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Producing and Using Stream of Electrons 1. Free electrons : The electrons, revolving in the outermost orbit of an atom, which are weakly held by the nucleus are called free electrons. 2. Thermions : The electrons emitted from the surface of metal, when heat energy is supplied to metal are called thermions. 3. Thermionic emission : The phenomenon due to which free surface of the metal emits electrons, on being heated is called thermionic emission. 4. Threshold energy or work function : The minimum heat energy required to make a metal to emit an electron (thermion), from its surface is called threshold energy or work function. 5. Electron volt (eV) : It is the smallest unit of work and is generally used to measure threshold energy. It is defined as the amount of work done, when a charge of 1.6 10 19 coulombs (charge of one electron), moves through two points in an electric field, at a potential difference of one volt. 1 eV = 1.6 10 19 V = 1.6 l0 19 J. 6. Factors determining the rate of emission of thermions : 1. It is inversely proportional to the work function of a material. 2. It is directly proportional to the temperature of the surface emitting thermions. 3. It is directly proportional to the surface area of the surface emitting thermions. Class-X 1 Key points 7. Characteristics of material acting as thermion emitter. 1. It must have low work function. 2. It must have high melting point. 8. Directly heated thermion emitter : A thermion emitter, which gets heated on the passage of electric current and when its temperature becomes more than the threshold temperature, it starts emitting electrons, is called directly heated thermion emitter. 9. Indirectly heated thermion emitter : A material, which itself does not get heated upon the passage of electric current, but when heated by some other source emits thermions is called indirectly heated thermion emitter. 10. Threshold temperature : The minimum temperature at which a particular material emits thermions from its surface on heating is called threshold temperature. 11. Diode valve : An electronic device, which allows the electrons to flow only in one particular direction is called diode valve. 12. Directly heated diode valve : A diode valve in which heating element acts as cathode is called directly heated diode valve. 13. Indirectly heated diode valve : A diode valve in which heat is produced by tungsten filament, which is used to emit thermions from cathode is called indirectly heated diode valve. 14. Space charge : The space around cathode which is filled with free thermions is called space charge. Class-X 2 Key points 15. Cathode ray tube is used for : (i) Studying features of wave motion, (ii) Investigating changes in p.d. of alternating current, (iii) Studying various wave forms, (iv) As a T.V. screen (v) As a monitor for computers. Class-X 3 Key points
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