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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?
0
\dfrac{\mu_0 ir}{2\pi a^2}
\dfrac{\mu_0 i}{2\pi r}
\dfrac{\mu_0 i}{2\pi r}\dfrac{c^2-r^2}{c^2-b^2}
\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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