The resistivity of copper is 1.7 × 10-8 Ωm. What length of copper wire of diameter 0.1 mm will have a resistance of 34Ω?
Given
Resistivity (ρ) = 1.7 × 10-8 Ωm
Diameter (d) = 0.1 mm = 0.1×10-3 m
(∵ 1m = 1000 mm)
Resistance(R) = 34 Ω
Area of cross section (A) = 
A = 
⇒ A =
Now
⇒ 
⇒ 
⇒ 
⇒ 
Hence the length of the wire is 15.7 m.
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