Answer:
V = I * R
R = 2 / 3.5 = .571 ohms maximum resistance of wire
R = ρ L / A where R is proportional to L and inversely proportional to A
A = ρ L / R minimum area of wire
ρ = 1 / μ = 1.67E-8 ohm-m resistivity inverse of conductivity
A = 1.67E-8 ohm-m * 225 m / .571 ohm = 6.68E-6 m^2
A = 6.68 mm^2 since 1 mm^2 = 10-6 m^2 or 1 mm = 10-3 m
A = Π r^2 = 6.68 mm^2
r = (6.68 / 3.14)^1/2 mm = 2.13 mm radius of wire
d = 2 * r = 4.26 mm
