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Saturday, 3 September 2011

Physics GRE - #32

As shown below, a coaxial cable having radii a, b, and c carries equal and opposite currents of magnitude i on the inner and outer conductors. What is the magnitude of the magnetic induction at a point P outside the cable at a distance r from the axis?


  1. 0
  2. \dfrac{\mu_0 ir}{2\pi a^2}
  3. \dfrac{\mu_0 i}{2\pi r}
  4. \dfrac{\mu_0 i}{2\pi r}\dfrac{c^2-r^2}{c^2-b^2}
  5. \dfrac{\mu_0 i}{2\pi r}\dfrac{r^2-b^2}{c^2-b^2}

Solution :

Choice 1 is the answer.

The currents cancel out the magnetic fields created by each other, resulting in a net field of zero. More formally, this is an application of Ampere's Law.

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