Prediction:
In pencil lead, as the hardness of the lead increases the higher the resistance. This is because the harder the pencil lead is, the greater the proportion of clay is in the lead, this means that it will need a large potential difference (voltage) across it, in order to get a current to flow through it.
Hypothesis:
Pencil lead is made from a combination of finely ground graphite and clay, mixed with water and pressed together it is then heated into thin rods at high temperatures. Graphite is largely made up of carbon, which is quite a good conductor of electricity, but clay is not. This means the more clay that is mixed with the graphite in the pencil lead the higher the resistance will be. The resistivity of a material is an individual property. It does not depend on the dimensions of the component only the materials from which it is made. In order to find the resistance of the pencil ohm’s law will be applied. The expected graph for the resistance graph will be:
The reason I predict the graph to look like this is because I assume that the pencil lead will be an ohmic resistor, this means that the current will be directly proportional to the voltage. The gradient of the graph is also the resistance of the pencil lead. Hence the finding of R, the calculated gradient or resistance will then be used along with the cross-sectional area and the length of the pencil lead the resistivity formula. But, this will only happen when there is no voltage drop when passing through it as in the preliminary experiment it has shown that this is not possible in real life. Since pencil lead allows electricity to move through the circuit, but some of the electrical energy will be lost to in the form of heat due to collisions that the electrons make with atoms and ions in the lead. This means that the graph could look like this:
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.
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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
Variables:
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.
The voltage being passed through the carbon rod will be kept at between a range of 2v-6v. However, due to the resistance around the circuit, the voltage that is applied originally before passing through the circuit is larger than when it has passed through the circuit. However, this does not matter to the experiment, as the potential difference is still constant in its range, and when finding the gradient only the straight line before going into a curve will be taken into account. This is an important part of the procedure.
Dependent variables:
For the individual experiment the dependent variables will be the current, whereas in the overall experiment the dependent variable is the resistance R in Ohms law. From the individual experiment current will be found, and then I will work out the resistance by plotting a graph and then the resistivity of the pencil lead by using the formula:
Controlled variables:
In order to make sure the experiment is a fair test I will need to do the following:
- The length will remain constant because the resistance varies with the length. If the length is increased, the resistance will also increase, as there are more positive ions to obstruct the electrons (current). The best way to achieve this is to keep the pencils used for each experiment the same, as well as constantly measure the length of the pencils with a veinier throughout the experiment.
- Pencil leads come in different hardness’s so I will have to keep the same hardness of the pencil leads that I decided to use. To do this I will use the same piece of lead for HB, 4B and 8B, throughout the experiment.
- I will use exactly the same equipment throughout the experiment as different wires have different diameters and length while the meters may all vary slightly. As well as the same voltmeter and ammeter, because each electronics might have a slightly different reading on them.
- The temperature of the pencil lead will affect the resistance. It is especially important that this remains the same as materials that are ohmic resistors, at high temperatures, can change the characteristic of the material (no longer an ohmic resistor). The higher the temperature the higher the resistance because the electrons move faster and the positive ions vibrate more due to an increase in kinetic energy. This means that there are more collisions between them leading to an increase in the resistance. To achieve keeping the temperature of the pencil lead constant I will let the pencil lead cool for 30 seconds when changing the volts from power supply. In addition, the 3 different trials for each pencil lead will not be repeated in order for each lead to cool down. Consequently, there will be a period of time between experiments.
Apparatus:
- Power pack – A range of 2V – 6V
- Venier- to measure the length of each pencil
- Micrometer – To measure cross section of Graphite.(area)
- Ammeter – To measure the current
- Voltmeter – To take readings of Voltage
- Crocodile Clips – Used to place them on the graphite at the exact length.
- Graphite Pencils- This is the conductor (HB,4B,8B)
- Length of each pencil- 14.5cm
- diameter of each pencil
- HB=2.09mm
- 4B=2.76mm
- 8B=2.83mm
- Wires
- Pencil Sharpener
- Digital Stop Watch
Method:
Safety
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.