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To investigate the factors which effect the resistance of a metal wire.

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Hannah Clapham                                                                   Physics Coursework 2003

To investigate the factors which effect the resistance of a metal wire

Aim: To find how different factors affect the resistance of a metal wire.

Hypothesis: For this investigation I think that the thinner the wire the higher the resistance of the wire.  This is because the electrons in the wire can flow more easily when the wire is thicker because there are less collisions and therefore less resistance so when the wire is therefore thinner there are more collisions and the electrons flow more slowly creating more resistance.

Knowledge that I already have is Ohms law and 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 and inversely proportional to the total resistance of the circuit. The law is usually expressed by the formula I = E/R, where I is the current in amperes, E is the electromotive force in volts, and R is the resistance in ohms. This enables us to work out

the resistance using the current and voltage.

Theory:Atoms are made up of a positively charged nucleus surrounded by negatively charged electrons. In solids, one or two of the electrons in each atom are used to form the bonds between atoms that hold the solid together. In a metal these bonding electrons are free to move through the entire solid. These are often referred to as “free electrons”. An electric current is a flow of charged particles and when a voltage is applied to a metal, the free electrons can flow through the metal.

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The tick marks along the barrel of the micrometer represent halves of millimetres. Every revolution of the knob will expose another tick mark on the barrel, and the jaws will open another half millimetre. There are 50 tick marks wrapped around the moving barrel of the micrometer. Each of these tick marks represents 1/100 millimetre.

To read the distance between the jaws of the micrometer, simply add the number of half-millimetres to the number of hundredths of millimetres. For example the above diameter is 2.62mm (2.50+0.12mm)

Method for measuring the diameter of the wire:

  1. Put the wire you want to measure between the jaws of the micrometer.
  2. Use the ratchet knob to close the jaws on the wire until it clicks. When it clicks it is sufficiently closed.
  3. Look at the fixed barrel and the moving barrel and record the two numbers. Now add these two numbers together and this is the diameter of the wire.
  4. Take away the zero error of the micrometer. To work out the zero error you close the jaws using the ratchet knob until it clicks. You then read off the measurement as though you would for a wire.
  5. Repeat the reading a further 2 times to improve accuracy by taking an average of the 3 readings.

Fair Test: To make sure it is a fair test I will use the same length of wire each time. I will also make sure that I use the same multimeter throughout my experiment to avoid any inaccuracy in changing equipment. The only thing that will change is the diameters of the wire.

The Data: I am going to collect data on the different resistances of varying diameters of wire. I will increase the diameter of the wire each time by 2 SWG, so the diameter will range from 18 SWG to 38 SWG.

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Evaluation:  The results I obtained from this experiment were initially not quite as clear as I had hoped, but after  changing my method for the thicker wires this caused the results to become a lot more accurate.  The accuracy of my results was very good as when using the variable resistor methid the results were taken  up to 3 decimal places and then measuring the diameter the micrometer reads to 100th of a millimetre. Although some of my data was not 100% accurate the  graphs did not show these results to be anomalous as they fitted almost perfectly to the best fit line. By taking each of the readings 3 times  I increased my accuracy as this meant I coul d take an average of my results and would decrease any anomalous results.

My experiment went well although there are a couple of areas which could be improved if I were to do this experiment again. The first of theses is  the straightening if the wire. When straightening out a 1 metre length of wire I found it very difficult to get the thicker wires straight which could have lead to some inaccuracy. To improve this I could use the follwing method. Clamp one end of the wire and  place a weight on the other other to stretch out the wire hopefully straightening out any bends and kinks in the wire. Although doing this would straighten the wire the wire may also stretch. I would therefore take the diameter reading after straightening the wire as if the wire had stretched with the weight the diameter may have decreased.

I would repeat this investigation I would investigate more factors such as heat and length so that I could see how these affect the wires resistance. I would also carry out this investigation further and compare the properties of other conductive metals so I could see how each factor affects metals with varying resistivity’s.  

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