Tuesday, February 26, 2019
Describe the electronic discharge?
Electronic discharge can be explain as:1) The passage of electric current through air is called electric discharge through the air.
2) When a small potential difference is applied across the two electrodes in dry air, at atmospheric pressure, no discharge passes through air is a bad conductor of electricity since it does not contain free positive or negative ions.
3) When a very strong potential difference is applied across the two electrodes in a discharge tube and the pressure of air/gas is lowered gradually, then a stage is reached at which the current beings to flow through the air with cracking noise. The potential difference at which it happens is called sparking potential.
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4) The value of sparking potential depends upon-
a) the nature of gas, b) the pressure of the gas,c) the nature of electrodes and d)distance between the electrodes.
5) The value of sparking potential V is proportional to the product of pressure (P) and spark length of the gas i.e. V ∝ Pl. It is called Paschan's law.
6) When the pressure in a discharge tube is lowered without any charge in the strong potential difference applied across the two electrodes fitted in the tube, the different interesting phenomenon are observed at different stages of pressure.
7) At the pressure of 0.01 m of Hg, cathode rays are produced in a discharge tube.
Electrons
1) These are negatively charged particles, of charge 1.6×10-19 C and mass 9.1×10-31 kg.2) These where discovered by J.J. Thomson while studying the phenomenon of discharge of electricity through gases under conditions of low pressure and high voltage.
3) Electrons are present in the atom of every element.
Cathode rays
Cathode rays were discovered by William Crooke's. These are the stream of negatively charged particles (i.e. electrons) shot out at a high speed from the cathode of a discharge tube at a pressure bellow 0.01 mm of mercury and a very high potential difference (≈104 V) is applied across the two electrodes.Properties of cathode rays
1) Cathode rays travel in straight line and cast sharp shadows of the objects placed in their path.2) Cathode rays shot out normally from the surface of cathode and their direction is independent of the position of the anode.
3) They possess a lot of kinetic energy and can exert mechanical pressure.
4) They produce heat when they fall on metals hence temperature of the metal rises.
5) Cathode rays are deflected by the electric fields.
6) Cathode rays are deflected by the magnetic fields.
7) Cathode rays are independent of the nature of gas or electrodes employed in the discharge tube.
8) The value of specific charge (e/m) of cathode rays is universally constant (i.e. e/m = 1.7592×1011 C/kg).
9) They produce fluorescence with certain materials.
10) They can affect a photographic plate.
11) They can travel with a speed ranging 1/30 to 1/10 th of velocity of light.
12) They can pass through the metal foils without puncturing them.
13) They ionize the gas through which they pass.
14) They can exhibit interference and diffraction phenomenon under suitable arrangement. Thus they may behave as waves.
15) Cathode rays are streams of fast moving electrons.
Positive rays
These are the streams of positive ions which are moving towards the cathode in a discharge tube. These rays were discovered by Goldstein in 1886. These are produced in a discharge tube having anode and perforated cathode facing each other. When in such a discharge tube pressure is reduced to 10-3 mm of Hg and a very high potential difference is applied across the two electrodes, the electrons moving from cathode to anode ionize atoms or molecules of gas present in the tube, resulting positive and negative charge on them. The positive ions are repelled by the anode and are attracted by the cathode and emerge out of the perforated cathode as positive rays.Properties of positive rays
1) The positive rays are made of positively charged particles (i.e. ionized gas atoms or molecules).2) The value of charge on positive rays in an integral multiple of electronic charge.
3) Being heavy, their velocity is much less than that of cathode rays.
4) The kinetic energy of positive rays is more than that of cathode rays.
5) Specific charge (q/m) of positive rays is much smaller than that of cathode rays and is not a universal constant.
6) They ionize the gas through which they pass and ionizing power of positive rays is more than that of cathode rays.
7) The penetrating power of positive rays is less than that of cathode rays.
8) Positive rays are deflected by electric and magnetic field showing that they carry positive charge with them.
9) Positive rays produce fluorescence with certain materials and affect photographic plate like cathode rays.
Free electrons in metals
These are loosely bound electrons of the atoms, which can move freely within the metal surface but cannot leave the metal surface at room temperature.Work function of metal
1) It is the minimum energy required by an electron to just escape from the metal surface as to overcome the restraining forces at the surface of metal.2) Work function of a metal is generally denoted by W0 and it is usually expressed volt (eV).
3) The value of work function of alkali metal is less as compared to other metals.
4) The work function of a metal will decreases as the atomic number of element increases.
5) When the temperature of a metal increases, its work function will decrease.
Electron emission
It is a phenomenon of emission of electrons from the surface of metal. The electron emission can be obtained from the following processes:1) Thermionic emission
2) Photoelectric emission
3) Field emission
4) Secondary emission