Velocity selector
A device using crossed electric and magnetic fields to select particles with a specific velocity. For particles moving perpendicular to both fields, the electric and magnetic forces balance when v = E/B, allowing only particles at this velocity to pass through undeflected.
Formula
v = E/B
Real World
Mass spectrometers used in NHS doping-control labs use a velocity selector to ensure only ions of one specific speed enter the deflection chamber, giving accurate mass readings.
Exam Focus
Show the balance of forces explicitly — write qE = qvB and cancel q — examiners expect the algebraic derivation, not just the formula.
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