Derive an expression for magnetic force $\vec{F}$ acting on a straight conductor of length $L$ carrying current I in an external magnetic field $\vec{B}$. Is it valid when the conductor is in zig-zag form? Justify.

A telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm . Calculate its magnifying power in normal adjustment and the distance of the image formed by the objective.

(a) Two energy levels of an electron in hydrogen atom are separated by 2.55 eV . Find the wavelength of radiation emitted when the electron makes transition from the higher energy level to the lower energy level.

(b) In which series of hydrogen spectrum this line shall fall?

The earth revolves around the sun in an orbit of radius $1.5 \times 10^{11} \mathrm{~m}$ with orbital speed $30 \mathrm{~km} / \mathrm{s}$. Find the quantum number that characterises its revolution using Bohr's model in this case (mass of earth $\left.=6.0 \times 10^{24} \mathrm{~kg}\right)$