The temperature coefficient of resistivity of iron is 5.0 10-3 / C; that of carbon is -0.50 10-3 / C. When an iron wire and a carbon rod, each having the same 10 W resistance at 20C, are cooled from that temperature to -80C, the new ratio of the resistance of the carbon rod to the resistance of the iron wire at the lower temperature is
a. -0.10.
b. +1.9.
c. +2.1.
d. -10.
e. +10.
Question 2
If a mile of 24-gauge copper wire has a resistance of 0.14 kW and the resistivity of copper is 1.7 10-8 W m, what is the diameter of the wire? (1 mile = 1.6 km)
a. 0.40 mm
b. 0.50 mm
c. 0.63 mm
d. 0.80 mm
e. 0.25 mm
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