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Resistance of Carbon Putty.

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Our aim was to investigate a set of factors that affected the resistance of a mass of carbon putty.

Preliminary work:

From my physics textbook ( Physics for you by Keith Johnson), I have discovered that there are several factors that affect the resistance of a conductor:

  • Surface area: Thick wires may be regarded as equivalent to a number of thinner wires of equal area joined in parallel. Doubling the area will therefore halve the resistance. This means that the surface area is inversely proportional to its resistance.
  • Length: Double the length of the wire will double the resistance as twice the length of the wire is equivalent to two equal resistance in the series.
  • Temperature: The resistance for a metallic conductor is a constant if the temperature and other physical properties remained constant. In general, for metallic wires, the higher the temperature, the larger its resistance. But for some materials e.g. carbon and semi conductors like silicon and germanium, the higher the temperature, the lower its resistance. The resistance of most of the alloys, e.g. manganin and constantan, is only affected slightly by a change in its temperature.

The properties of Carbon:

  • Name: carbon
  • Symbol: C
  • Atomic number: 6
  • Group number: 14
  • Group name: (none)
  • Period number: 2
  • Atomic weight: 12.0107 (8) gr
  • Standard state: solid at 298 K
  • Colour: graphite is black, diamond is colourless
  • Classification: Non-metallic
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These readings clearly show a pattern in that as we decrease the length of the putty, the potential difference gets bigger and the resistance decreases. What can also be seen is that from a length of 14.5 cm where the resistance was 16.5 ohms to a length of 7.5 cm where the resistance was 7.7 ohms the resistance has almost halved. This process was very time consuming, as we had to first measure the current and then calculate the resistance. As we were also provided with an Ohmmeter I decided to measure the resistance directly.  


I am investigating the length of the carbon putty as this factor is connected to the physical effects of the putty.It is much harder to record the temperature of the conductor than to measure the length of the carbon putty. And the experiment to investigate this factor should hopefully give a good straightforward result.


I predict that as I increase the length of the putty the resistance will also increase. From all the information collected in my preliminary work I predict that as I double the length of carbon putty the resistance will also double


I ensured this was a safe experiment by:

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One error with this experiment is that the wire that connects the putty to the Power pack has its own resistance and this could have affected results. We tried to overcome this by always using the same wires, and therefore this fault was built into the system. Another error that could have occurred is the fact that the diameter of the putty was achieved by hand rolling it. This could have led to un-evenness due to different hand pressures and could also produce air bubbles.

I would like to extend this experiment by checking the resistance of putty at both above and below the length used in this one to see if the graph I have drawn is accurate. This would be above 12cms and below 2cms.

 If I was able to further extend my experiment I would like to change the diameter of the putty and see if this made any difference to the resistance.  I could use both thicker diameter than 2.2cm and/or thinner than 2.2cm.  This would enable me to compare all the results and see if the pattern held true for all thicknesses of putty.  I could use a Kenwood mincer or a spaghetti maker to ensure that the putty was the same diameter all along its length. .

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