Cathode Rays and Discovery of Electron :
Michael Faraday showed that chemical changes occur when electricity is passed though an electrolyte. He stated that electricity is made up of particles called atoms of electricity. G.J. Stoney suggested the name of electron for the atoms of electricity. However, the real credit for the discovery of electrons goes to J.J. Thomson. In mid `1850`s many scientists mainly Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it. The electrical discharge through the gases could be observed only at very low pressures
and at very high voltages. The pressure of different gases could be adjusted by evacuation.
When sufficiently high voltage is applied across the electrodes, current starts flowing as a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles. The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays, after passing through anode, strike the zinc sulphide coating, a bright spot on the coating is developed (same thing happens in a television set). See fig.1.
`text(The results of these experiments are summarised below)` :
(i) The cathode rays start from cathode and move towards the anode.
(ii) These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines.
(iv) In the presence of electric or magnetic field, the behaviour of cathode rays is similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.
(v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that electrons are basic constituent of all the atoms.
(vi) Cathode rays produce heating effect. When these rays are made to strike on a metal foil, the latter gets heated.
(vii) Cathode rays produce `X`-rays when they strike on surface of hard metals such as tungsten, copper molybdenum etc.
(viii) Cathode rays can pass through thin foils of metals like aluminium. However, these are stopped if the foil is quite thick.
(ix) Cathode rays ionize the gas through which they pass.
`text(J. J. Thomson)` `(1856-1940)` :
Sir J. J. Thomson confirmed these findings in `1897`. Thomson performed a series of experiments in which he was able to determine the charge/mass ( `e//m` ) ratio of the Particles that make up the cathode ray by measuring the deflection of the rays with varying magnetic and electric fields. Thomson performed the same experiments using different metals cathode and anode as well as different gases inside the tube.
`text(This value)` `(e/m)` `text(is found to be)` `1.76 xx 10^(22)` `text(coulomb per kg.)`
The e/m ratio for electron was found to be same irrespective of the nature of cathode and nature of gas taken in discharge tube. This shows that the electrons are universal constituent of all matter.
Quantization of Charge :
R.A. Millikan (`1868-1953`) devised a method known as oil drop experiment (`1906-14`), to determine the charge on the electrons. Millikan concluded that the magnitude of electrical charge, `q`, on the droplets is always an integral multiple of the electrical charge, `e`.
That is, `q = n e`, where `n = 1, 2, 3 ...............` That means charge is quantised.
He found that the value of `e` is `- 1.6 xx 10^(-19)` C. The present accepted value of electrical charge is `- 1.6022 xx 10^(-19)` C.
The mass of the electron (`m_e`) was determined by combining these results with Thomson's value of `e//m` ratio.
`text(Calculation of Mass of an electron)` : With the help of the experiments discussed above, it is possible to calculate the values of `e//m` ratio and also the charge (`e`) on the electron. The mass (`m`) of the electron can be calculated as follows.
Mass of election (`m`) `= e/(e//m) = (1.60 xx 10^(-19)C)/(1.76 xx 10^(8)Cg^(-1)) = 9.10 xx 10^(-28) g = 9.10 xx 10^(-31) kg`
An electron may be defined as : A fundamental particle present in an atom, which carries one unit negative charge `(1.60 xx 10^(-19))` and negligible mass `(9.10 xx 10^(-28) g)` which is `1//1837` of the mass of an atom of hydrogen.
and at very high voltages. The pressure of different gases could be adjusted by evacuation.
When sufficiently high voltage is applied across the electrodes, current starts flowing as a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles. The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays, after passing through anode, strike the zinc sulphide coating, a bright spot on the coating is developed (same thing happens in a television set). See fig.1.
`text(The results of these experiments are summarised below)` :
(i) The cathode rays start from cathode and move towards the anode.
(ii) These rays themselves are not visible but their behaviour can be observed with the help of certain kind of materials (fluorescent or phosphorescent) which glow when hit by them. Television picture tubes are cathode ray tubes and television pictures result due to fluorescence on the television screen coated with certain fluorescent or phosphorescent materials.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines.
(iv) In the presence of electric or magnetic field, the behaviour of cathode rays is similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.
(v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that electrons are basic constituent of all the atoms.
(vi) Cathode rays produce heating effect. When these rays are made to strike on a metal foil, the latter gets heated.
(vii) Cathode rays produce `X`-rays when they strike on surface of hard metals such as tungsten, copper molybdenum etc.
(viii) Cathode rays can pass through thin foils of metals like aluminium. However, these are stopped if the foil is quite thick.
(ix) Cathode rays ionize the gas through which they pass.
`text(J. J. Thomson)` `(1856-1940)` :
Sir J. J. Thomson confirmed these findings in `1897`. Thomson performed a series of experiments in which he was able to determine the charge/mass ( `e//m` ) ratio of the Particles that make up the cathode ray by measuring the deflection of the rays with varying magnetic and electric fields. Thomson performed the same experiments using different metals cathode and anode as well as different gases inside the tube.
`text(This value)` `(e/m)` `text(is found to be)` `1.76 xx 10^(22)` `text(coulomb per kg.)`
The e/m ratio for electron was found to be same irrespective of the nature of cathode and nature of gas taken in discharge tube. This shows that the electrons are universal constituent of all matter.
Quantization of Charge :
R.A. Millikan (`1868-1953`) devised a method known as oil drop experiment (`1906-14`), to determine the charge on the electrons. Millikan concluded that the magnitude of electrical charge, `q`, on the droplets is always an integral multiple of the electrical charge, `e`.
That is, `q = n e`, where `n = 1, 2, 3 ...............` That means charge is quantised.
He found that the value of `e` is `- 1.6 xx 10^(-19)` C. The present accepted value of electrical charge is `- 1.6022 xx 10^(-19)` C.
The mass of the electron (`m_e`) was determined by combining these results with Thomson's value of `e//m` ratio.
`text(Calculation of Mass of an electron)` : With the help of the experiments discussed above, it is possible to calculate the values of `e//m` ratio and also the charge (`e`) on the electron. The mass (`m`) of the electron can be calculated as follows.
Mass of election (`m`) `= e/(e//m) = (1.60 xx 10^(-19)C)/(1.76 xx 10^(8)Cg^(-1)) = 9.10 xx 10^(-28) g = 9.10 xx 10^(-31) kg`
An electron may be defined as : A fundamental particle present in an atom, which carries one unit negative charge `(1.60 xx 10^(-19))` and negligible mass `(9.10 xx 10^(-28) g)` which is `1//1837` of the mass of an atom of hydrogen.
