Funny Wire

Metal wire A is 100 cm 100\text{ cm} long and has a diameter of 2 mm 2 \text{ mm} . Its resistance from end to end is 3.00 Ω 3.00\ \Omega .

Metal wire B is also 100 cm 100\text{ cm} long, but its diameter increases linearly from 1 mm 1 \text{ mm} at one end to 3 mm 3 \text{ mm} at the other end.

If the wires are made of the same material, what is the resistance of wire B (in Ω \Omega )?


The answer is 4.00.

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1 solution

Arjen Vreugdenhil
Apr 26, 2016

The resistance is proportional to R 0 L d x ( D ( x ) ) 2 , R \propto \int_0^L \frac{dx}{(D(x))^2}, where x x runs along the length of the wire and D ( x ) D(x) is the diameter of the wire at that point.

Taking L = 1 L = 1 , we have D A ( x ) = 2 D_A(x) = 2 and D B ( x ) = 1 + 2 x D_B(x) = 1 + 2x . So R A 0 1 d x 2 2 = 1 4 ; R B 0 1 d x ( 1 + 2 x ) 2 = [ 1 2 ( 1 + 2 x ) ] 0 1 = ( 1 6 ) ( 1 2 ) = 1 3 R_A \propto \int_0^1 \frac{dx}{2^2} = \frac 1 4; \\ R_B \propto \int_0^1 \frac{dx}{(1+2x)^2} = \left[-\frac 1 {2\cdot (1+2x)}\right]_0^1 \\ = \left(-\frac 1 6\right) -\left(- \frac 1 2\right) = \frac 13 where the proportionality constants are equal. Therefore

R B R A = 1 / 3 1 / 4 = 4 3 , \frac{R_B}{R_A} = \frac{1/3}{1/4} = \frac 4 3, so that R B = 4.00 Ω R_B = \boxed{4.00}\ \Omega .

Sir, are you a professor? Your solutions, activity makes me feel that you are a teacher.

Aditya Kumar - 5 years, 1 month ago

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Yes, I am a teacher. I used to teach Physics at a college.

Arjen Vreugdenhil - 5 years, 1 month ago

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Wow nice! Which college?

Aditya Kumar - 5 years, 1 month ago

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@Aditya Kumar Grand Valley State University, Allendale, MI

Arjen Vreugdenhil - 5 years, 1 month ago

Solved this exactly the same way! Great problem, Arjen!

tom engelsman - 5 years, 1 month ago

Assuming the cross section is changing linearly over the wire (what I did confused) one gets another result namely:

1.5 * ln(9) Ω \Omega = 3.3 Ω \Omega .

Andreas Wendler - 5 years, 1 month ago

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