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College Physics 2e

Section Summary

College Physics 2eSection Summary

12.1 Flow Rate and Its Relation to Velocity

  • Flow rate QQ is defined to be the volume VV flowing past a point in time tt, or Q=VtQ=Vt where VV is volume and tt is time.
  • The SI unit of volume is m3m3.
  • Another common unit is the liter (L), which is 103m3103m3.
  • Flow rate and velocity are related by Q=A v ¯ Q=A v ¯ where AA is the cross-sectional area of the flow and v ¯ v ¯ is its average velocity.
  • For incompressible fluids, flow rate at various points is constant. That is,
    Q 1 = Q 2 A 1 v ¯ 1 = A 2 v ¯ 2 n 1 A 1 v ¯ 1 = n 2 A 2 v ¯ 2 . Q 1 = Q 2 A 1 v ¯ 1 = A 2 v ¯ 2 n 1 A 1 v ¯ 1 = n 2 A 2 v ¯ 2 .

12.2 Bernoulli’s Equation

  • Bernoulli’s equation states that the sum on each side of the following equation is constant, or the same at any two points in an incompressible frictionless fluid:
  • Bernoulli’s principle is Bernoulli’s equation applied to situations in which depth is constant. The terms involving depth (or height h ) subtract out, yielding
  • Bernoulli’s principle has many applications, including entrainment, wings and sails, and velocity measurement.

12.3 The Most General Applications of Bernoulli’s Equation

  • Power in fluid flow is given by the equation P1+12ρv2+ρghQ=power,P1+12ρv2+ρghQ=power, where the first term is power associated with pressure, the second is power associated with velocity, and the third is power associated with height.

12.4 Viscosity and Laminar Flow; Poiseuille’s Law

  • Laminar flow is characterized by smooth flow of the fluid in layers that do not mix.
  • Turbulence is characterized by eddies and swirls that mix layers of fluid together.
  • Fluid viscosity ηη is due to friction within a fluid. Representative values are given in Table 12.1. Viscosity has units of ( N/m 2 ) s ( N/m 2 ) s or Pa s Pa s .
  • Flow is proportional to pressure difference and inversely proportional to resistance:
  • For laminar flow in a tube, Poiseuille’s law for resistance states that
  • Poiseuille’s law for flow in a tube is
  • The pressure drop caused by flow and resistance is given by

12.5 The Onset of Turbulence

  • The Reynolds number NRNR can reveal whether flow is laminar or turbulent. It is
  • For NRNR below about 2000, flow is laminar. For NRNR above about 3000, flow is turbulent. For values of NRNR between 2000 and 3000, it may be either or both.

12.6 Motion of an Object in a Viscous Fluid

  • When an object moves in a fluid, there is a different form of the Reynolds number NR=ρvLη(object in fluid), NR=ρvLη(object in fluid), which indicates whether flow is laminar or turbulent.
  • For NRNR less than about one, flow is laminar.
  • For NRNR greater than 106106, flow is entirely turbulent.

12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes

  • Diffusion is the movement of substances due to random thermal molecular motion.
  • The average distance xrmsxrms a molecule travels by diffusion in a given amount of time is given by

    where DD is the diffusion constant, representative values of which are found in Table 12.2.

  • Osmosis is a process by which molecules of a solvent pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus, equalizing the solute concentrations on each side of the membrane.
  • Dialysis is the transport of any other molecule through a semipermeable membrane due to its concentration difference.
  • Both processes can be reversed by back pressure.
  • Active transport is a process in which a living membrane expends energy to move substances across it.
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