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A uniform magnetic field of $3000 \mathrm{G}$ is established along the positive $Z$-direction. A rectangular loop of sides 10 $\mathrm{cm}$ and $5 \mathrm{~cm}$ carries a current of $12 \mathrm{~A}$. What is the torque on the loop in the different cases shown in figure?
What is the force on each case? Which case corresponds to stable equilibrium?


What is the force on each case? Which case corresponds to stable equilibrium?


Solution:
2409 Upvotes
Verified Answer
(a) Let us detail each case separately,

Dipole moment $(\vec{M})$ is along $+\mathrm{x}$ direction
$$
\begin{aligned}
&\vec{M}=10 \times 5 \times 10^{-4} \times 12 \hat{i}=0.06 \hat{i} \\
&\vec{B}=3000 \times 10^{-4} \hat{k}
\end{aligned}
$$
Torque $\vec{C}=\vec{M} \times \vec{B}=-1.8 \times 10^{-2} \hat{J}$ N.m
Torque is $1.8 \times 10^{-2} \mathrm{~N}-\mathrm{m}$ along $-\mathrm{y}$ direction, Net force or the coil is zero, coil is not in equilibrium.
(b) Dipole moment is along $+\mathrm{x}$ direction
$$
\vec{M}=0.06 \hat{i}, \vec{B}=0.3 \hat{k}
$$
Torque $\vec{\tau}=\vec{M} \times \vec{B}$
So, torque is $1.8 \times 10^{-2} \mathrm{~N}-\mathrm{m}$ along $-y$ direction.
Net force on the coil is zero, coil is not in equilibrium.
(c) Dipole moment is along $-y$ direction
$\vec{M}=-0.06 \hat{j}, \hat{B}=0.3 \hat{k}$
Torque $\vec{\tau}=\vec{M} \times \vec{B}=-1.8 \times 10^{-2} \hat{i} \mathrm{~N}-\mathrm{m}$.
So, the torque is $1.8 \times 10^{-2} \mathrm{Nm}$ along $-x$ direction.
Net force on the coil is zero, coil is not in equilibrium.
(d) Dipole moment is at an angle $150^{\circ}$ with the $+x$ direction.
$$
=1.8 \times 10^{-2} \sin \frac{\pi}{2}=1.8 \times 10^{-2} \text { N.m }
$$
At an angle $240^{\circ}$ with the $+x$ direction, net force on the coil is zero, coil is not in equilibrium.
(e) Dipole moment is along $+\mathrm{z}$ direction.
Torque $\vec{\tau}=\vec{M} \times \vec{B}=1.8 \times 10^{-2} \hat{k} \times \hat{k}=0$
Potential energy $U=\vec{M} \cdot \vec{B}$
$$
U=-1.8 \times 10^{-2}(\hat{k} \hat{k})=-1.8 \times 10^{-2} \mathrm{~J}
$$
negative sign shows equilibrium is stable.
(f) Dipole moment is along $-\mathrm{z}$ direction
Torque $\vec{\tau}=\vec{M} \times \vec{B}=1.8 \times 10^{-2}(-\hat{k} \cdot \hat{k})=0$
Potential energy, $U=-\vec{M} \cdot \vec{B}=+1.8 \times 10^{-2} \mathrm{~J}$ Positive energy so, equilibrium is unstable.

Dipole moment $(\vec{M})$ is along $+\mathrm{x}$ direction
$$
\begin{aligned}
&\vec{M}=10 \times 5 \times 10^{-4} \times 12 \hat{i}=0.06 \hat{i} \\
&\vec{B}=3000 \times 10^{-4} \hat{k}
\end{aligned}
$$
Torque $\vec{C}=\vec{M} \times \vec{B}=-1.8 \times 10^{-2} \hat{J}$ N.m
Torque is $1.8 \times 10^{-2} \mathrm{~N}-\mathrm{m}$ along $-\mathrm{y}$ direction, Net force or the coil is zero, coil is not in equilibrium.
(b) Dipole moment is along $+\mathrm{x}$ direction
$$
\vec{M}=0.06 \hat{i}, \vec{B}=0.3 \hat{k}
$$

Torque $\vec{\tau}=\vec{M} \times \vec{B}$
So, torque is $1.8 \times 10^{-2} \mathrm{~N}-\mathrm{m}$ along $-y$ direction.
Net force on the coil is zero, coil is not in equilibrium.
(c) Dipole moment is along $-y$ direction
$\vec{M}=-0.06 \hat{j}, \hat{B}=0.3 \hat{k}$
Torque $\vec{\tau}=\vec{M} \times \vec{B}=-1.8 \times 10^{-2} \hat{i} \mathrm{~N}-\mathrm{m}$.
So, the torque is $1.8 \times 10^{-2} \mathrm{Nm}$ along $-x$ direction.
Net force on the coil is zero, coil is not in equilibrium.
(d) Dipole moment is at an angle $150^{\circ}$ with the $+x$ direction.

$$
=1.8 \times 10^{-2} \sin \frac{\pi}{2}=1.8 \times 10^{-2} \text { N.m }
$$
At an angle $240^{\circ}$ with the $+x$ direction, net force on the coil is zero, coil is not in equilibrium.
(e) Dipole moment is along $+\mathrm{z}$ direction.

Torque $\vec{\tau}=\vec{M} \times \vec{B}=1.8 \times 10^{-2} \hat{k} \times \hat{k}=0$
Potential energy $U=\vec{M} \cdot \vec{B}$
$$
U=-1.8 \times 10^{-2}(\hat{k} \hat{k})=-1.8 \times 10^{-2} \mathrm{~J}
$$
negative sign shows equilibrium is stable.
(f) Dipole moment is along $-\mathrm{z}$ direction

Torque $\vec{\tau}=\vec{M} \times \vec{B}=1.8 \times 10^{-2}(-\hat{k} \cdot \hat{k})=0$
Potential energy, $U=-\vec{M} \cdot \vec{B}=+1.8 \times 10^{-2} \mathrm{~J}$ Positive energy so, equilibrium is unstable.
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