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Question:

A rectangular loop of sides 10 cm and 5 cm carrying a current I and 12 A is placed in different orientations as shown in the figures above. If there is a uniform magnetic field of 0.3 T in the positive z direction, in which orientations the loop would be in (i) stable equilibrium and (ii) unstable equilibrium?

(A) and (B), respectively

(B) and (C), respectively

(A) and (C), respectively

(B) and (D), respectively

Solution:

U=−→M.→B=−MBcosθ
So, Umin when θ=0° → Stable equilibrium
Umax when θ=180° → Unstable equilibrium