Question

A free charged particle moves through a magnetic field. The particle may undergo a change in :

• A. Speed
• B. Energy
• C. Direction of motion
• D. None of these

Answer 1

The correct option is C Direction of motion

Answer is C.

Since the magnetic force always acts at right angles to the charge's motion, the magnetic force can do no work on the charge. The B-field cannot speed up or slow down a moving charge; it can only change the direction in which the charge is moving.
The general path of a moving charge in a constant magnetic field is that of a helix with its axis parallel to the direction of the magnetic field.
If we stand in such a way that we are looking directly into the oncoming magnetic field, the a positively charged particle will be seen to rotate in clockwies circle where as a negatively charged particle will rotate in counterclockwise circle.
The component of velocity of the charged particle that is parallel to the magnetic field is unaffected, i.e. the charge moves at a constant speed along the direction of the magnetic field.