The magnitude of electric field intensity $E$ such that an electron placed in it would experience an electrical force equal to its weight is given by:

Four identical charges are placed at the four corners of a square. If the electric field intensity at the center of the square due to any one charge is $E$,then the resultant electric field intensity at the center of the square will be .......

Two charges $Q$ and $-3Q$ are placed at a certain distance $x$. If the electric field at the position of charge $Q$ due to charge $-3Q$ is $E$,then what is the electric field at the position of charge $-3Q$ due to charge $Q$?

Three equal charges $+q$ each are placed at the three vertices of an equilateral triangle. The electric field at the centroid of the triangle is . . . . . . . ('$r$' is the length of the side of the triangle).

Explain electric field and its source as well as magnetic field and its source.

Total charge $-Q$ is uniformly spread along the length of a ring of radius $R$. $A$ small test charge $+q$ of mass $m$ is kept at the centre of the ring and is given a gentle push along the axis of the ring.
$(a)$ Show that the particle executes a simple harmonic oscillation.
$(b)$ Obtain its time period.