Phy Chapter 6 Morden Physics Caltec S.4

CHAPTER TWELVE MODERN PHYSICS ELECTRONS Atomic structure An atom consists of a nucleus at its centre where its mass is concentrated. The nucleus consists of neutrons and protons and is surrounded by electrons which move around it in circular paths called orbits or shells. In an atom, there are equal numbers of electrons and protons. Electrons carry a negative charge and protons and electrons in an atom form an electrically neutral combination. Thermionic emission When a metal is heated, the free electrons in it gain an extra energy which enables them to break through the surface of the metal and become free electrons. The process by which electrons are emitted from hot metal surface is called thermionic emission. Rectification A semiconductor diode rectifies, i.e. converts a.c. to d.c. a) Half wave rectification The diode removes the negative half-cycles of a.c. to give a varying but one-way (direct) p.d. across R, the ‘load’ requiring a d.c. supply, Figure b above. Full wave rectification Both the half-cycles of the a.c to be rectified are used. In the bridge circuit of fig. a, the current flows the solid arrows when X is positive and Y negative and the broken arrows on the negative half-cycles when the positive of X and Y are reversed. During both half-cycles, current flows in R and in the same direction giving a p.d. as shown below, Cathode rays Streams of electrons moving at high speed are called cathode rays. CATHODE RAY OSCILLOSCOPE (C.R.O) The C.R.O is one of the most important scientific instruments ever to be developed. It consists, like a television set. A cathode ray tube has three main parts, as in figure below. a) Electron gun This consists of a heater, a cathode C, another electrode called the grid G, and two or three anodes A1, …