A square coil ABCD of side ' L ' is carrying a current in clockwise direction. A straight conductor carrying current $\mathrm{I}_2$ (upward direction) is kept parallel to side AB at a distance $\frac{\mathrm{L}}{3}$ in the plane of $A B C D$. The net force on the coil ABCD is ( $\mu_0=$ magnetic permeability)

In a common emitter transistor amplifier circuit, the current gain $\alpha=0.8$. If the base current changes by $3 \mu \mathrm{~A}$ then the collector current changes by

A ray of light is incident on first face of an equilateral glass prism at an angle of incidence ' $I$ '. The emergent ray just grazes along the adjacent face. The refractive index of the the prism is $\sqrt{2}$. The value of angle of incidence at first face is $\left(\sin 45^{\circ}=\frac{1}{\sqrt{2}}, \sin 90^{\circ}=1\right)$

In hydrogen atom, the energy of electron in first and third orbit is ' $E_1$ ' and' $E_3$ ' respectively. If $E_3=x E_1$ then the value of $x$ will be