× × × × X × × × × × × × × × × × B₁ × 3 τω × × × × × × × × × × × × × velocity selector

A velocity selector is a device that can choose the velocity of a charged particle moving through a region in which the electric field is perpendicular to the magnetic field, and with both fields perpendicular to the initial velocity of the particle (see image below). To make the charged particle travel straight through the parallel plates, the downward deflection due to the electric field must equal the upward deflection due to the magnetic field. The force of gravity is negligible and can be ignored. Suppose you want to design a velocity selector that will allow protons to pass through, undeflected, only if they have a speed of 425 m/s. The magnetic field is 0.04 T, and the separation of the electrically charged plates is 0.12 m. Calculate the magnitude of the electric potential difference you would need to apply across the two plates, in V.