Question

A copper rod, 0.4 m long and 4.0 mm in diameter, has a resistance of 550 μΩ at 20 °C. Calculate the resistivity of copper at that temperature. If the rod is drawn out into a wire having a uniform diameter of 0.8 mm, calculate the resistance of the wire when its temperature is 60 °C. Assume the resistivity to be unchanged and the temperature coefficient of resist- ance of copper to be 0.004 26/°C

Answer 1

Answer:

Correct answer : 0.0173 x 10^-6 ohm m, 0.398ohm My steps as follows:

the resistivity of copper : 0.0173 x 10^-6 ohm m ( I've got it, no problem!)

the resistance of the wire :

Area : π x [(0.8/2)x10^-3]^2

RO= Pl/A =[ 0.0173 x 10^-6 x 0.4] / 3.14 x 1.6 x 10^-7 =0.01377ohm

R1 /R2= (1+0.00426x20) / (1+0.00426x60) R1/0.01377ohm = 0.8652

R1 = 0.1184ohm (wrong answer!!)