Metal wire A is
long and has a diameter of
. Its resistance from end to end is
.
Metal wire B is also long, but its diameter increases linearly from at one end to at the other end.
If the wires are made of the same material, what is the resistance of wire B (in )?
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The resistance is proportional to R ∝ ∫ 0 L ( D ( x ) ) 2 d x , where x runs along the length of the wire and D ( x ) is the diameter of the wire at that point.
Taking L = 1 , we have D A ( x ) = 2 and D B ( x ) = 1 + 2 x . So R A ∝ ∫ 0 1 2 2 d x = 4 1 ; R B ∝ ∫ 0 1 ( 1 + 2 x ) 2 d x = [ − 2 ⋅ ( 1 + 2 x ) 1 ] 0 1 = ( − 6 1 ) − ( − 2 1 ) = 3 1 where the proportionality constants are equal. Therefore
R A R B = 1 / 4 1 / 3 = 3 4 , so that R B = 4 . 0 0 Ω .