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To find out which factor or factors determine the resistance of a length of carbon in the form of a pencil.

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Physics Coursework

Aim: To find out which factor or factors determine the resistance of a length of carbon in the form of a pencil.

Introduction: The reason we are investigating carbon is because it has very interesting properties and because it is such an important element in electronics. What makes carbon so interesting is that it is a semi-conductor, this means that it behaves like an insulator when cold but when warm it becomes poor conductors. Also carbon in the form of graphite, which we are using in this investigation is very unusual because it is a non-metal which conducts. In fact it is the only non-metal pure element that conducts electricity and this is the reason why it is so useful in electronic circuits.

Theory: Resistance in terms of this experiment is the slowing of the flow of current through the carbon track by anything in the carbon track. To explain this definition fully we first have to explain conduction, which is the flow of current through a conductor. The current flows by means of free electrons through the conductor and so the more free electrons or the larger the “sea” of free electrons the better conductor the material is. EG. Iron has less free electrons than Gold and so gold is a better conductor.  

The electricity put into the material then gives the free electrons a charge and so they begin to move randomly and crash into other free electrons. This happens all along the length of the conductor and so the electrons begin to move from the negative connection to the positive connection (this happens because electrons are negatively charged). This is how the current flows.  The resistance in the carbon track is caused by these charged free electrons crashing into fixed particles (nuclei and protons)

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Equipment: Using a digital multimeter in the ohm-meter mode.

Advantage:Simple and quick to use and has different ranges to measure both large and small resistances accurately. It’s very safe to use as it only puts a small current through the example.

Disadvantage:Accuracy not that good for small resistances and the resistance of the connecting wires has to be taken into consideration.

 Best choice:Digital multimeter in the ohmmeter mode.

Reasons:I have chosen this because it is far quicker to use meaning that I can repeat readings to get more reliable results and the results also have a smaller percentage error. Although it is not particularly good at measuring small resistances, I expect that carbon being a semi-conductor will have quite high resistances.

The smallest value of length I will measure will be 2cm and the largest will be 20cm. I will take a reading every 2cm and so this will give me a total of 10 readings.

The smallest value of width I will measure will be 0.5cm and the largest will be 5cm. I will take a reading every 0.5cm and so this will give me a total of 10 readings.

To make sure the readings are reliable I will repeat this at least two times and more if time permits. To make sure the readings are as precise as possible I will measure both the length and the width to the nearest ½mm.

Precautions and special techniques:When I connect the carbon track to the ohmmeter, I have to decide between these two connection ideas.

Idea:A sharp pointed connector

Advantage:Can easily measure accurately and confidently between the two connectors because the connector is very thin.


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Another improvement to the experiment I would make would be improving the accuracy of the carbon track. This would mean that the carbon would be uniformly thick throughout the line and would go exactly up to the line and not pass it. This is so that we could know that the resistance was of exactly 5cm was not the resistance of between 4.5cm and 5.5cm. To do this I would make a template for the carbon track, which, could be coloured in ensuring that the carbon would no go past where it is, desired which happens no matter how hard you try to stay within the lines. An example of this is in the carbon track I drew, which appears to be quite good in that no carbon went over the lines but on closer inspection it in place is over a millimetre past the line.

My carbon line in normal view:


As you can see the line appears perfect.

Magnified view of Carbon track:


This closer view shows that the carbon track is by no means perfect and this degree inaccuracy means that the results may not be what they should.

The patterns in my graphs were fairly obvious to find and I believe that when I did use a straight fit best line instead of a curve it was acceptable because it was more fitting for the readings.

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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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