Electrons moving with different speeds enter a uniform magnetic field in a direction perpendicular to the field. They will move along circular paths.

The acceleration of an electron at a moment in a magentic field $\vec B\, = \,2\hat i + 3\hat j + 4\hat k$ is $\vec a\, = \,x\hat i - 2\hat j + \hat k$. The value of $x$ is

A charged particle is moving in a uniform magnetic field in a circular path. Radius of circular path is $R$. When energy of particle is doubled, then new radius will be

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 small block of mass $m$, having charge $q$ is placed on frictionless inclined plane making an angle  $\theta$ with the horizontal. There exists a uniform magnetic field $B$ parallel to the inclined plane but  perpendicular to the length of spring. If $m$ is slightly pulled on the inclined in downward direction and released, the time period of oscillation will be (assume that the block does not leave contact with the plane)

A particle of charge per unit mass $\alpha$ is released from origin with a velocity $\bar{v}=v_0 \vec{i}$ in a uniform magnetic field $\bar{B}=-B_0 \hat{k}$. If the particle passes through $(0, y, 0)$ then $y$ is equal to