are either very thin or very long. E.G the filament across a bulb is quite thin, as it needs to resist the flow of particles for the bulb to glow (with a thin wire there will be more collisions between the electrons and the atoms as there will be less space to move). A further example, which shares relevancies to my experiment, is the national grid. With the cables between the pylons being so long, the wires create a lot of resistance. Therefore, a high voltage (400 000v) is used to transmit enough power – in this case the long length of the cables causes a high resistance.  

Apparatus list

The apparatus in which I will need in order to undertake this experiment are:              

             -1 meter ruler                                                    -Power Pack                                -Voltmeter                                                  -Connecting Wires                 -Ammeter                                                  -Sellotape                        Nichrome Wire (alloy of nickel and chromium)                          -Crocodile Clips                                                                                                                                                                                                                                                                                                                                         Circuit Diagram                                                                           

This is a diagram of the circuit in which I will set up in order to undertake this experiment:

Method

  • I will first gather together the equipment in which I will need to use for this experiment.

  • I will then set up the equipment as illustrated in the diagram (figure 1).

  • I will firstly measure out 5 meters of the Nichrome Wire, as this is the length in which I am going to use for the experiment. I have chosen to use a length of five meters, as I believe a wide range will give me more accurate and reliable results than if the wire was only of 1 or 2 meters in length.

  • I will then Sellotape the Nichrome Wire to the table, changing the wires direction every 1-meter so that it does not take up too much of my workspace.

  • I will then assemble the circuit, connecting the Voltmeter in parallel and the Ammeter in series. I will then connect the crocodile clips to the power pack and to the ammeter and attach them to either side of the Nichrome wire.

  • I will then switch on the Power pack.

  • I will firstly perform a preliminary test in order to check that the circuit works. I will also use the broad set of results as a benchmark for when I undertake my real test to check that I am doing it right.

  • I will now be ready to carry out the test.

  • The resistance of the wire is what I want. I will devise a results table and using this I will record the voltage and the current passing through the wire at various different points. Once I have these results I will then be able to calculate the resistance of the wire at my chosen points by using ohms law (R=V÷I).

  • To acquire my results I will move the crocodile clip, connected to the positive terminal, up every 10cm on my 500cm wire. In doing this I will record 50 different results, with a wide range of results this should make my experiment more reliable as there can be a good spread of results so that I am able to arrive at a good conclusion. I have chosen to use these lengths (every 10cm) because they are easily measured by the meter ruler and give a good range of results.

  • Once I have my first full set of results I will repeat the test a further two times. I will then calculate the average current for my set of results by adding together my results and dividing them by three. This will increase the accuracy of my results and I will also be able to observe any anomalous results.

  • Once I have my results recorded in my results table I will then plot my results onto a length and resistance graph.    

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Preliminary Test

Before I start my real test I will first undertake a preliminary test in order to test that the circuit works and also to get used to using it and recording data. I will record some brief results in order to use as a benchmark for when I undertake my real test in order to check that I am doing it right. My results from my preliminary test are as follows:

My table from my preliminary test will give me a rough and brief, scaled down guide to what my real results table should look like and provides ...

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