Time period of a block suspended from the upper plate of a parallel plate capacitor by a spring of stiffness $k$ is $T$. When block is uncharged. If a charge $q$ is given to the block them, the new time period of oscillation will be

A cube of side $'a'$ has point charges $+Q$ located at each of its vertices except at the origin where the charge is $- Q$. The electric field at the centre of cube is

Figures below show regular hexagons, with charges at the vertices. In which of the following cases the electric field at the centre is not zero

Explain electric field and also electric field by point charge.

Give physical meaning of electric field.