An Investigation of the Resistivity of Charcoal

1. An Investigation of the Resistivity of Charcoal Presented by: Si Hongbo Lin Siqi Wang Chengxu Arun Balasubramaniam

2. Resistivity • Electrical resistivity (also known as specific electrical resistance) is a measure of how strongly a material opposes the flow of electric current. • where • ρ/ Ωm, is the resistivity • R/ Ω, is the electrical resistance of a uniform specimen of the material • l/m, is the length of the specimen • A/m², is the cross-sectional area of the specimen http://en.wikipedia.org/wiki/Image:Resistivity_geometry.png

3. Theories & Principles • Ohm’s Law: • where I is the current, V is the potential difference, and R is a constant called the resistance • Resistivity equation:

4. Hypothesis • Charcoal sticks are not made of pure carbon, but mixed with soil. • Hence, the resistivity measured would be higher than the resistivity of pure carbon • Resistivity of carbon is about 3.5×10-5Ωm • Therefore, I hypothesize that the resistivity of charcoal would be larger than the conventional resistivity of carbon.

5. Objective & Setups • To investigate the resistivity of charcoal. • 6 thin charcoal sticks with the same diameter but different lengths

6. Procedure • Measuring the length of the charcoal stick. • Adjust the rheostat to the highest resistance. Close the switch. • Adjust the rheostat to certain extent, and record down the voltage V and ampere A shown. • Calculate the resistance values using Ohm’s Law. • Plot a graph of lnR vs. lnl. Determine the average resistivity of charcoal.

7. Results & Data Cross-sectional area = 0.00005 m²

8. Results & Analysis lnA = -9.9 Plot a graph of lnR vs. lnl, so the y-intercept is

9. Results & Graph Plot a graph of lnR vs. lnl We can get the y-intercept to be (0 , 1.65), which is

10. Conclusion • We have gotten the experimental value of the resistivity of charcoal, which is about 2.6 x 10-4 Ωm. • The value is larger than the resistivity of carbon, but still very low. • The hypothesis is accepted. The objective is fulfilled.

11. Precautions • Experimental precautions: • Charcoal sticks: homogeneous • Temperature • rheostat adjustment • Safety precautions: • keep the voltage within a safety range • To protect the ammeter and voltmeter

12. Evaluation • Assumptions: • There is no temperature change. • The charcoal sticks are in regular shape and are thin cylinders. • The charcoal sticks are homogeneous: the percentage of carbon and soil is consistent for all 6 sticks. The carbon and soil are evenly dispersed.

13. Evaluation • Assumptions: • The resistance of the voltmeter is extremely large, since its resistance would affect the ampere values recorded. • The experiment is done under constant voltage, since the power supply may work inconsistently.

14. Evaluation • Error sources: • Temperature changes affect the resistances of most conductors, since the speed of the delocalized electrons or ions varies with temperature. • High voltage and current cause more heat, due to Joule’s Law: • Inconsistent voltage supply

15. Evaluation • Improvement & Extension: • To use electronic multi-meters, which can measure the variables more accurately, instead of classical mechanical ammeters and voltmeters. • To get a better and more consistent power supply. • To get purer charcoal, so that eventually we can measure the resistivity of carbon approximately.

16. Evaluation • Personal Comments: • This is the 1st experiment on electricity we have ever carried out since last year. • Experiments (high school level) on electricity & magnetism are relatively easier to conduct. The results are easier to recorded. • From this experiment, we revised the knowledge on resistance, and its related matters.