Question

can someone explain the solution to me for this question

Answer 1

The electric field caused by the potential difference across the plates gives the ion a known kinetic energy, (1/2) m v^2.

When it enters the magnetic field, it is made to begin a circular trajectory that depends on the field and the velocity and if it has a specific radius (r), it is captured. r is very sensitive to the exact mass of the ion, m, and thus various elements and their isotopes can be separated, distinguished.

Answer 2

This is sometimes also referenced as the "Lorentz Force".