Question

A 1.0mm diameter, 20cm long copper wire carries a 3.0A current. What is the potential difference between the ends of the wire?

Answer 1

$$
R=\rho\cfrac{l}{A}
$$
source - http://en.wikipedia.org/wiki/Electrical_resistivity#Resistors_or_conductors_with_uniform_cross-section

Where
$$
A=\pi r^2~m^2\\
r=\cfrac{.001}{2}~m\\
l=0.20~m\\
\rho=16.78\times 10^{-9}~\Omega\cdot m
$$
source - http://en.wikipedia.org/wiki/Electrical_resistivity

Potential Difference:
$$
Ans. V=IR\\
\text{where }I=3.0~A
$$

Make sense?