Question

What is the unit of viscosity? Please explain.

Answer 1

Hint :To calculate the units of viscosity in both the S.I. system and the C.G.S. system, you should first know about the formula of the viscosity and then, by using it use can easily calculate its units from the respective quantities involved in the chemical formula.

Complete Step By Step Answer:
First of all, let’s discuss viscosity. By the term viscosity, we simply mean the resistance to the flow of liquids while moving.
We can easily calculate the viscosity by using the formula as;
$F = \eta A\dfrac{{du}}{{dz}}$
Here F is the force, eta ($\eta $) is the co-efficient of viscosity, A is the area, du is the velocity and dz is the distance between the two layers.
Then; The S.I. units of viscosity can be calculated as;
We know that Force is measured in Newton.
A in square meters.
Velocity in meter per second and distance in meters.
Thus;
The S.I. unit of co-efficient of viscosity is as;
$\eqalign{
  & \eta = \dfrac{{{\text{F}} \times {\text{dz}}}}{{{\rm A} \times {\text{du}}}} \cr
  & {\text{ = }}\dfrac{{{\rm N}{\text{ewton}} \times m \times {\text{s}}}}{{{{\text{m}}^2} \times m}} \cr
  & {\text{ = }}\dfrac{{{\rm N}{\text{ewton}} \times {\text{s}}}}{{{{\text{m}}^2}}} \cr
  & {\text{ = Ns}}{{\text{m}}^{ - 2}} \cr} $
The Ns is common; known as the pascal so, therefore, The S.I. unit of Viscosity is $P{m^{ - 2}}$.
And the C.G.S. unit of viscosity is poise (P).

Note :
Viscosity can be measured by using the capillary viscometer. In this method, capillary tubes are being used and time is measured which the liquid took to pass through the tube.
There are two mains factors on which the viscosity depends;-
1. Temperature:- The viscosity of the liquids decreases with the rise in temperature whereas that of the gases increases with the rise in temperature.
2. Intermolecular force of attractions:- Stronger the intermolecular force of attraction; greater is the viscosity of the liquid and vice-versa.