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Testing the Resistivity of Pencil Lead.

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Introduction

RESISTIVITY OF PENCIL LEAD

CANDIDATE’S NAME: BAŞAK BAYRAMBAŞ

SUPERVISOR’S NAME: MİNE GÖKÇE ŞAHİN

CRITERIA TO BE EVALUATED:

DESIGN

DATA COLLECTION & PROCESSING

CONCLUSION & EVALUATION


DESIGN

Research Question: How does length of pencil lead affect the resistance of the pencil lead when the pencil is connected to power supply from two ends and factors like temperature of the medium, type of lead, voltage of power supply etc. are kept constant?

1)The resistivity depends strongly on the presence of impurities in the material

2)Resistivity and Temperature Coefficients at 20oC reference

The electrical resistance of a wire is greater for a longer wire and less for a wire of larger cross sectional area. The resistance depend on the material of which it is made and can be expressed as:

R = ρ L / A         (1)

where

R= resistance (ohm)

ρ= resistivity coefficient (ohm m)

L= length of wire (m)

A= cross sectional area of wire (m2)

The factor in the resistance which takes into account the nature of the material is the resistivity. Since it is temperature dependent, it can be used to calculate the resistance of a wire of given geometry at different temperatures.

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Middle

Bind one end of conducting wire to power supply and other to the end of pencil lead.Bind one end of other conducting wire to 9.0cm away from the other one which is bound to pencil lead before, measure the distance between two conducting wires on the pencil lead with ruler. Then bind other end of conducting wire to power supply.Keep voltage of the power supply constant at 1.0V. Get measurements of current as soon as you turn on the power supply.Record data you have taken to data table.Repeat this process for different lengths (8, 7, 6, 5, 4, 3, 2, 1, 0 cm) of pencil lead between two conducting wires.Make sure you make at least 5 trials to get more precise results.

image19.jpg

Figure 2: Sample figure of the experiment

DATA COLLECTION & PROCESSING

Length (±0.1cm)

Current(±0.01A)

9.0

0.10

8.0

0.12

7.0

0.13

6.0

0.15

5.0

0.17

4.0

0.20

3.0

0.26

2.0

0.27

1.0

0.38

0.0

0.82

Table 2: Length (cm)

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Conclusion

image15.png

where R is resistance and L is length of pencil lead. By using another formula which is image16.png

 , another equation which is as below is obtained:
image04.png

        As a result of these steps shown above and the experiment, resistivity coefficient of pencil lead is found as image17.png

. This uncertainty shows that the experiment has lots of error sources and limitations.

        First and the most significant one these error sources could be itself of the pencil lead. In this experiment it is tried to be found the resistivity coefficient of the pencil lead. However, the lead used in pencil may not be pure. That’s why the results are much more than expected. Using pure lead would be a solution for such this error.

        Another error source can be temperature. High temperature would cause pencil lead to expand and this would also increase its resistance. As a result, calculations would be made with greater values and the results would be more observable and accurate.


[1]http://www.engineeringtoolbox.com/resistivity-conductivity-d_418.html

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