A high voltage transmission line has an aluminum cable of diameter 3.0 cm , 200 km long . What is the resistance of this cable?
Use the relation R = ρ l/A
A= πr2 =πd2/4
= 3.14 x 3 x 10-2x 3 x 10-2/4
ρ= 2.82×10−8 ohm m
l = 200km = 200000 m = 2 x 105m
You can now just substitute the values and get the answer.