State if an alpha particle [alpha particles are positively charged] experiences any force in a magnetic field if [i] it is placed in the field and is at rest. [ii] it moves in the magnetic field parallel to field lines. [iii] it moves in the magnetic field perpendicular to field lines . Justify your answer in each case.

Explanation

F = qvB sin `theta`
i] When the alpha particle is at rest. its velocity v = 0 and hence, magnetic force f = 0 .
ii] When the alpha particle moves parallel to the magnetic field, `theta = 0^(@)` and hence sin `theta` = F = 0 .
iii] When the alpha particle moves perpendicular to the magnetic field, `theta = 90^(@)` . Then, sin `theta = l` and F = qv B, i.e., a maximum .
Magnetic field exerts a force on the alpha particle perpendicular to both the magnetic field and its direction of motion . This force provides the centripetal force to make the particle move in a plane perpendicular to the magnetic field .