Answer:
Because the force F is always at 90 degrees to the velocity of the particle.
Explanation:
The factors that determine the path of a particle in a uniform magnetic field depend on
The magnetic flux density
The charge on the particle
The speed of the particle
Why is the trajectory of the positively charged particle curving down, to the bottom of the screen, when moving in the magnetic field?
Because the force F is always at 90 degrees to the velocity of the particle.
This can be illustrated by the equation
F = BQVSINØ
The magnetic force is always acting perpendicular to the particle velocity and its direction can be found using the left hand rule.