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Unformatted text preview: Devices that use magnetic force on a moving charge (22.4) LORENTZ FORCE: expression for total magnetic and electric force on a charge In space containing electric and magnetic field together: o force on moving charge is Force) (Lorentz B v q E q F r r r r + = o For special choice of fields & particle velocity, Lorentz force can be zero Important for devices used to analyse molecules in chemistry, physics, biochemistry, medicine, etc. VELOCITY SELECTOR Contains region of space with uniform electric and magnetic fields o perpendicular to each other o perpendicular to path of charged particle In selector region: o B v q F r r r = B is up by RHR o E q F r r = E is down Net force on charge is zero IF E q B v q = So particles with speed B E v = are undeflected o Can be directed through hole in barrier to give beam of particles with same speed regardless of mass and charge (other speeds miss hole) o Can THEN analyse for charge and mass + + + + + + Barrier with hole E (down) B (into screen) + q v MASS SPECTROMETER...
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