Since, the magnetic field is brought to zero in time $\Delta t$, the magnetic flux linked with the ring also reduces from maximum to zero. This, in turn, induces an emf in ring by the phenomenon of EMI. The induces emf causes the electric field $E$ generation around the ring.

The induced emf $=$ electric field $E \times(2 \pi b)($ Because $V=E \times d)$ ..... (i)

By Faraday's law of EMI

The induced emf $=$ rate of change of magnetic flux

$=$ rate of change of magnetic field $\times$ area

$$=\frac{B л a^2}{\Delta t}\quad\text{... (ii)}$$

From Eqs. (i) and (ii), we have

$$2 \pi b E=\mathrm{e} m f=\frac{B \pi a^2}{\Delta t}$$

Since, the charged ring experienced a electric force $=Q E$ This force try to rotate the coil, and the torque is given by

$$\begin{aligned} \text { Torque } & =b \times \text { Force } \\ & =Q E b=Q\left[\frac{B \pi a^2}{2 \pi b \Delta t}\right] b \\ & =Q \frac{B a^2}{2 \Delta t} \end{aligned}$$

If $\Delta L$ is the change in angular momentum

$$\Delta L=\text { Torque } \times \Delta t=Q \frac{B a^2}{2}$$

Since, initial angular momentum $=0$

Now, since $\quad$ Torque $\times \Delta t=$ Change in angular momentum

Final angular momentum $=m b^2 \omega=\frac{Q B a^2}{2}$

$$\omega=\frac{Q B a^2}{2 m b^2}$$

On rearranging the terms, we have the required expression of angular speed.

Here, the component of magnetic field perpendicular the plane $=B \cos \theta$

Now, the conductor moves with speed $v$ perpendicular to $B \cos \theta$ component of magnetic field. This causes motional emf across two ends of rod, which is given by $=v(B \cos \theta) d$

This makes flow of induced current $i=\frac{v(B \cos \theta) d}{R}$ where, $R$ is the resistance of rod. Now, current carrying rod experience force which is given by $F=i B d$ (horizontally in backward direction).

Now, the component of magnetic force parallel to incline plane along upward direction $=F \cos \theta=i B d \cos \theta=\left(\frac{v(B \cos \theta) d}{R}\right) B d \cos \theta$ where, $v=\frac{d x}{d t}$

Also, the component of weight $(\mathrm{mg})$ parallel to incline plane along downward direction $=m g \sin \theta$.

Now, by Newton's second law of motion

$$\begin{aligned} m \frac{d^2 x}{d t^2} & =m g \sin \theta-\frac{B \cos \theta d}{R}\left(\frac{d x}{d t}\right) \times(B d) \cos \theta \\ \frac{d v}{d t} & =g \sin \theta-\frac{B^2 d^2}{m R}(\cos \theta)^2 v \\ \frac{d v}{d t} & +\frac{B^2 d^2}{m R}(\cos \theta)^2 v=g \sin \theta \end{aligned}$$

But, this is the linear differential equation. On solving, we get

$$v=\frac{g \sin \theta}{\frac{B^2 d^2 \cos ^2 \theta}{m R}}+A \exp \left(-\frac{B^2 d^2}{m R}\left(\cos ^2 \theta\right) t\right)$$

$A$ is a constant to be determine by initial conditions.

The required expression of velocity as a function of time is given by

$$=\frac{m g R \sin \theta}{B^2 d^2 \cos ^2 \theta}\left(1-\exp \left(-\frac{B^2 d^2}{m R}\left(\cos ^2 \theta\right) t\right)\right)$$

The conductor of length $d$ moves with speed $v$, perpendicular to magnetic field $B$ as shown in figure. This produces motional emf across two ends of rod, which is given by $=v B d$. Since, switch $S$ is closed at time $t=0$. capacitor is charged by this potential difference. Let $Q(t)$ is charge on the capacitor and current flows from $A$ to $B$.

Now, the induced current

$$I=\frac{v B d}{R}-\frac{Q}{R C}$$

On rearranging the terms, we have

$$\frac{Q}{R C}+\frac{d Q}{d t}=\frac{v B d}{R}$$

This is the linear differential equation. On solving, we get

$$\begin{aligned} & Q=v B d C+A e^{-t / R C} \\ & \Rightarrow \quad Q=v B d C\left[1-e^{-t / R C}\right] \quad \text { (At time } t=0, Q=0=A=-v B d c \text { ). } \end{aligned}$$

Differentiating, we get $I-\frac{v B d}{R} e^{-t / R C}$

This is the required expression of current.

The conductor of length $d$ moves with speed $v$, perpendicular to magnetic field $\mathbf{B}$ as shown in figure. This produces motional emf across two ends of rod, which is given by $=v B d$.

Since, switch $S$ is closed at time $t=0$. current start growing in inductor by the potential difference due to motional emf.

Applying Kirchhoff's voltage rule, we have

$$-L \frac{d I}{d t}+v B d=I R \text { or } L \frac{d I}{d t}+I R=v B d$$

This is the linear differential equation. On solving, we get

$$\begin{aligned} I & =\frac{v B d}{R}+A e^{-R t / 2} \\ \text { At } t & =0 \quad I=0 \\ \Rightarrow\quad A & =-\frac{v B d}{R} \Rightarrow I=\frac{v B d}{R}\left(1-e^{-R t / L}\right) \end{aligned}$$

This is the required expression of current.

The magnetic flux linked with the metallic ring of mass $m$ and radius $l$ falling under gravity in a region having a magnetic field whose $z$-component of magnetic field is $B_z=B_0(1+\lambda z)$ is

$$\phi=B_z\left(\pi l^2\right)=B_0(1+\lambda z)\left(\pi l^2\right)$$

Applying Faraday's law of EMI, we have emf induced given by $\frac{d \phi}{d t}=$ rate of change of flux

Also, by Ohm's law

$$B_0\left(\pi l^2\right) \lambda \frac{d z}{d t}=I R$$

On rearranging the terms, we have $\quad I=\frac{\pi l^2 B_0 \lambda}{R} v$

Energy lost/second $$=I^2 R=\frac{\left(\pi l^2 \lambda\right)^2 B_0^2 v^2}{R}$$

This must come from rate of change in $\mathrm{PE}=m g \frac{d z}{d t}=m g v$ [as kinetic energy is constant for $v=$ constant]

Thus, $$m g v=\frac{\left(\pi l^2 \lambda B_0\right)^2 v^2}{R} \text { or } v=\frac{m g R}{\left(\pi l^2 \lambda B_0\right)^2}$$

This is the required expression of velocity.