We need to find the resistance of the wire.
The resistivity and resistance are related as
R =
l / A
Where l is the length of the wire and A is that area of
cross-section.
R = 2.7 x 10-8 x l / 4.3 x 10-6
R = 0.6279 x 10-2 l
The power and the resistance are related as
P = V2 / R
We need to find the length of the wire that would develop a power
of 45 W when connected to a voltage of 20 V
45 = 202 / 0.6279 x 10-2 l
l = 1415.64 m
l = 1.416 km
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