# Exiting from a magnetic field

The above shows the path (green arrows) of a charged particle in an $$xy$$-plane, where a uniform magnetic field exists penetrating through the $$xy$$-plane for the left half. In the beginning when the charged particle is inside the magnetic field, the path is horizontal in the $$+x$$ direction. After getting out the magnetic field, at the height of $$H$$, the path becomes a projectile motion, ending on the ground at last. If the magnetic field intensity is $$B$$, the mass of the charged particle is $$m,$$ and the quantity of the electric charge of the particle is $$q,$$ then what is the distance $$L$$ above? (Gravitational acceleration is $$g$$, and the amount of air resistance is negligible.)

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