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Radioactivity Summary Notes An atom has a central core called nucleus which contains protons (positively charged particles) and neutrons (neutral particles). Negatively charged electrons revolve around the nucleus in orbits located at different distances. The electrons very close to the nucleus are called bound electrons. They are strongly attracted by the protons in the nucleus. The electrons moving in orbits lying away from the nucleus are weakly attracted by the nucleus, i.e., these electrons are loosely bound electrons are called free electrons. Metals which are good conductors, have a large number of free electrons. If we increase the kinetic energy of the free electrons, they start coming out from the surface of a metal. The minimum amount of energy required to emit electrons from the surface of the metal. The minimum amount of energy required to emit electrons from the surface of the metal is called work function or threshold energy of that metal. The process of emitting or ejecting out free electrons from the surface of a metal is called electron emission. The phenomenon of emission of free electrons from a metal surface when it is heated to a high temperature, is called thermionic emission. The emitted electrons are called thermions. The number of electrons emitted per second, i.e., the rate of emission of electrons from a metal surface depends on the following factors: Nature of the surface of metal: Less the work function, more is rate of emission of electrons. Temperature of the surface: Higher the temperature, more is the rate of emission of electrons. Surface area of the material: Greater the surface area, greater is the rate of emission of electrons. A substance which is used as an electron emitter must have the following properties: Its work function should be low so that the electrons can be emitted at low temperature. Its melting point should be high so that the substance does not melt easily. A cathode ray tube is a device used for studying the current and voltage waveform in various electric circuits. A cathode ray tube works on the principles of: Thermionic emission Deflection of electron beam by the electric and magnetic fields. Fluorescence produced by the electron beam. A cathode ray tube mainly consists of: The electron gun The deflecting system The fluorescent screen A cathode ray tube is used: Mainly to convert an electrical signal into a visual signal. For checking the waveform of an electrical signal. In demonstrating the features of wave motion. For measuring the short time intervals. www.vedantu.com 1
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