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To investigate the resistance of a conducting material as its length changes.

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Introduction

Christopher de Wardt                                                         Dulwich College

Physics Coursework 2002

Planning

Introduction

Aim: To investigate the resistance of a conducting material as its length changes.

        I am carrying out this experiment to find out if as a result of changing the length of a conducting material – graphite paper in my case, the resistance changes directly because of this change. In order to make sure that the results, leading to further conclusions, are reliable, I must make sure that any other factors that could possibly affect resistance are kept constant.

Variable Studied - Length

        I will be using a circuit method in this investigation; there are a number of different factors that I must take into account when investigating the resistance of the graphite paper. Obviously, the first is length, which I will decrease from its starting length in equal steps as it approaches zero centimetres long; this is the main variable being considered in my investigation. The width of the paper must also be kept constant, as it may affect the results, and causing the investigation to become unfair.  To evaluate the resistance of the graphite paper I will record my observations in terms of voltage across the paper and the current of the circuit. Because

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Middle

20

3.35

30

3.29

40

3.24

50

3.19

60

3.14

70

3.09

80

3.05

image10.png

Conclusion of preliminary experiment:

As the maximum temperature change of the lab environment is only likely to be around a few degrees at the very most, my results suggest that the decrease of accuracy of my main experiment will be very small. For example, my graph shows that if there was a change of temperature from 20ºC to 22ºC in the lab, the accuracy of my results would 0.012kΩ too high, which would be a percentage error of around +0.36%. Therefore, in the context of this experiment, temperature will have almost no effect on the reliability of my results in the lab environment.

Method of main experiment

Apparatus:

                • 2 Volt cyclon cell

                • Analogue ammeter

                • Digital voltmeter

                • 45 cm strip of graphite-coated paper

                • Rheostat (potential divider)

                • Circuit wires

                • Modified Bulldog clips (2)

image06.png

The circuit should be assembled as shown above; the slide on the rheostat should always be adjusted so that the digital voltmeter reads 1 volt.

The current should be recorded for the following lengths of graphite paper:

Voltage/V

Length/cm

1

45

1

40

1

35

1

30

1

25

1

20

1

15

1

10

1

5

As shown in the table, and as I wrote earlier, voltage must be kept constant to make this a fair experiment. Also, the width of the graphite paper must be kept constant throughout the experiment – easily done by simply using the same piece of graphite paper cut to shorter and shorter lengths.

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Conclusion

-1 and as voltage was kept constant at 1 Volt in this experiment, effectively R=I-1. Because of this, and the line of best fit seems to suggest a ‘squared’ relationship, I believe a graph showing resistance against length would show a direct proportionality.

image12.png

        This graph shows that Resistance increases with length; it shows that resistance is directly proportional to length. Specifically for my width of graphite paper it shows that R= ~70*length. The fact that resistance has been shown to be directly proportional to length - in this case - fully supports my prediction.

This allows me to conclude that the scientific basis for my prediction was sound, and that the number of collisions between electrons and atom sites determines the value of resistance in a conductor.

Evaluation

        As I found that my results fell in line with previous scientific knowledge, it suggests that my experiment was carried out accurately, and that the use of repeat readings is a good way of getting closer to the ‘ideal’ values which previous scientific study suggests I should be finding. The accuracy of my results also shows that my conclusions have a firm basis; by comparing my own personal results with others carrying out similar experiments, they seem to be similar enough to ensure that they are accurate. In order to make sure these results are reliable, even further amounts of readings could be taken using different sets of equipment to ensure that the experiment remains fair.

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