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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

Asked in JEE Main 25th June 1st Shift 2022 · Thick wires and cables

• (1) B∝ r²
• (2) B∝ r
• (3) B∝1/(r²)
• (4) B∝1/r

Step-by-step solution

Why the other options are wrong

• (1) Uses enclosed current ∝ r² but forgets to divide by the loop length 2π r
• (3) Uses a point-source inverse square law
• (4) Applies the outside-wire result inside

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