Question

a proton is moving horizontally at 7.9 x 10^5 m/s. It passes through a vertical magnetic field that points downward. In which direction is the proton deflected by the magnetic field? A. up B. to the right of its velocity C. down D. to the left of its velocity

Answer 1

There are four key characteristics of the magnetic force on a moving charge.

First, Its magnitude is proportional to the magnitude of the charge. 
Second, the magnitude of the force is also proportional
Third, the magnetic force depends on the particle’s
velocity. 
Fourth, we find by experiment that the magnetic force $\vec{F}$ does not have the same direction as the magnetic field $\vec{F}$ but instead is always perpendicular to both $\vec{B}$ and the velocity $\vec{v}$ 

So, in a mathematical language this is given by:

$\vec{F}=q\vec{v} \times \vec{B}$

So, this is a cross product. Therefore, applying the Right-hand rule:

The answer is:

D. to the left of its velocity.

Explaining this in other words:

"The force $\vec{F}$ is directed into the plane of the paper, so this is the direction of the proton"