20.3.6—How electric and magnetic fields can be used in velocity selection

In a velocity selector, electric and magnetic forces oppose. Particles pass undeflected when qE=qvB, so v=E/B.

This condition assumes perpendicular, uniform fields and the correct orientation; faster or slower particles deflect.

E=2.0×10⁴ N C⁻¹ and B=0.50 T select v=4.0×10⁴ m s⁻¹.

The selector does not select charge sign or mass directly; it selects speed, with curvature after the selector used for further analysis.