The electron

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The electron
D.K.
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Leaving Certificate Physics Topics

• Mechanics
• Temperature
• Heat
• Waves
• Vibrations and Sound
• Light
• Electricity
• Modern Physics The electron
• Option 1 Particle Physics
• Option 2 Applied Electricity

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Cathode rays

• Cathode rays were first observed in 1869 by the
  German physicist Johann Hittorf.
• They seemed to come from the negative terminal
  (the cathode) of an evacuated tube.
• They cause some substances to fluoresce,
  including glass to a some extent.
• They could be deflected by a magnetic field or an
  electric field.
• In 1897 British physicist J. J. Thomson showed
  the rays were composed of a previously unknown
  negatively charged particle, which was later
  named the electron.
• Cathode ray tubes (CRTs) were long used as the
  picture tubes of television sets, oscilloscopes,
  heart monitors etc.

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Thermionic emission

• Around 1873 Frederick Guthrie discovered that a
  red hot conductor would readily lose a negative
  charge but would hold a positive charge.
• Thomas Edison noticed the same effect in 1880.
  Using the effect he made a device that could
  conduct electricity in one direction only and
  filed a patent based on the ideathe first ever
  patent for an electronic device.
• The British physicist John Ambrose Fleming,
  working for the British "Wireless Telegraphy"
  Company, discovered that thermionic emission
  could be used to detect radio waves and developed
  a vacuum tube diode which he patented in 1904.

Edison
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Learning outcomes

• The discovery of thermionic emission and of
  cathode rays led to the discovery of the
  electron.
• Applications Cathode ray tube, radio and
  television
• Photoelectric emission and how Einstein developed
  the quantum energy concept to explain the effect.
• Application photocell
• X-rays were discovered (1895, Röntgen)and the
  method of producing them developed and refined.
• Application medical uses as well as research and
  industrial uses.
• Spectra explained in terms of electron
  transitions between discrete energy levels (1913,
  Bohr).
• Application diagnostic research tool and laser
  applications.

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Discharge Tubes

• Callan developed high voltages
• Geissler developed effective vacuum pumps
• Discharge tubes developed
• Cathode rays discovered
• Experiments revealed them to be particles...then
  named electrons

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Thermionic emission

• The emission of electrons from a metal when it is
  heated
• . . . . discovered by Edison
• This paved the way for a revolution in
  communication
• . . . . i.e. radio and television

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Cathode Ray Tube

• Electron gun - deflection plates - fluorescent
  screen
• Similar to Thomsons experiment of 1897 where he
  identified the electron,
• Used in cathode ray oscilloscopes for measuring
  voltages and frequencies.
• Adapted for use in first generation televisions

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Electron gun

• Accelerating and focussing effect on the electron
  beam
• Electron volt as a unit of energy

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Recap questions

• What property of cathode rays suggested that they
  were charged particles?
• What is meant by the term space charge?
• What is the purpose of the grid in a cathode ray
  tube?
• How does the grid function?
• How does a cylindrical anode focus the electron
  beam?
• What happens when cathode rays strike the
  fluorescent screen
• What is an electron volt?

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Photoelectric effect

• It was found that no electrons were emitted
  unless the frequency of light was above a certain
  value, called the threshold frequency.
• Electrons were emitted immediately there was no
  delay after first exposure.

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Elements of Einsteins interpretation
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Photocell
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How x-rays are produced

• What is the role of the heater current?
• What is the role of the anode voltage?
• Beware! X-rays are ionising radiation.

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What happens at an atomic level to liberate an
x-ray?

• Rapid deceleration of an electron . or .
• Dislodge an inner electron, with subsequent
  replacement by an outer electron

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X-rays and crystals

• X-rays have shorter wavelength than light
• The tiny separation between the layers of atoms
  in crystals is required to cause diffraction of
  x-rays
• Much has been learned about crystals using x-rays

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X-rays and medicine

• Doctors diagnose broken bones using x-rays
• Doctors treat certain cancers with x-rays
• Dentists diagnose hidden tooth decay using x-rays

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Summary

• Cathode rays were discovered when discharge tubes
  were developed
• J.J. Thomson identified in 1897 that the cathode
  rays were particles (electrons).
• Thermionic emission was found to be an efficient
  way to obtain electrons.
• The photoelectric effect initially proved
  puzzling until Einstein explained it in 1905.
• X-rays were discovered by Roentgen in 1895.
• X-rays are very useful but can be dangerous
  because they are ionising.
• X-ray production is the converse of the
  photoelectric effect.