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Discovery of the Electron

Discovery of the Electron. Thomson J. J , Cathode Rays, Philosophical Magazine, 44 (1897) 293. Mustafa Amin University of Texas at Arlington Department of Physics. e. e. Outline. Introduction Historical Background Thompson’s Experiment Millikan’s Experiment Conclusion. e.

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Discovery of the Electron

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  1. Discovery of the Electron Thomson J. J , Cathode Rays, Philosophical Magazine, 44 (1897) 293 Mustafa Amin University of Texas at Arlington Department of Physics e e

  2. Outline • Introduction • Historical Background • Thompson’s Experiment • Millikan’s Experiment • Conclusion e e

  3. Introduction • The Atom is no longer indivisible!!! • Electron, the first fundamental particle • Birth of Elementary Particle Physics e e

  4. Background • Electric Discharge & green phosphoroscence • Something travels in straight lines Mica Cross experiment by Sir William Crookes • (1897) Cathode Rays: Aetherial or Material ? Material: Deflection by magnet Negative charge (Perrin) Deflection by a static electric field? Aetherial: Penetration of thin metal sheets (Hertz)

  5. Thompson’s Experiment • Charge carried by the Cathode Rays • Electrostatic Deflection of Cathode rays • Determination of e/m e

  6. Charge carried by cathode rays Cathode rays pass from the tube in the upper left into the larger bulb, where they are deflected with a magnetic field. When they are bent so as to enter the slits in the cylinders, the electrometer measures the charge transferred to the cylinder.[1] Thomson J. J , Cathode Rays, Philosophical Magazine, 44 (1897) 293

  7. Deflexion of Cathode rays by Electrostatic Field and Velocity C Cathode A, B Anode and Ground D, E Capacitor plates Velocity • qvB = qE or v = E/B • v ~ c / 3 ~1000 times the velocity of Hydrogen!! Thomson J. J , Cathode Rays, Philosophical Magazine, 44 (1897) 293

  8. Determination of e/m Displacement 2s = a t2 = ( Ee/m )( l2/v2 ) Charge to mass ratio e/m = (2s/E)( v2/l2 ) ~ 17000000 (c.g.s units) ~17000 times the e/mH for Hydrogen

  9. Magnitude of e e/m 17000 times the e/mH for Hydrogen • e large ? • m small? • Wilson’s Experiment • Townsend’s Experiment • Millikan’s Experiment Conclusion: Small m !!!! + + e + + + +

  10. Millikan’s Experiment With E m g = q E Without E m g = K vT , K = 6p hr q ~ n (1.6 * 10-7 C) Quantized charge !!!! http://www.kingsu.ab.ca/~brian/templeton

  11. Conclusion Charge e = 1.6 * 10-19 C Mass m = 9.1* 10-31 kg Spin s = +1/2, -1/2 (Uhlenbeck, Goudsmith)

  12. "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?"-- J.J. Thomson.

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