Question

What is the resistance of a 1.3-m-long copper wire that is 0.30 mm in diameter?

Answer 1

The resistance R of a wire is given by:
$R= \frac{\rho L}{A}$
where 
$\rho$ is the resistivity of the material
L is the length of the wire
A is the cross-sectional area of the wire.

For the wire in the problem, the resistivity is (copper resistivity) $\rho=1.68 \cdot 10^{-8} \Omega m$. The length of the wire is L=1.3 m, while the cross-sectional area is
$A=\pi r^2 = \pi ( \frac{d}{2})^2 = \pi ( \frac{0.30 \cdot 10^{-3} m}{2} )^2 =7.07 \cdot 10^{-8} m^2$

so the resistance of the wire is:
$R= \frac{(1.68 \cdot 10^{-8} \Omega m)(1.3 m)}{7.07 \cdot 10^{-8} m^2}=0.31 \Omega$