'show that a 2.5m length of wire with cross section area 0f 1.0mm and resistance of 0.75ohms has a resistivity o f 3.0 ×10-7'
cross section must be in mm^2.
So,
l = 2.5m
A = 1.0 mm^2 = 0.01 cm^2 = 10^-6 m^2
R = 0.75 ohm
Then, resistivity (r) is
r = (R.A)/l
= (0.75.10^-6)/2.5
= 3x10^-7 ohm.m