electron gun

Examples of electron gun in the following topics:

• Cathode Ray Tube, TV and Computer Monitors, and the Oscilloscope

  • A cathode ray tube consists of a vacuum tube that contains one or more electron guns used to excite phosphors on a screen to produce images.
  • The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns (a source of directed electrons) and a fluorescent screen used to view images.
  • It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images.
  • These phosphors are struck by incoming electrons from the electron gun, absorb energy, and then re-emit some or all of that energy in the form of light (this process is called phosphorescence) .
  • Cutaway rendering of a color CRT: 1) Three Electron guns (for red, green, and blue phosphor dots) 2) Electron beams 3) Focusing coils 4) Deflection coils 5) Anode connection 6) Mask for separating beams for red, green, and blue part of displayed image 7) Phosphor layer with red, green, and blue zones 8) Close-up of the phosphor-coated inner side of the screen

• de Broglie and the Wave Nature of Matter

  • It took three years for Clinton Davisson and Lester Germer to observe diffraction patterns from electrons passing a crystalline metallic target (see ).
  • The experiment included an electron gun consisting of a heated filament that released thermally excited electrons, which were then accelerated through a potential difference (giving them a certain amount of kinetic energy towards the nickel crystal).
  • To avoid collisions of the electrons with other molecules on their way towards the surface, the experiment was conducted in a vacuum chamber.
  • To measure the number of electrons that were scattered at different angles, an electron detector that could be moved on an arc path about the crystal was used.
  • The detector was designed to accept only elastically scattered electrons.

• Electron Configurations

  • The electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals.
  • The electron configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals.
  • Electron configurations describe electrons as each moving independently in an orbital, in an average field created by all other orbitals.
  • For atoms or molecules with more than one electron, the motion of electrons are correlated and such picture is no longer exact.
  • However, the electronic wave function is usually dominated by a very small number of configurations and therefore the notion of electronic configuration remains essential for multi-electron systems.

• Multielectron Atoms

  • However, when more electrons are involved, each electron (in the $n$-shell) feels not only the electromagnetic attraction from the positive nucleus, but also repulsion forces from other electrons in shells from '1' to '$n$'.
  • This causes the net force on electrons in the outer electron shells to be significantly smaller in magnitude.
  • Therefore, these electrons are not as strongly bonded to the nucleus as electrons closer to the nucleus.
  • Each has 10 electrons, and the number of nonvalence electrons is two (10 total electrons minus eight valence electrons), but the effective nuclear charge varies because each has a different number of protons:
  • A multielectron atom with inner electrons shielding outside electrons from the positively charged nucleus

• Electron Microscopes

  • An electron microscope is a microscope that uses an electron beam to create an image of the target.
  • This idea is used in the electron microscope which is a type that uses electrons to create an image of the target.
  • The original form of electron microscopy, transmission electron microscopy, works in a similar manner using electrons.
  • In the electron microscope, electrons which are emitted by a cathode are formed into a beam using magnetic lenses (usually electromagnets).
  • This electron beam is then passed through a very thin target.

• Diffraction Revisited

  • Here h is Planck's constant and p the relativistic momentum of the electron.
  • De Broglie's formula was confirmed three years later for electrons (which have a rest-mass) with the observation of electron diffraction in two independent experiments.
  • Experiments are usually performed using a transmission electron microscope or a scanning electron microscope.
  • This means that the incident electrons feel the influence of both the positively charged atomic nuclei and the surrounding electrons.
  • Typical electron diffraction pattern obtained in a transmission electron microscope with a parallel electron beam.

• Electric Charge in the Atom

  • Both protons and electrons have charge that is quantized.
  • The negatively charged electronic cloud indicates the regions of the space where electrons are likely to be found.
  • If a neutral atom gains an electron, it becomes negative.
  • If a neutral atom loses an electron, it becomes positive.
  • The steady flow of electrons is called current.

• Specialty Microscopes and Contrast

  • There are many types of microscopes: optical microscopes, transmission electron microscopes, scanning electron microscopes and scanning probe microscopes.
  • Microscopes can also be classified based on whether they analyze the sample by scanning a point at a time (scanning electron microscopes), or by analyzing the entire sample at once (transmission electron microscopes).
  • Transmission Electron Microscope: The TEM passes electrons through the sample, and allows people to see objects that are normally not seen by the naked eye .
  • Scanning Electron Microscopes: Referred to as SEM, these microscopes look at the surface of objects by scanning them with a fine electron beam .
  • The electron beam of the microscope interacts with the electrons in the sample and produces signals that can be detected and have information about the topography and composition.

• The Thomson Model

  • Thomson proposed that the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons' negative charges.
  • Thomson, who discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model.
  • In Thomson's model, the atom is composed of electrons (which Thomson still called "corpuscles," though G.
  • Stoney had proposed that atoms of electricity be called electrons in 1894) surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively charged "plums" surrounded by positively charged "pudding" .
  • Rutherford, Thomson, electrons, nuclei, and plums.

• The Electron-Volt

  • The electron volt is defined as the amount of energy gained or lost by the charge of an electron moved across a one-volt electric potential difference.
  • Applying a potential difference to an electron gives it energy, which manifests itself in motion of the electron through it.
  • Given that the electron has both mass and velocity, it has momentum.
  • Given that mass is equivalent to energy, the electron volt can measure mass.
  • In plasma physics, the electron volt can be used as a unit of temperature.