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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 (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. Thisisknown 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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