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Chapter 28b - Magnetic fields(cont We found last time that...

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We found last time, that the force , , experienced by a charged particle , of charge q, moving with velocity through magnetic field , , is given by, Magnetic fields (cont.) B F G B G B F q(v B) = × G G G This lets us control the path of charged particles using magnetic fields. v G Since electric fields also exert forces on charged particles, E F qE = G G We have two independent means of controlling the trajectory of charged particles. The net force on a charged particle in an electric and/or magnetic field can be expressed as, E B F F F qE q(v B) q E (v B) = + = + × = + × G G G G G G G G G
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The textbook describes (section 28-4) how J. J. Thomson exploited this control in his discovery of the electron . In an evacuated tube he used an cathode ray gun to fire what at the time were only understood to be charged particles through a region of crossed electric and magnetic fields.
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The crossed electric and magnetic fields were arranged to exert opposing forces on the particles so that the balance of these fields controlled the vertical position of the particles when they exited the field region, determining where they hit the phosphor coated screen .
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