where , ( dv / dx ) = rate of change of velocity with distance called velocity gradient , A = area of cross-section and = coefficient of viscosity .
SI unit of η is Nsm -2 or pascal-second or decapoise . Its dimensional formula is [ ML -1 T -1 ].
The knowledge of the coefficient of viscosity of different oils and its variation with temperature helps us to select a suitable lubricant for a given machine .
Viscosity is due to transport of momentum . The value of viscosity ( and compressibility ) for ideal liquid is zero .
The viscosity of air and of some liquids is utilised for damping the n . ving parts of some instruments .
The knowledge of viscosity of some organic liquids is used in determining the molecular weight and shape of large organic moleculars like proteins and cellulose .
Variation of Viscosity The viscosity of liquids decreases with increase in temperature
where , η0 and ηt is are coefficient of viscosities at 0 ° C t ° C , α and β are constants .
The viscosity of gases increases with increase in temperatures as η ∝ √T
The viscosity of liquids increases with increase in pressure but the viscosity of water decreases with increase in pressure .
The viscosity of gases do not changes with pressure .