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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

1. inversely proportional to r in the region r < a and uniform throughout in the region r > a
2. directly proportional to r in the region r < a and inversely proportional to r in the region r > a
3. zero in the region r < a and inversely proportional to r in the region r > a
4. uniform in the region r < a and inversely proportional to distance r from the axis, in the region r > a

Correct answer: directly proportional to r in the region r < a and inversely proportional to r in the region r > a

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