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JEE Main202225 Jun 2022Morning ShiftPhysicsMagnetic Effects of CurrentActual

A long straight wire with a circular cross-section having radius R , is carrying a steady current I . The current I is uniformly distributed across this cross-section. Then the variation of magnetic field due to current I with distance r r < R from its centre will be

Options

  1. AB ∝ r
  2. BB ∝ 1 r
  3. CB ∝ r 2
  4. DB ∝ 1 r 2

Correct answer

A. B ∝ r

Step-by-step solution

Using Ampere's law and noting that the steady current is uniformly distributed over the cross-section of the cylindrical wire. B = μ 0 I r 2 π R 2 ,       r ≤ R μ 0 I 2 π r ,       r > R Thus, magnetic field B ∝ r from the centre for r < R .

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