Discovery%20of%20the%20Electron

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Discovery of the Electron
Thomson J. J , Cathode Rays, Philosophical
Magazine, 44 (1897) 293

• Mustafa Amin
• University of Texas at Arlington
• Department of Physics

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Outline

• Introduction
• Historical Background
• Thompsons Experiment
• Millikans Experiment
• Conclusion

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Introduction

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

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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)

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Thompsons Experiment

• Charge carried by the Cathode Rays
• Electrostatic Deflection of Cathode rays
• Determination of e/m

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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
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Deflexion of Cathode rays by Electrostatic Field
and Velocity
Thomson J. J , Cathode Rays, Philosophical
Magazine, 44 (1897) 293

• 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 !!

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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

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Magnitude of e

• e/m 17000 times the e/mH for Hydrogen
• e large ?
• m small?
• Wilsons Experiment
• Townsends Experiment
• Millikans Experiment
• Conclusion Small m !!!!

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Millikans Experiment
With E m g q E  Without E
m g K vT ,
K 6p h r   q n (1.6 10-7 C)
Quantized charge !!!!
http//www.kingsu.ab.ca/brian/templeton
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Conclusion
Charge e 1.6 10-19 C Mass m 9.1
10-31 kg Spin s 1/2, -1/2 (Uhlenbeck,
Goudsmith)
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"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.