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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
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