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PGTRB : LATEST STUDY MATERIAL:PHYSICS: ATOMIC PHYSICS.

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 PGTRB : LATEST STUDY MATERIAL:
PHYSICS: 
ATOMIC PHYSICS.

  • Gases in general are poor conductors of electricity. This is because they do not have free charged particles in large numbers.
  • The study of electric discharge through gases valuable information regarding the structure of atoms.
  • The discovery of X-rays by Roentgen.
  • 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.
  • When cathode rays strike a metals, X-rays, are emitted.
  • Cathode rays ionize the gases through which they pass. 
  • Cathode rays affect the photographic plates.
  • Cathode rays travel with high velocities.
  • Canal rays are streams of positive ions of the gas enclosed in the discharge tube. The mass of each ion is nearly equal to the mass of the gas atom.
  • Canal rays are deflected by electric and magnetic fields.
  • Canal rays travel in straight lines.
  • The velocity of canal rays is much smaller than the velocity of cathode rays.
  • Canal rays affect photographic plate.
  • Canal rays can penetrate through paper and thin foils of aluminums.
  • Canal rays cause fluorescence. They ionize the gas through which they pass.
  • The oil drop attains a terminal velocity (v) the net downward force acting on the oil drop is equal to the viscous force.
  • The charge of an electron was found to be 1.602*10^-19 coulomb.
  • The X-rays are produced in a Coolidge tube by making a fast moving electron beam to strike on a metal target having high atomic weight like tungsten kept in vacuum.
  • The photo electric effect may be defined as the phenomenon of the emission of electrons when electromagnetic radiations like X-rays, ultra violet rays and even visible light is incident on certain elements.
  • Photo electric current first increase slowly with the applied voltage, then more rapidly and finally reaches a steady maximum value called the saturation current.
  • The anode potential is gradually reduced to zero and negative till the current reduces to zero.
  • The potential which is just sufficient to bring the photo electric current to zero irrespective of the intensity of the incident light is called stopping potential or cut off potential (Vo).
  • For light of any given frequency the photo-electric current is directly proportional to the intensity of light.
  • The maximum kinetic energy of the photo electrons is directly proportional to the frequency of the incident light.
  • Matter in motion must be associated by waves called de-Broglie waves.
  • An electron microscope is operated only in high vacuum. This prohibits the use of the microscope to study living organisms which evaporate and disintegrate under such conditions.
  • At normal pressure gases do not conduct electricity.
  • The condition of electricity in a gas will vary with pressure. The behaviour 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 are fast moving electrons.
  • Canal rays are positive ions. They move in opposite direction to that of cathode rays in the same discharge tube.
  • The velocity of canal rays are less than that of cathode rays.
  • According to Millikan only very high viscous liquids like glycine 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.
  • 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.
  • In non-radiating orbits the angular momentum possessed by a electron is equal to nh /2pi.
  • Bohr atom model is successful in explaining the spectral lines of hydrogen and hydrogen like atoms.
  • Most gases conduct electricity at very low pressure.
  • Discharge tube requires a potential about 50000volts.
  • Cathode rays produce heat energy when they strike a material surface.
  • When fast moving electrons strike metal, X-rays are produced.
  • e/m of an electron was determined by J.J.Thomson.
  • In Millikan's experiment, the downward force acting on the oil drop is viscous force.
  • The tube which produces X-rays is Coolidge tube.
  • The device used to measure the wavelength of X-rays is Bragg spectrometer.
  • Electron microscope is operated in high vacuum.






 

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