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Investigating factors affecting the resistance of a metal wire.

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Rhodri Williams        11X        Science Group C

 Investigating into factors affecting the resistance of a metal wire



  Metals can be described as regular rows and columns of positive ions surrounded by a sea of electrons. Each atom in a piece of metal loses one, two or three electrons, which can then move around in the metal:


                                Positive ionimage00.png


Metals conduct because these electrons can be pushed along by a power supply. Resistance comes about because the electrons in a wire continually collide with the stationary ions. So if there is a high resistance, electrons collide with the ions more, so less pass each point each second, so the current goes down.

  A longer wire has more atoms, so more ions, while offering no increase in cross sectional area. Because of this, there are more ions with which the electrons can collide, so there is a higher resistance.

  Different materials will have different resistances, as metals have different distances between atoms, different sized atoms, and different numbers of electrons released. A large distance between atoms means that electrons can get past more easily, so a lower resistance. In a metal with large atoms, there would be a greater chance of an electron hitting one, so a higher resistance than a metal with small atoms. The more electrons released, the more that can pass each point every second, so the lower the resistance.

  An increase in temperature would cause the atoms to vibrate more, so the electrons would have a higher chance of colliding with an ion, so the resistance would increase.

 Increasing the cross section area would decrease resistance.

...read more.


  •   Finding a method of straightening out the wire. There are two proposed methods:
  1. Hanging the piece of wire from a stand, and attaching weights to it.image08.jpg

  This method works well, but it must be insured that excess weight is not used, as this stretches the wire. Stretching the wire reduces its diameter, so also reduces its cross sectional area, so the measurements taken for a stretched wire are invalid. It is not necessary to use this method for the thinner wires, as they can easily be stretched out by hand using the following method.

  1. Straightening out the wire across a meter rule:

  It is difficult to straighten out the thicker wires with this method, because a large force is needed to do so, and it is easier to provide such a force with the previous method. However, for the thinner wires, this method works perfectly, as they can be straightened out very easily.image09.jpg

Proposed method:

  1.   Straighten out a 1 metre length of nichrome wire by attaching it to a stand clamped to a table and attaching weights weights to the bottom of the wire, or by pulling it across a metre rule. If the method with the weights is used, care should be taken not to let the weights fall on a person's foot, as this could seriously injure them, and the weights should be not so heavy as to stretch the wire.
  2.   Put crocodile clips one metre apart on the wire, and to attach them to a circuit with a power supply with a low voltage and very sensitive ammeter (or a milliammeter if the current is small enough)
...read more.


  There are enough accurate results to be quite sure of my conclusions. Out of the 20 average values taken, only three are not close to the manufacturer's values. That is an 85% success, but it would have been helpful to take readings with more, different diameter wires, but due to time and material constrictions, this was not possible. However, the method I used was not the best possible. If the wire was straightened out along a ruler, human error could result in the crocodile clips being attached slightly wrong, and they are not very thin at the tip, so exactly a meter is very hard to measure out. Therefore, obtaining much thinner crocodile clips would help to make results more accurate. Also, using weights to straighten out the thicker wires would stretch them slightly, giving them slightly lower cross sectional surface areas. To avoid this, it would be necessary to purchase the thicker wires not in coils, but in straight lengths, to give more accurate readings. The measuring instruments used were not the most accurate in existence. However, the most accurate equipment is very expensive, and thus could not be afforded for this investigation.

  There are many other variables that could affect the resistance of a wire. Temperature is one, and this could be tested by insulating the wire being tested, and submerging it in water baths of various temperatures while taking readings. Testing wires of different materials would show whether wires of the same length and diameter but of different metals have the same resistance or not. Varying the length of a wire being tested would show whether the length of a wire affects its resistance.

...read more.

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