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.