Define stokes formula
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The force of viscosity on a small sphere moving through a viscous fluid is given.
where:
Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particleη is the dynamic viscosity (Some authors use the symbol μ)R is the radius of the spherical objectv is the flow velocity relative to the object.
In SI units, Fd is given in newtons, η in Pa·s, Rin meters, and v in m/s.
Stokes' law makes the following assumptions for the behavior of a particle in a fluid:
Laminar FlowSpherical particlesHomogeneous (uniform in composition) materialSmooth surfacesParticles do not interfere with each other.
Note that for molecules Stokes' law is used to define their Stokes radius.
The CGS unit of kinematic viscosity was named "stokes" after his work.
ApplicationsEdit
Stokes' law is the basis of the falling-sphere viscometer, in which the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. A series of steel ball bearings of different diameters are normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses glycerine or golden syrup as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. Several school experiments often involve varying the temperature and/or concentration of the substances used in order to demonstrate the effects this has on the viscosity. Industrial methods include many different oils, and polymer liquids such as solutions.
The importance of Stokes' law is illustrated by the fact that it played a critical role in the research leading to at least three Nobel Prizes.[3]
Stokes' law is important for understanding the swimming of microorganisms and sperm; also, the sedimentation of small particles and organisms in water, under the force of gravity.[4]
In air, the same theory can be used to explain why small water droplets (or ice crystals) can remain suspended in air (as clouds) until they grow to a critical size and start falling as rain (or snow and hail).[5] Similar use of the equation can be made in the settlement of fine particles in water or other fluids
where:
Fd is the frictional force – known as Stokes' drag – acting on the interface between the fluid and the particleη is the dynamic viscosity (Some authors use the symbol μ)R is the radius of the spherical objectv is the flow velocity relative to the object.
In SI units, Fd is given in newtons, η in Pa·s, Rin meters, and v in m/s.
Stokes' law makes the following assumptions for the behavior of a particle in a fluid:
Laminar FlowSpherical particlesHomogeneous (uniform in composition) materialSmooth surfacesParticles do not interfere with each other.
Note that for molecules Stokes' law is used to define their Stokes radius.
The CGS unit of kinematic viscosity was named "stokes" after his work.
ApplicationsEdit
Stokes' law is the basis of the falling-sphere viscometer, in which the fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to descend through the liquid. If correctly selected, it reaches terminal velocity, which can be measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. A series of steel ball bearings of different diameters are normally used in the classic experiment to improve the accuracy of the calculation. The school experiment uses glycerine or golden syrup as the fluid, and the technique is used industrially to check the viscosity of fluids used in processes. Several school experiments often involve varying the temperature and/or concentration of the substances used in order to demonstrate the effects this has on the viscosity. Industrial methods include many different oils, and polymer liquids such as solutions.
The importance of Stokes' law is illustrated by the fact that it played a critical role in the research leading to at least three Nobel Prizes.[3]
Stokes' law is important for understanding the swimming of microorganisms and sperm; also, the sedimentation of small particles and organisms in water, under the force of gravity.[4]
In air, the same theory can be used to explain why small water droplets (or ice crystals) can remain suspended in air (as clouds) until they grow to a critical size and start falling as rain (or snow and hail).[5] Similar use of the equation can be made in the settlement of fine particles in water or other fluids
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