what is force of adhesion and force of cohesion? explain with an example. Also explain the causes which give rise to surface tension?

Asked by mehreennisar | 6th Oct, 2010, 07:55: PM

Expert Answer:

Dear student
Adhesion and cohesion, attractive forces  between material bodies. A distinction is usually made between an adhesive force, which acts to hold two separate bodies together (or to stick one body to another) and a cohesive force, which acts to hold together the like or unlike atoms, ions, or molecules of a single body. However, both forces result from the same basic properties of matter. A number of phenomena can be explained in terms of adhesion and cohesion.
 
For example, surface tension in liquids results from cohesion, and capillarity results from a combination of adhesion and cohesion.
Surface tension, is the tendency of liquids to reduce their exposed surface to the smallest possible area. A drop of water, for example, tends to assume the shape of a sphere.

To understand what causes the surface tension in the liquids refer the accompanying figure in which two molecules of the liquid, A and B, are considered. The molecule A is completely inside the fluid and it is attracted by cohesive forces of the surrounding molecules in all directions. The resultant force acting on molecule is zero. Similarly, the resultant forces acting on all the molecules located below the surface of the fluid is zero.

Now let us consider the molecule B located at the surface of the fluid. There are downward forces of attraction on this molecule due to other molecules. Thus there is net resultant force acting on molecule B in the downward direction. In a similar manner all the molecules on the surface of the fluid experience net downward force. Thus the free surface of the fluid acts like a very thin film under tension of the surface of the liquid acts as though it is an elastic membrane under tension, thus causing the surface tension.

We hope that clarifies your query.

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Answered by  | 8th Oct, 2010, 05:55: AM

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