If a force f is exerted on a charged particle when the particle enters a magnetic field b at a velocity v, which of the following is always true?
a. vectors f, b and v are all perpendicular to each other
b. vectors f and b are parallel
c. vectors f and b are perpendicular
d. vectors f and v are be parallel
e. vectors b and v may perpendicular

The correct option is 'c': The Force vector (F) and the magnetic field vector (B) are always perpendicular.

Explanation:

The magnetic force that acts on a charged particle moving in a magnetic field is given by

As we can see that vector cross product is involved hence we conclude that Force vector (F) is always perpendicular to both the velocity vector (v) and the magnetic field vector (B) which may be at any angle with respect to each other.

Hence we conclude that vectors F and B are always perpendicular.