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Cohesion and Adhesion

المؤلف:  GEORGE A. HOADLEY

المصدر:  Essentials of Physics

الجزء والصفحة:  P-23

2025-10-20

628

Cohesion is the mutual attraction that particles of the same kind have for each other at molecular distances. It is measured by the force required to pull a body apart.

If the attraction is between particles of different kinds, it is called adhesion. Both of these attractions are molecular; there is no essential difference between them, and no need of two names. Cohesion is very great in solids, and serves to give them form and strength. In liquids cohesion is not strong enough to determine the form, except in very small quantities, when they take the form of drops. In gases cohesion is very slight. In many cases adhesion is greater than cohesion, as in the case of two boards glued together, or two pieces of china cemented to each other; if they are again broken, the break will be more likely to take place in the board or china than in the joint. Finely divided matter is often made into a solid body by compression, as in the making of emery wheels. If a piece of rubber gum is cut with a knife, the two pieces may be made to cohere perfectly by pressure.

FIG 1

Demonstration. - Press together two pieces of plate glass as in Fig. 1, and see whether both can be lifted by raising the upper piece by the corner. Put two or three drops of water between the plates, and they will cling together more firmly.