The radius of third stationary orbit of electron for Bohr's atom is R. The radius of fourth stationary orbit will be:

A rectangular loop of length $$2.5 \mathrm{~m}$$ and width $$2 \mathrm{~m}$$ is placed at $$60^{\circ}$$ to a magnetic field of $$4 \mathrm{~T}$$. The loop is removed from the field in $$10 \mathrm{~sec}$$. The average emf induced in the loop during this time is

An electric charge $$10^{-6} \mu \mathrm{C}$$ is placed at origin $$(0,0)$$ $$\mathrm{m}$$ of $$\mathrm{X}-\mathrm{Y}$$ co-ordinate system. Two points $$\mathrm{P}$$ and $$\mathrm{Q}$$ are situated at $$(\sqrt{3}, \sqrt{3}) \mathrm{m}$$ and $$(\sqrt{6}, 0) \mathrm{m}$$ respectively. The potential difference between the points $\mathrm{P}$ and $\mathrm{Q}$ will be :

A convex lens of focal length $$40 \mathrm{~cm}$$ forms an image of an extended source of light on a photoelectric cell. A current I is produced. The lens is replaced by another convex lens having the same diameter but focal length $$20 \mathrm{~cm}$$. The photoelectric current now is :