Question

What length of copper wire 1 mm in diameter is needed to make a 30 ohm resistor at 18 degrees C.

Answer 1

the resistivity of Copper at 20 °C is 1.68 * 10^-8 , it's temperature coefficient is 0.0039 ℃^-1

∆T = 18 -20 = -2℃

= 1.68×10^-8×(1+0.0039×(-2))

= 1.667×10^-8

Cross section area is

A = πd²/4

= π ×(1×10^-3 m)²/4

= 0.785 ×10^-6 m²

= 30×0.785×10^-6/(1.667×10^-8)

=  1413 m