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At the very end of a long solenoid, the magnetic field along the axis is
Aμ₀nI
Bμ₀nI/2
C2μ₀nI
DZero
Answer & Solution
Correct answer: B. μ₀nI/2
1. The on-axis field of a finite solenoid involves angles subtended at its two ends.
2. Deep inside (both ends far), the formula gives B = μ₀nI.
3. At an end face, one angle is 90° while the other approaches 0°.
4. The field at the end of a long solenoid is therefore B = μ₀nI/2, half the interior value.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", §4.8.1_
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