Physics – Viscosity of liquids -Important Points

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A- Viscosity of liquids

* The resistance offered by fluids (liquids as well as gases) to relative motion between its different layers is called viscous force. This property is called viscosity.

* When two parallel layers of a liquid are moving with different velocities, they experience tangential forces which tend to retard the faster layer and accelerate the slower layer.
These forces are (F) called viscous forces.

* The viscous forces are similar to frictional forces which resist relative motion between two bodies in contact.

B- Coefficient of viscosity

* Newton found that the viscous force is directly proportional to the common area (A) of the liquid layers in contact.

* Directly proportional to their relative velocity (v1 – v2 ) Inversely proportional to the distance (x) between them. This can be represented by the following formula:

* Where is a constant known as coefficient of viscosity The unit of coefficient of viscosity is N s m-2 or Poise.

C- Viscosity and its applications in Everyday Life

* The viscosity of sea water makes the waves subside during a storm.

* The motion of falling raindrops is opposed by the viscous force offered by air. Hence the rain drops falls slowly.

D- Lubricants

* A lubricant is a substance used to reduce friction. high molecular weight compounds such as hexanol are added as viscosity index improvers.

* Friction is reduced by using lubricants.

* Friction reduces the efficiency of a machine by converting mechanical energy into heat energy and causes much wear and tear of the moving parts.

E- Reynold’s Number

Reynold found that

* The critical velocity is directly proportional to the coefficient of viscosity and inversely proportional to the density of the liquid and radius of the tube.

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