I'm gonna make him an offer he can't resist(ance)

A conductor of length l l has a uniform cross section.

The radius of cross section varies linearly from a a to b b .The resistivity of the material is ρ \rho .

Find the resistance of the conductor across its ends in Ohms .

Details and Assumptions

  • ρ \rho = 6.28 × 6.28\times 1 0 8 10^{-8} Ω \Omega m
  • l l (length of conductor) =10 cm
  • a a =2 cm and b b =5 cm


The answer is 0.000002.

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2 solutions

Chew-Seong Cheong
Aug 20, 2014

The resistance of the conductor across its ends is given by: R = 0 l ρ d x π r 2 R=\int_0^l{\cfrac{\rho dx}{\pi r^2}}

where x x is the distance along the axis from the left end and r r is the radius of the cross section at x x , and r = a + b a l x = 0.02 + 0.3 x r = a + \cfrac{b-a}{l}x = 0.02+0.3x

Therefore,

R = 0 l ρ d x π r 2 = ρ π 0 l d x ( 0.02 + 0.3 x ) 2 \displaystyle R=\int_0^l{\cfrac{\rho dx}{\pi r^2}}=\cfrac{\rho}{\pi}\int_0^l{\cfrac{dx}{(0.02+0.3x)^2}}

= ρ 0.3 π [ 1 0.02 + 0.3 x ] 0 l = 6.28 × 1 0 8 0.3 × 3.1416 [ 1 0.02 1 0.05 ] \quad = \cfrac{\rho}{0.3\pi}\left[\cfrac{-1}{0.02+0.3x}\right]_0^l= \cfrac{6.28\times 10^{-8}}{0.3\times 3.1416}\left[\cfrac{1}{0.02}-\cfrac{1}{0.05}\right]

= 6.28 × 1 0 8 × 30 0.3 × 3.1416 = 2 × 1 0 6 Ω \quad = \cfrac{6.28\times 10^{-8}\times 30}{0.3\times 3.1416}=\boxed{2\times 10^{-6}\Omega}

Sir, it wouldn't be possible to integrate everytime for such problems in exams due to time constraints . So, I suggest that you mention the final general formula (derived from the same integration method you used) as R = ρ l π a b R = \dfrac{\rho l}{\pi a b} , where ρ \rho is the resistivity of the truncated cone, l l is the height (or length perpendicular to the faces of the truncated cone), and a , b a , b are the cross-sectional areas of the end faces of the truncated cone.

Anirudha Nayak
Mar 21, 2014

USE calculus

HOW

zalmey khan - 7 years, 2 months ago

a more accurate answer would be 1.63E-6 ohms.

Roland Patalinjug - 7 years, 1 month ago

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