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For a current loop placed in a uniform field, the orientation of stable equilibrium is the one where:
AThe area vector A is perpendicular to B
BThe area vector A is antiparallel to B
CThe plane of the loop contains B
DThe area vector A is parallel to B
Answer & Solution
Correct answer: D. The area vector A is parallel to B
1. The torque is $\boldsymbol{\tau} = \mathbf{m}\times\mathbf{B}$ with $\mathbf{m} = I\mathbf{A}$, vanishing when m is parallel or antiparallel to B.
2. Parallel alignment is stable: a small rotation produces a restoring torque.
3. Antiparallel alignment is unstable: a small rotation produces a torque that grows.
4. In the stable orientation the flux of the total field is maximum, so A ∥ B.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.157_
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