PGTRB (PHYSICS) – ATOMIC PHYSICS

ATOMIC PHYSICS

  • 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 (2dsin
    Ɵ) 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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