# Math

Problem:

What is the resistance of a 3.9-m length of copper wire 1.1 mm in diameter? The resistivity of copper is 1.68×10^−8Ω⋅m.

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1. R = PL/A.

P = 1.68*10^-8 Ohm*m.
L = 3.9 m.
r = 1.1*10^-3/2 = 5.5*10^-4 m.
A = pi*r^2 = 3.14*(5.5*10^-4)^2 = 9.5*10^-7 m^2.

R = 1.68*10^(-8)*3.9/(9.5*10^(-7)) = 0.069 Ohms.

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