PGTRB (PHYSICS) - ATOMIC PHYSICS

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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. 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.