Pencil “lead” is actually carbon. What is the resistance of the 0.680 mm-diameter, 4.40 cm-long lead from a mechanical pencil?
3 Answers
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RickB was almost correct. You multiply the resistivity by the length and divide by the cross-sectional area. A long, thin resistor will have a greater resistance than a short, wide resistor made of the same material.
Also, different forms of carbon have different resistivity. You’ll want to look up the resistivity of graphite.
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Look up the resistivity of carbon in a table.
Multiply by the pencil lead’s cross-sectional area (remember A=πr²).
Divide by the pencil lead’s length.
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Resistivity of Carbon (graphite) constant at RT = 3.5e -5
Formulas:
Area = (pi(r^2))
r = (0.680/2/(1000))
pi = 3.14
Resistivity = Res. Constant (σ) * Length (L) / Area (A)
Resistivity = 3.5e -5 * .0440 m / 3.62984e -7 = 4.2426 Ω
