$A$ proton and an $\alpha$-particle enter a uniform magnetic field perpendicularly with the same speed. If a proton takes $25 \ \mu s$ to make $5$ revolutions,then the periodic time for the $\alpha$-particle would be ........ $\mu s$.

As shown in the figure,the uniform magnetic field between the two identical plates is $B$. There is a hole in the bottom plate. If a particle of charge $q$,mass $m$,and energy $E$ enters this magnetic field through this hole,then the particle will not collide with the upper plate provided:

An electron having charge $1.6 \times 10^{-19} \ C$ and mass $9 \times 10^{-31} \ kg$ is moving with $4 \times 10^6 \ m \ s^{-1}$ speed in a magnetic field of $2 \times 10^{-1} \ T$ in a circular orbit. The force acting on the electron and the radius of the circular orbit is $.........$

An electron is moving along the positive $x$-axis. $A$ uniform electric field exists towards the negative $y$-axis. What should be the direction of a magnetic field of suitable magnitude so that the net force on the electron is zero?

Which of the following particles will describe the smallest circle when projected with the same velocity perpendicular to the magnetic field?

Given below are two statements. One is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A) :$ An electron in a certain region of uniform magnetic field is moving with constant velocity in a straight line path.
Reason $(R) :$ The magnetic field in that region is along the direction of velocity of the electron.
In the light of the above statements,choose the correct answer from the options given below.