JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Magnetic Effects of Current

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A solid/long straight wire of circular cross-section with radius 'a' carries a steady current I uniformly distributed across the cross-section. The plot of magnitude of magnetic field B with distance r from the centre of the wire is given by: [graph options in source]

(A)[graph - correct: B increases linearly to r=a then falls as 1/r (correct plot)]✓ Correct
(B)[graph - refer to source PDF]
(C)[graph - refer to source PDF]
(D)[graph - refer to source PDF]

Answer: (A)

Solution

For a solid wire of radius a with uniform current, Ampere's law gives

B = \frac{μ _{0} I r}{2 π a ^{2}}

for

r < a

(rising linearly) and

B = \frac{μ _{0} I}{2 π r}

for

r > a

(falling as

1/ r

). B rises linearly to

r = a

, then decreases as

1/ r

JEE Main · 2025 · 24 Jan 2025 · Shift 2 · Single correct option

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