Measuring The Resistivity Of A Pencil Lead.
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PHYSICS COURSEWORK 16TH JANUARY 2003 Measuring The Resistivity Of A Pencil Lead Hypothesis The resistance of two objects is not always the same due to "resistivity". The resistance of a uniform conductor depends on : * Its length (L) * Its cross-sectional area (A) * The material of which it is made Therefore, the relationship between these three variables and resistance is as follows: Where : - R = Resistance P = Resistivity L = Length A = Cross-sectional Area Resistivity is defined as a materials opposition to the flow of electric current of a cubic metre of a material. To measure the resistivity of pencil lead I will need the following apparatus....... * 1 x Power Pack (Ranging from 2V - 12V) * 1 x Voltmeter (Reading up to 20V, giving readings to 0.00 with a ?3% error reading) * 1 x Ammeter (Reading 20m/10A, giving readings to 0.00 with a ?0.5% error reading) * 1 x Rheostat * 1 x Micrometer (with an error reading of ? 0.005mm) * 1 x Metre rule (with mm graduations, with an error reading of ? 0.5mm) * 2 x Crocodile Clips * 2 x Identical HB Pencils * 6 x Electrical Wires The above apparatus will be set-up as below : Method Once the apparatus has been set-up as above I will first carry out some preliminary experimentations to help me carry out my main investigation. From the preliminary experiments I aim to find the following : * What settings the power pack should be set at i.e.
Variables The variable in this experiment will be length. The reason I have chosen this as a variable is because it is the easiest to record and to carry out the experiment. I could have chosen to vary the pencil's diameter or hardness (i.e. 1B 2B 3B etc). The variables I will be keeping constant in this experiment are as follows : * Current @ 0.1A * Voltage from the power pack @ 4V * Hardness @ HB * Diameter @ 2.05mm From my preliminary work I found that the maximum length I could get out of my pencil was 175mm and the minimum was 55mm. I also decided to use 10mm intervals as any larger would have not given me enough results and any smaller would have been too hard to get accurate lengths. If I used the range at 10mm intervals I would have obtained 13 results. I felt this was too much so I decided to have a maximum of 175mm and a minimum of 75mm. This gives me eleven results and I feel that this is sufficient to obtain a good set of results. Pencil Length Voltage Resistance Average Resistance Units mm V ? ? Error % ? % ?3% ?3.5% ?3.5% NCE + - NCE + - 175mm 162mm 155mm 145mm 135mm 125mm 115mm 105mm 95mm 85mm 75mm Key : NCE = Not Considering Errors Errors The main sources of error will be * In the voltmeter with a percentage error of ?3% * In the ammeter with a percentage error of ?0.5%
This would have affected the results if, for instance, the crocodile clips were touching the table desk. My graph is quite accurate I feel as there are few anomalies and this allows me to make a strong conclusion as the results are as expected. The data does justify a strong conclusion as it is what I would have expected and there are very few anomalies. My results aren't perfect as there are always factors affecting the recording of results. The biggest factor is the human error. As I was taking the results in this experiment there leave's a big gap for human error this means that my results were not perfect. To improve my experiment I would have recorded all of the data directly to a computer. This would allow for more accurate timing and instant results. As mentioned earlier I would have removed the pencil lead from the pencil in future experiment as to make sure that the crocodile clips didn't touch the encasing wood of the pencil. Extension There are many extensions that could be done to this experiment such as : * Using softer or harder pencils (i.e. 1B, 2B 3B etc) * Changing the value of the current thus the temperature of the pencil * Changing the temperature of the lead (keeping the current at a fixed value) * Use different brands of pencil (might be composed of different materials) The same set-up could be used for these experiments so the same recording methods and results could be used.
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