An $\alpha$-particle of mass $m$ moves on a circular path of radius $r$ in a plane inside and normal to a uniform magnetic field $B$. The time taken by this particle to complete one revolution is . . . . . . .

In which of the following cases is no force exerted by a magnetic field on a charge?

$A$ charged particle is released from rest in a region of uniform electric and magnetic fields which are parallel to each other. The particle will move on a

$A$ charged particle with specific charge $S$ moves undeflected through a region of space containing mutually perpendicular uniform electric and magnetic fields $E$ and $B$. When the electric field is switched off,the particle will move in a circular path of radius:

An electron is orbiting in a circular orbit under the influence of a constant transverse magnetic field of strength $B$. Assuming that Bohr's postulate regarding the quantisation of angular momentum holds good for this electron,find the kinetic energy of the electron in the $n^{th}$ orbit. ($m_e =$ mass of electron,$e =$ magnitude of charge of electron)

$A$ charge moves in a circular path perpendicular to a magnetic field. The time period of revolution is independent of