A wire of cross-sectional area A, length L 1 , resistivity 1 , and temperature coefficient

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A wire of cross-sectional area A, length L1, resistivity ρ1, and temperature coefficient α1 is connected end to end to a second wire of the same cross-sectional area, length L2, resistivity ρ2, and temperature coefficient α2, so that the wires carry the same current.

(a) Show that if ρ1L1α1 + ρ2L2α2 = 0, the total resistance R is independent of temperature for small temperature changes.

(b) If one wire is made of carbon and the other is copper, find the ratio of their lengths for which R is approximately independent of temperature.

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