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 :

An electron is projected with uniform velocity along the axis inside a current carrying long solenoid. Then :

A metal wire of uniform mass density having length $$L$$ and mass $$M$$ is bent to form a semicircular arc and a particle of mass $$\mathrm{m}$$ is placed at the centre of the arc. The gravitational force on the particle by the wire is :

An effective power of a combination of 5 identical convex lenses which are kept in contact along the principal axis is $$25 \mathrm{D}$$. Focal length of each of the convex lens is: