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  • At normal pressure gases do not conduct electricity (poor conductors of electricity).
  • The conduction of electricity in a gas will vary with pressure. The behavior is studied with the help of a discharge tube.
  • When the pressure in the discharge tube is about 0.1mm of mercury cathode rays are produced.

  • Cathode Rays:

  • Cathode rays are fat moving electrons.
  • Cathode rays are emitted normally from the cathode surface. Their direction is independent of the position of the anode.
  • Cathode rays travel in straight line.
  • Cathode rays possess momentum and kinetic energy
  • Cathode rays produce heat when they strike a material surface.
  • Cathode rays produce fluorescence when they strike a number of crystals, minerals and salts.
  • Cathode rays strike a metal, X-rays are emitted.
  • Cathode rays affect the gases through which they pass.
  • Cathode rays affect the photographic plates.
  • Cathode rays are deflected from their straight line path by both electric and magnetic field. The direction of deflection shows that they are negatively charged particle.
  • Cathode rays travel with high velocities.
  • Cathode rays are found to be electrons which are fundamental constituents of all atoms.

  • Canal rays (Positive rays):

  • Canel rays are positive ions. They move in opposite direction to that of cathode rays in the same discharge tube
  • The mass of each ion is nearly equal to the mass of the gas atom.
  • They are deflected by electric and magnetic fields. Their deflection is opposite to that of cathode rays.
  • They travel in straight line.
  • The velocity of canel rays are less than that of cathode rays.
  • They affect photographic plate.
  • They can penetrate through paper and thin foils of aluminums.
  • They cause fluorescence.
  • They ionize the gas through which they pass.

  • J.J.Thomson:

  • In 1887 J.J.Thomson measured the specific charge –e/m (charge per unit mass) of the cathode ray particle.
  • The value of e/m was independent of the gas used and also independent of the electrodes.
  • A fine beam of cathode rays to the combined action of crossed electric and magnetic field e/m can be calculated.
  • Uniform magnetic field is produced perpendicular to the plane of the paper an outwards through out the region between P1P2
  • According to Millikan only very high viscous liquids like glycerin are used so that the oil drops acquire terminal velocity.
  • X-rays are produced when fast moving electrons strike a metal target of high atomic weight.
  • X-rays are produced in a Coolidge tube.
  • X-rays are invisible.
  • X-rays are allowed pass through gases.
  • X-rays can be diffracted by crystals.
  • There are two types of X-ray spectra:

  • (i)                  Continuous spectra and
  • (ii)                Characteristic spectra

  • When the path difference (2dsinƟ) is an internal multiple of wavelength (nλ), there will be a constructive interference, which is known as Bragg’s law (2dsinƟ=nλ).
  • Particle nature and wave nature of matter are called dual nature of matter.
  • When light is focused on certain material electrons are ejected from the surface. This  is  known as Photo electric effect.
  • The photo electric effect can be explained on the basis of quantum theory of radiation.
  • Photons behave like particles. They possess energy and momentum only. They travel with the velocity of light.
  • The photo electric effect starts only for a minimum frequency of light known as threshold frequency.
  • According to De Broglie moving particles like electrons under certain conditions behave like a wave.
  • Electron microscope is based on matter waves.
  • Prout suggested that all elements are made up of hydrogen atoms.
  • J.J.Thomson proposed that the atom is made up of a number of electrons embedded in a sphere of positive charge.
  • Rutherford suggested that the electrons are revolving around the nucleus in circular orbits.
  • According to Bohr the electrons revolving around the nucleus are fixed to specific orbits from which they do not radiate.
  • If non- radiating orbits the angular momentum possessed by a electron is equal to Bohr atom model is successful in explaining the spectral lines of hydrogen and hydrogen like atoms.

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