An experiment to find how the resistance of a pencil lead varies which the change in length

Authors Avatar

Nick. Thorburn                                        MJ                                            15 March 2004

An experiment to find how the resistance of a pencil lead varies which the change in length

        The problem with which I have been presented is to find out the most efficient type of pencil lead with which to conduct electricity. I will look at these variables:

  • The type of pencil lead, i.e. HB, 5H, 6H.
  • The length in cm.
  • The cross-sectional diameter.
  • The temperature in oC.

In the preliminary work I will establish how I will conduct my actual experiment, and I will also decide what my dependent and independent variables will be. There are four initial factors which I am going to discuss below; these will be all the possible variables.

Firstly I can vary the length. This seems the most convenient. I can measure this accurately, to the nearest millimetre, although it may not give me a wide enough range of variables. I will need relatively little special equipment, only two crocodile clips which will be moved along a pencil lead glued next to a scale, against which I can check my measurements. As the length increases I should expect the resistance to increase, due to the larger distance the electrons have to travel.

Secondly I can vary the temperature of the pencil lead. As it becomes hotter, I will expect resistance to decrease, and as it cools off again it should increase. This is because the heat would give the electrons increased kinetic energy, which means they would move through the atoms in the pencil lead faster. This could give me a very wide range of readings, but it is difficult to heat and monitor something solid like a pencil lead, and it would take too long to get accurate temperatures, to the nearest degree. As I don’t have enough time, it would be likely that temperature readings could be off by at least two or three degrees, as I wouldn’t be able to let the lead cool down if I overshot.

Thirdly I could vary the cross-sectional area of the pencil lead. However, the stores do not actually have any of varying diameter, and I would have to cut them down myself. This would have led to inaccuracies in the cutting, and thus an unfair test. It would require a large number of diameters to actually gain a good range of results, and these were not available. I would expect the resistance to decrease when the diameter increased, and as it became narrower, I would expect resistance to increase. This is due to the narrowness of the pencil lead, which cause more electrons to collide with graphite molecules in the course of their flowing through the lead.

Finally, I could vary the type of pencil lead – 2H, 5H and 8H. As the pencil lead “hardness” increased, the resistance would increase due to the pencil becoming denser. This would almost certainly not give a great enough number of results, or a great enough range.

I decided upon length, due to ease of use (a ready-prepared lead with attached ruler), accuracy (to the nearest millimetre), the range and number of results (30+ with repeats) and the amount of equipment needed. This is particularly useful, as I would only require two crocodile clips in addition to the other apparatus.

My variable for this investigation will be how the length of the pencil lead affects its resistance.

Preliminary work

After having performed my practical preliminary experimental, these were my results.

Join now!

        From these results I decided to use 8H, as it showed the greatest range. I will also measure the resistance starting at 1cm, ending at 10cm, with increments of 1cm Results will be repeated twice, and all anomalies will be repeated until a satisfactory result is obtained. However, for analysis purposes I will use the average of these resistances in order to produce more accurate results.

        The maximum current I am likely to obtain from 8H is:

          Imax  =    Vmax          =        6V

     

...

This is a preview of the whole essay