JEE Advanced 2013 Physics question

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed coaxially inside an infinite solenoid of radius 2R. The solenoid has n turns per unit length and carries a steady current I. Consider a point P at a distance r from the common axis. The correct statement(s) is (are)

(A)In the region 0 < r < R, the magnetic field is non-zero.Correct
(B)In the region R < r < 2R, the magnetic field is along the common axis.
(C)In the region R < r < 2R, the magnetic field is tangential to the circle of radius r, centered on the axis.
(D)In the region r > 2R, the magnetic field is non-zero.Correct

Answer: (A),(D)

Solution

The hollow cylinder produces no field inside it (r < R) and a tangential field outside it (r > R). The ideal solenoid produces a uniform axial field inside (r < 2R) and zero outside. (A) For r < R only the solenoid field exists: . True.

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Question ID VK-026190