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JEE Advanced · Physics · 20. Magnetism & Current

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed co-axially 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):

  1. A In the region 0<r<R, the magnetic field is non-zero.
  2. B In the region R<r<2R, the magnetic field is along the common axis.
  3. C In the region R<r<2R, the magnetic field is tangential to the circle of radius r, centered on the axis.
  4. D In the region r>2R, the magnetic field is non-zero.
Verified Solution

Answer & Solution

Correct Answer

(D) In the region r>2R, the magnetic field is non-zero.

Step-by-step Solution

Detailed explanation

If we consider a point at a distance r from a common axis such that 0<r<R, the magnetic field is non-zero because inside infinite solenoid magnetic field is, B = μ0ni and due to the hollow cylinder it is zero.  Hence, net magnetic field is non-zero. 

If 2R<r, then magnetic field due to an infinite solenoid is zero but there will be a non-zero magnetic field because of hollow cylinder. Hence, net magnetic field is non-zero.

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