Question

A charged partical enters at right angle into a uniform magnetic field. what is the nature of charge partical if it experience a force in a direction pointing vertically out of page.

Answer 1

Let the particle of charge q enter from Left to right horizontally on the page with a velocity v. Let the magnetic field B be from down upwards vertically along the page.

Now the particle experiences a force F coming perpendicularly out of the page.

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

So In that case charge q is a positive charge.