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In a coaxial straight cable, the central conductor and the outer conductor carry equal currents in opposite directions. The magnetic field is zero

$\begin {array} {1 1} (a)\;Outside\: the\: cable & \quad (b)\;In\: between\: the\: two\: conductors \\ (c)\;Inside\: the\: inner \: conductor & \quad (d)\;Inside\: the\: outer\: conductor \end {array}$

Construct an Ampere’s loop in the concerned regions to see the value of $i_p$.
At a pont outside the co-axial tube at a diatance R from the central axis consider a circular path.
Then from Amperes Law closed integral of $B .dl=\mu_0 I(net)$
but the net current is zero as the current in the two cables flow in opposite directions.
therefore Magnetic field is zero outside
Ans : (a)

edited Sep 18, 2014