To compare EMF of two cells using potentiometer the balancing lengths obtained are 200 cm and 150 cm . The least count of scale is 1 cm . The percentage error in the ratio of EMFs is $\_\_\_\_$

Two charges $7 \mu \mathrm{C}$ and $-2 \mu \mathrm{C}$ are placed at $(-9,0,0) \mathrm{cm}$ and $(9,0,0) \mathrm{cm}$ respectively in an external field $E=\frac{\mathrm{A}}{r^2} \hat{r}$, where $A=9 \times 10^5 \mathrm{~N} / \mathrm{C} . \mathrm{m}^2$. Considering the potential at infinity is 0 , the electrostatic energy of the configuration is $\_\_\_\_$ J.

A prism of angle $75^{\circ}$ and refractive index $\sqrt{3}$ is coated with thin film of refractive index 1.5 only at the back exit surface. To have total internal reflection at the back exit surface the incident angle must be $\_\_\_\_$

$\left(\sin 15^{\circ}=0.25\right.$ and $\left.\sin 25^{\circ}=0.43\right)$

The current passing through a conducting loop in the form of equilateral triangle of side $4 \sqrt{3} \mathrm{~cm}$ is 2 A . The magnetic field at its centroid is $\alpha \times 10^{-5} \mathrm{~T}$. The value of $\alpha$ is $\_\_\_\_$ .

(Given : $\mu_{\mathrm{o}}=4 \pi \times 10^{-7}$ SI units)