As we know,
Baxis=2(R2+x2)3/2μ0iR2where,R=a
Bcentre =2aμ0i
∴Baxis =2(a2+r2)3/2μ0ia2
∴ fractional change in magnetic field =2aμ0i2aμ0i−2(a2+r2)3/2μ0ia2=1−[1+(a2r2)]3/21
≈1−[1−23a2r2]=23a2r2
JEE Main 2021 — Physics Electromagnetism
The fractional change in the magnetic field intensity at a distance r from centre on the axis of current carrying coil of radius a to the magnetic field intensity at the centre of the same coil is: (Take r<a)
Held on 26 Aug 2021 · Verified 6 Jul 2026.
32r2a2
23r2a2
23a2r2
32a2r2
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