As you can see from my preliminary results my resistance did not fluctuate at a current of 0. 1A at each extremity of the pencils length. From these results I can see that the temperature did not rise and thus affect the results. This tells me that my settings for my investigation should be : 1. Power pack set at 4V 2. Current at 0. 1A Method Now I have completed the preliminary work I am able to start my main investigation. I will start the experiment by setting up the apparatus as shown in my diagram.
Once the apparatus is set up I will sharpen the pencil at both ends to its first length. I will then place the crocodile clips on the exposed pencil lead and switch on the power pack. I will then slide the rheostat so that the ammeter reads 0. 1A. I will leave the apparatus on for 30 seconds then switch off and record the readings on the voltmeter. I will do this five times for each length. Once I have my reading for the length I will sharpen the pencil down to the next length and continue to record the results five times at 30 second intervals.
Once I have obtained all my results I will cut open the pencil with Stanley Knife to expose the lead. Once I have the lead I will measure it with a micrometer three times and then average out to get the diameter of the pencil lead. This will be used later on to calculate resistivity. Safety There are few safety issues in this experiment but they still need to be taken into consideration. One safety issue to take care of is the current through the circuit, if this current is too high the wooden encasing the pencil lead could ignite and cause a fire hazard.
The other safety issue to consider is when cutting open the pencil to measure the lead’s diameter. I will be using a very sharp Stanley Knife so care needs to be taken as not to cut ones self. 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.
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.