Q1. The resistance of a copper wire 200 m long is 21 Ω. If its thickness (diameter) is 0.44 mm, its specific resistance is approximately:
A 1.6 × 10^-8 Ω-m Correct
B 1.2 × 10^-8 Ω-m
C 1.4 × 10^-8 Ω-m
D 1.8 × 10^-8 Ω-m
Explanation
Specific resistance is found from rho = RA/L. The diameter is 0.44 mm, so the radius is 0.22 mm, or 0.22 × 10^-3 m. The cross-sectional area is pi r^2, about 1.52 × 10^-7 m^2. Substituting R = 21 Ω and L = 200 m gives rho = 21 × 1.52 × 10^-7 / 200, which is about 1.6 × 10^-8 Ω-m. The other values are lower or higher than this calculation.