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L26_a - Devices that use magnetic force on a moving...

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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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L26_a - Devices that use magnetic force on a moving...

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