$A$ proton moving with a velocity $2.5 \times 10^7 \ m/s$ enters a magnetic field of intensity $2.5 \ T$ making an angle $30^{\circ}$ with the magnetic field. The force on the proton is

The charge on a particle $Y$ is double the charge on particle $X$. These two particles $X$ and $Y$,after being accelerated through the same potential difference,enter a region of uniform magnetic field and describe circular paths of radii $R_1$ and $R_2$ respectively. The ratio of the mass of $X$ to that of $Y$ is

An ionized gas contains both positive and negative ions. If it is subjected simultaneously to an electric field along the $+x$ direction and a magnetic field along the $+z$ direction,then

$A$ charge $q$ is moving in a magnetic field. Then,the magnetic force does not depend upon:

Given the fact that:
$A)$ Magnetic field exerts force only on a moving charge.
$B)$ Electric field exerts force on both stationary and moving charge.
$C)$ Magnetic field exerts force on a charge moving parallel to the direction of the field.
Which of the following is true?

$A$ charged particle of mass $m$ and charge $q$ travels on a circular path of radius $r$ that is perpendicular to a magnetic field $B$. The time taken by the particle to complete one revolution is