Question

Write a short note on Viscosity.

Answer 1

1. Liquids (fluids) have a tendency to flow.
2. Viscosity measures the magnitude of internal friction in a liquid or fluid to flow as measured by the force per unit area resisting uniform flow.
3. Different layers of a liquid flow with different velocities. This called laminar flow. Here, the layers of molecules in the immediate contact of the fixed surface remains stationary. The subsequent layers slip over one another. Strong intermolecular forces obstruct the layers from slipping over one another, resulting in friction between the layers.
4. Viscosity is defined as the force of friction between the successive layers of a flowing liquid. It is also the resistance to the flow of a liquid.
5. When a liquid flows through a tube, the central layer has the highest velocity, whereas the layer along the inner wall in the tube remains stationary. This is a result of the viscosity of a liquid. Hence, a velocity gradient exists across the cross-section of the tube.
   
   Laminar flow of a liquid (fluid) through a tube/pipe
6. Viscosity is expressed in terms of the coefficient of viscosity, ‘η’ (Eta). The SI unit of viscosity coefficient is N sm^–2 (newton second per square meter). In the CGS system, the unit (η) is measured in poise.
   1 poise = 1 g cm^–1s^–1 = 10^–1 kg m^–1s^–1