Magnetic Effects & Magnetism

JEE Main level, single correct, medium

232 of 276

Question 232 of 276

A rectangular loop of sides 10cm10cm and 5cm5cm, carrying a current II of 12A12A, is placed in different orientations as shown in the figure below.
(a)
(b)

(c)
(d)
If there is a uniform magnetic field of
0.3T0.3T in the positive zz direction, in which orientations the loop would be in (i)(i) stable equilibrium and (ii)(ii) unstable equilibrium?

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Answer: (D) (b) and (d), respectively.

For a magnetic dipole placed in a uniform magnetic field, the torque is given by τ⃗=M⃗×B⃗\vec{\tau }=\vec{M}\times \vec{B}, and potential energy UU is given as,

U=−M⃗⋅B⃗=−MBcos⁡θU=-\vec{M}\cdot \vec{B}=-MB\cos \theta

When
M⃗\vec{M} is in the same direction as B⃗\vec{B} then τ⃗=0\vec{\tau }=0 and UU is minimum = −MB-MB as θ=0o\theta ={0}^{o}.⇒\Rightarrow Stable equilibrium is (b).
When
M⃗\vec{M} the opposite direction of B⃗\vec{B}, then τ⃗=0\vec{\tau }=0 and UU is max = +MB+MB
As,
θ=180o\theta ={180}^{o} ⇒\RightarrowUnstable equilibrium in (d)

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