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JEE · NEET Previous Year Question

A long straight wire of circular cross-section (radius a) is carrying steady current I. The current I is uniformly distributed across this cross-section. The magnetic field is

Topic: JEE PYQ

Answer: B. Full step-by-step solution below.

  1. Ainversely proportional to r in the region r < a and uniform throughout in the region r > a
  2. Bdirectly proportional to r in the region r < a and inversely proportional to r in the region r > a
  3. CZero in the region r < a and inversely proportional to r in the region r > a
  4. Duniform in the region r < a and inversely proportional to distance r from the axis, in the region r > a

Correct answer: B

Solution

(b) The formula to calculate the magnetic field inside the conducting wire is given by
... (a)
The formula to calculate the magnetic field outside the conducting wire is given by
. . . (b)
From equation (a), it can be concluded that the magnetic field is directly proportional to the radial distance inside the wire, whereas from equation (b), it can be concluded that the magnetic field is inversely proportional to the radial distance outside the wire.

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