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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

1. B ∝ r 2
2. B ∝ r
3. B ∝ 2 r 1
4. B ∝ r 1

Correct answer: B ∝ r

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