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РЕГИСТРАЦИЯ ЭКСКУРСИЯ

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Прочтите текст про себя (контрольное время чтения — 3,5 минуты). A Kinetic Theory of Liquids

In a liquid, molcculcs arc closc to cach other, and consequently the forccs exerted on one molcculc by its neighbours arc substantial. Thus, the problem of analysing the motion of a single molcculc is exceedingly difficult, for cach is constantly in "collision", subjcct to the forccs for as many as twelve nearest neighbours. What then can wc say about molccular motion in liquids? One of the most revealing observations in this respect was made by the botanist Robert Brown in 1827. Brown discovered that very tiny particles (10"4 cm diameter) suspended in a liquid undergo inccssant randomly directed motion. These motions occur without any apparent external causc such as stirring or convcction, and arc evidently associated with an intrinsic property of all liquids. A wealth of experimental observation has confirmed the idea of this Brownian motion being a direct manifestation of the thermal motion of molcculcs. When it is suspended in a liquid, a very small particlc constantly experiences collisions with all the molcculcs surrounding it. If the particlc is small enough, so few molcculcs will be able to collidc with it, that at any particular instant the number striking it from one side may be different from the number striking it from the other sides; consequently, the particlc will be displaced. Subsequently, another unbalance of collisional forccs may occur, this time displacing the particlc in a different direction. The great majority of these displacements arc so small that they cannot be dctcctcd individually, but the motion which is observed is a result of many of the smaller random displacements. In csscncc, a Brownian particlc is a "molcculc" thought to be large enough to be observable, but small enough to cxccutc observable random thermal motion.

Analysis of the motion of Brownian particles shows that their average kinetic energy is 3/2 kT. Since cach particle is to be considered as one of the molcculcs of the liquid wc can conclude that the average kinetic energy of a molcculc in a liquid is also 3/2 kT — cxactly the same as the kinetic energy of a gaseous molcculc at tfie same temperature.


19.01.2015; 01:31
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