Assertion (A): In a Young's double-slit experiment, interference pattern is not observed when two coherent sources are infinitely close to each other.

Reason (R): The fringe width is proportional to the separation between the two sources.

Assertion (A): An alpha particle is moving towards a gold nucleus. The impact parameter is maximum for the scattering angle of $180^{\circ}$.

Reason (R): The impact parameter in an alpha particle scattering experiment does not depend upon the atomic number of the target nucleus.

(a) Four point charges of $1 \mu \mathrm{C},-2 \mu \mathrm{C}, 1 \mu \mathrm{C}$ and $-2 \mu \mathrm{C}$ are placed at the corners $\mathrm{A}, \mathrm{B}, \mathrm{C}$ and D respectively, of a square of side 30 cm . Find the net force acting on a charge of $4 \mu \mathrm{C}$ placed at the centre of the square.

OR

(b) Three point charges, 1 pC each, are kept at the vertices of an equilateral triangle of side 10 cm . Find the net electric field at the centroid of triangle.

Derive an expression for magnetic force $\vec{F}$ acting on a straight conductor of length $L$ carrying current I in an external magnetic field $\vec{B}$. Is it valid when the conductor is in zig-zag form? Justify.