A velocity selector is used to insert charged particles into a mas spectrometer

Question

A velocity selector is used to insert charged particles into a mas spectrometer at a specific speed. The velocity selector has uniform electric and magnetic fields that are perpendicular to one another and to the motion of the charged particle. If the particle is at the right velocity the magnetic force and electric force will cancel and the particle will continue in a straight line path. The velocity of the particle is: v=E/B. For higher or lower velocities, the particle is deflected and will not pass into the mass spectrometer.

Assuming the particle is positively charged, what are the directions of the forces due to the electric field and the magnetic field?

How does the kinetic energy of the particle change as it travels in the velocity selector?

Explanation / Answer

Force due to electric field will be qE and due to magnetic field q vB .

and at right velocity,

both forces are same in magnitude and opposite in direction.

q E = q v B

so Net force due to these forces will be zero. ........Ans

Net force is zero hence KE of particle will not change throughout the velocity selector. ....Ans

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