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An experiment to find the Resistivity of Pencil leads in different degree of hardness

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An experiment to find the Resistivity of Pencil leads in different degree of hardness


A lead pencil, is not really made of lead, instead, it is made from graphite, which is a form of carbon. Carbon is one of a class of substances that will conduct electricity, like a conductor, but will also block some current.

(the parameter in about why under what circumstances it does not conduct)


The aim of this experiment is to find out the resistivity with different hardness of graphite (pencil leads, which relates to quantity of carbon in the lead). To measure the resistance of the lead, the ohm’s law equation will be used R= V/I (despite heat having been produced)

  , then

Using the equation: A (Cross-sectional area) R (resistance)/ L (Length) =ρ (Resistivity)

ρ=    A R     image00.png


Preliminary Work:

In order to find out the range of voltage to be used, the following data shows the result from the experiment of HB pencil lead.

Power Supply Unit (volts)

Current (amp)

Voltage (volts)































From the table above, it shows there is a significant dropped in the amount of voltage after the

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Clay that is a good insulator of electricity so I think this may affect the resistance characteristic but if the graph were to look like this it is because carbon is a semi conductor. This is due to that there is a distinct possibility that the pencil lead may have the characteristics of a semi conductor, as carbon is a semi conductor. From the graph above, to find the gradient I would use the results from the point (0,0) up until the line starts to curve.

  • Pure graphite is classed as a semi-metal and has a typical electrical resistivity of 3x10-5 ohm-m. Most pencil cores are a mixture of graphite and clay. (Predict the proportion of clay to graphite from your results?)
  • The Higher the resistance of a pencil lead, the lower the resistivity it has


Independent variables:

  • Different types of pencil??

When conducting this experiment I will alter only one variable at a time and keep the other variable constant at a fixed measurement. This will enable me to draw conclusions from my results. When changing the hardness of the pencil, HB, 4B and 8B, I will keep the length of the pencil the same as possible.

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Set HB pencil in a parallel circuit(with voltmeter and ammeter)



Change the unit on the power supply from 2-6



Record both readings on voltmeter and ammeter



Repeat the same steps for 4B, and 8B.


Repeat the same methods order again for each pencil for second trial.


Repeat the same methods and order again for each pencil for final trial.


To maintain safety in the lab, current passing through the graphite should be checked out that it must not be over 1 amp, this is because the current will make the electrons flowing flow at a faster rate creating friction through the graphite which could make it burn, which means that we should not use a range of 10v or above with the power supply. Also the experiment will be well away from any taps, and liquid. This is precautions, to prevent injury. But the risks from this experiment are not that great.

...read more.

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