In hydrogen like system the ratio of coulombian force and gravitational force between an electron and a proton is in the order of :

A charge $$q$$ is placed at the center of one of the surface of a cube. The flux linked with the cube is:

An infinitely long positively charged straight thread has a linear charge density $$\lambda \mathrm{~Cm}^{-1}$$. An electron revolves along a circular path having axis along the length of the wire. The graph that correctly represents the variation of the kinetic energy of electron as a function of radius of circular path from the wire is :