________________
40. (a)
(ii) Photoelectric emission: In this process, as already discussed
the additional energy is supplied by means of electromagnetic radiation. The emitted electrons are known as photo
electrons. (iii) Secondary emission: In this process, the fast moving
electrons on collision with the metal surface knock out
electrons, called the secondary electrons. (iv) Field emission: In this process, the electrons are emitted
by the metal surface on subjecting it to a very strong electric
field (108 volt per metre)" Prof. Dr. G.R. Jain, op.cit., p. 7 "Modern investigations have shown conclusively that all matter is composed of molecules which, in the case of gases, are travelling in all directions with high speeds. Theoretically a piece of chalk may be broken into two pieces, those two into four, and so on to infinity. In reality, matter cannot be subdivided beyond a certain point without loosing its identity. The smallest particle into which matter may be subdivided without destroying its characteristic properties is called a molecule.” "No one has ever seen a molecule; these particles being so small that even the best microscope fails to reveal them. The diameter of a molecule has been measured to be one ten-millionth an inch 1/10,000,000 inch. A drop of water is about 1/8 of an inch. Satish K. Gupta, op.cit., p. 1221"Molecules An atom is electrically a neutral system. In an atom, electrons revolve around the positive nucleus. The net electric field of an atom even at a small distance from the centre of the atom (nearly equal to 10-1°m) is almost zero. Atoms of most of the elements do not exist as atoms as such. When atoms come sufficiently close to each other, such that electron cloud of one atom overlaps that of the other, they interact with each other electrically. The atoms then, remain together rather than to exist as separate neutral atoms. When the atoms are held together because of the above fact, they are said to form molecules. The molecule so formed is said to be stable, if energy is required to split it into its constituent atoms. The minimum energy required to split a molecule into its constituent atoms is called dissociation energy. Atoms can combine in many ways to form molecules:
(b)
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