![]() So we can approximately say, then, liquids flow in an incompressible manner. Most of the liquids are nearly incompressible as the density of a liquid remains almost constant as the pressure changes. Ideal fluid and Viscosityįluid flow can be compressible or incompressible. This tendency to resist a fluid flow is referred to as the viscosity of a fluid.įluids like air have low viscosities, thicker fluids like water are more viscous, and fluids like honey and motor oil are characterized by high viscosity. This opposing force also exists when two adjacent layers of fluid have to move relative to each other. Similarly, a fluid flowing past a stationary surface experiences a force opposing the flow. When a block slides across a rough floor, a frictional force between the block and floor opposes the motion. Viscosity can be described in terms of the resistance between adjacent layers in laminar flow. Syrup and engine oil are very viscous, while runny liquids such as water and petrol and all gases have low viscosities. The viscosity of a fluid relates to its stickiness and thus to its resistance to flow. ![]() This mechanism is similar to the one by which an object sliding on a rough horizontal surface loses kinetic energy. ![]() Viscosity causes part of the kinetic energy of a fluid to be converted to internal energy. This internal friction, or viscous force, is associated with the resistance that two adjacent layers of fluid have to move relative to each other. The chains tend to align with the applied stress.Īt high temperatures there is viscous flow.The term viscosity is commonly used in the description of fluid flow to characterize the degree of internal friction in the fluid. There is an elastic region ending in brittle fracture.Īt normal temperatures polymers have a non-linear elastic region with a smaller modulus (slope) and a yield point followed by limited plasticity before fracture.Īt warm temperatures the modulus decreases further and there is a larger easy glide region due to secondary bonds breaking and reforming. Temperature has a stronger effect on the secondary bonds than the primary bonds.Īt low temperatures, polymers like most materials, are brittle. Polymers are long chains of atoms joined together with primary bondsīut cross linked between chains with secondary bonds. ![]() In the above graph) and the material is then said to be a solid. The change to a glass is smooth with a gradual increase in viscosity.Īt some point there will be a large increase in viscosity and this is called the glass transition temperature (T g The change to a crystal is abrupt with a sudden increase in the viscosity coefficient. Link with long range order to form a crystal. If liquid SiO 2 cools slowly then the tetrahedrons can jostle past each other into their lowest energy configurations and The tetrahedrons are linked in random orientations by secondary bonds, thus forming a glass. If liquid SiO 2 cools quickly then the tetrahedral units do not have time to move to their lowest energy configuration and Viscous drag acts not only between the fluid and the upper plate but also between adjacent laminas of fluid.Ībove 1600° SiO 2 is a liquid with its tetrahedral base units in random motion. The thin layers (or laminas) are held back by viscous drag between the surfaces of the layers.įor example, if two flat solid plates are separated by a viscous fluid,Īn external force is needed to slide the top plate at constant speed over the fixed lower plate. Laminar flow occurs when a fluid can be pictured as split into thin layers which slide smoothly over each other. The easiest case to consider is laminar flow. In fluid flow, viscosity is the force that opposes motion in and of a fluid. With solids, friction is the force that opposes motion between two surfaces pressed together. ![]() Poiseuille's Law and Laminar flow in a tube In this lecture the following are introduced: This is an introduction to Electricity, Strength of Materials and Waves. Peter's Index Physics Home Lecture 6 Course Index Lecture 8 ![]()
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