Mechanical Properties of Fluids | Class 11 Physics Notes

9.5 Viscosity

Viscosity is the property of fluids that causes resistance to relative motion between adjacent layers, analogous to internal friction. When a fluid flows, layers slide past each other with different velocities, generating viscous forces opposing the motion. For example, honey is more viscous than oil because it requires more force to move layers at the same speed. In laminar flow between two parallel plates, the fluid velocity varies linearly from zero at the stationary plate to maximum at the moving plate. The coefficient of viscosity (η) quantifies this resistance and is defined as the ratio of shear stress to the rate of shear strain (velocity gradient). Its SI unit is pascal-second (Pa·s). Viscosity decreases with temperature for liquids but increases for gases. Stokes' law describes the viscous drag force on a sphere moving through a fluid, proportional to velocity, viscosity, and sphere radius. The terminal velocity of falling spheres in viscous fluids depends on these parameters. Viscosity plays a crucial role in fluid flow, engineering applications, and biological systems.

📊 Diagram: Fig. 9.12 (a) A layer of liquid sandwiched between two parallel glass plates, in which the lower plate is fixed and the upper one is moving to the right with velocity v; (b) velocity distribution for viscous flow in a pipe; Fig. 9.13 Measurement of the coefficient of viscosity of a liquid.

🔗 Connection: This section leads to the study of surface tension, another important property of fluids related to molecular forces at interfaces.

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| Specific Gravity | | No | No | Psubstance, Scalar