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College Physics for AP® Courses

12.1 Flow Rate and Its Relation to Velocity

College Physics for AP® Courses12.1 Flow Rate and Its Relation to Velocity
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  1. Preface
  2. 1 Introduction: The Nature of Science and Physics
    1. Connection for AP® Courses
    2. 1.1 Physics: An Introduction
    3. 1.2 Physical Quantities and Units
    4. 1.3 Accuracy, Precision, and Significant Figures
    5. 1.4 Approximation
    6. Glossary
    7. Section Summary
    8. Conceptual Questions
    9. Problems & Exercises
  3. 2 Kinematics
    1. Connection for AP® Courses
    2. 2.1 Displacement
    3. 2.2 Vectors, Scalars, and Coordinate Systems
    4. 2.3 Time, Velocity, and Speed
    5. 2.4 Acceleration
    6. 2.5 Motion Equations for Constant Acceleration in One Dimension
    7. 2.6 Problem-Solving Basics for One Dimensional Kinematics
    8. 2.7 Falling Objects
    9. 2.8 Graphical Analysis of One Dimensional Motion
    10. Glossary
    11. Section Summary
    12. Conceptual Questions
    13. Problems & Exercises
    14. Test Prep for AP® Courses
  4. 3 Two-Dimensional Kinematics
    1. Connection for AP® Courses
    2. 3.1 Kinematics in Two Dimensions: An Introduction
    3. 3.2 Vector Addition and Subtraction: Graphical Methods
    4. 3.3 Vector Addition and Subtraction: Analytical Methods
    5. 3.4 Projectile Motion
    6. 3.5 Addition of Velocities
    7. Glossary
    8. Section Summary
    9. Conceptual Questions
    10. Problems & Exercises
    11. Test Prep for AP® Courses
  5. 4 Dynamics: Force and Newton's Laws of Motion
    1. Connection for AP® Courses
    2. 4.1 Development of Force Concept
    3. 4.2 Newton's First Law of Motion: Inertia
    4. 4.3 Newton's Second Law of Motion: Concept of a System
    5. 4.4 Newton's Third Law of Motion: Symmetry in Forces
    6. 4.5 Normal, Tension, and Other Examples of Force
    7. 4.6 Problem-Solving Strategies
    8. 4.7 Further Applications of Newton's Laws of Motion
    9. 4.8 Extended Topic: The Four Basic Forces—An Introduction
    10. Glossary
    11. Section Summary
    12. Conceptual Questions
    13. Problems & Exercises
    14. Test Prep for AP® Courses
  6. 5 Further Applications of Newton's Laws: Friction, Drag, and Elasticity
    1. Connection for AP® Courses
    2. 5.1 Friction
    3. 5.2 Drag Forces
    4. 5.3 Elasticity: Stress and Strain
    5. Glossary
    6. Section Summary
    7. Conceptual Questions
    8. Problems & Exercises
    9. Test Prep for AP® Courses
  7. 6 Gravitation and Uniform Circular Motion
    1. Connection for AP® Courses
    2. 6.1 Rotation Angle and Angular Velocity
    3. 6.2 Centripetal Acceleration
    4. 6.3 Centripetal Force
    5. 6.4 Fictitious Forces and Non-inertial Frames: The Coriolis Force
    6. 6.5 Newton's Universal Law of Gravitation
    7. 6.6 Satellites and Kepler's Laws: An Argument for Simplicity
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  8. 7 Work, Energy, and Energy Resources
    1. Connection for AP® Courses
    2. 7.1 Work: The Scientific Definition
    3. 7.2 Kinetic Energy and the Work-Energy Theorem
    4. 7.3 Gravitational Potential Energy
    5. 7.4 Conservative Forces and Potential Energy
    6. 7.5 Nonconservative Forces
    7. 7.6 Conservation of Energy
    8. 7.7 Power
    9. 7.8 Work, Energy, and Power in Humans
    10. 7.9 World Energy Use
    11. Glossary
    12. Section Summary
    13. Conceptual Questions
    14. Problems & Exercises
    15. Test Prep for AP® Courses
  9. 8 Linear Momentum and Collisions
    1. Connection for AP® courses
    2. 8.1 Linear Momentum and Force
    3. 8.2 Impulse
    4. 8.3 Conservation of Momentum
    5. 8.4 Elastic Collisions in One Dimension
    6. 8.5 Inelastic Collisions in One Dimension
    7. 8.6 Collisions of Point Masses in Two Dimensions
    8. 8.7 Introduction to Rocket Propulsion
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  10. 9 Statics and Torque
    1. Connection for AP® Courses
    2. 9.1 The First Condition for Equilibrium
    3. 9.2 The Second Condition for Equilibrium
    4. 9.3 Stability
    5. 9.4 Applications of Statics, Including Problem-Solving Strategies
    6. 9.5 Simple Machines
    7. 9.6 Forces and Torques in Muscles and Joints
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  11. 10 Rotational Motion and Angular Momentum
    1. Connection for AP® Courses
    2. 10.1 Angular Acceleration
    3. 10.2 Kinematics of Rotational Motion
    4. 10.3 Dynamics of Rotational Motion: Rotational Inertia
    5. 10.4 Rotational Kinetic Energy: Work and Energy Revisited
    6. 10.5 Angular Momentum and Its Conservation
    7. 10.6 Collisions of Extended Bodies in Two Dimensions
    8. 10.7 Gyroscopic Effects: Vector Aspects of Angular Momentum
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  12. 11 Fluid Statics
    1. Connection for AP® Courses
    2. 11.1 What Is a Fluid?
    3. 11.2 Density
    4. 11.3 Pressure
    5. 11.4 Variation of Pressure with Depth in a Fluid
    6. 11.5 Pascal’s Principle
    7. 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement
    8. 11.7 Archimedes’ Principle
    9. 11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
    10. 11.9 Pressures in the Body
    11. Glossary
    12. Section Summary
    13. Conceptual Questions
    14. Problems & Exercises
    15. Test Prep for AP® Courses
  13. 12 Fluid Dynamics and Its Biological and Medical Applications
    1. Connection for AP® Courses
    2. 12.1 Flow Rate and Its Relation to Velocity
    3. 12.2 Bernoulli’s Equation
    4. 12.3 The Most General Applications of Bernoulli’s Equation
    5. 12.4 Viscosity and Laminar Flow; Poiseuille’s Law
    6. 12.5 The Onset of Turbulence
    7. 12.6 Motion of an Object in a Viscous Fluid
    8. 12.7 Molecular Transport Phenomena: Diffusion, Osmosis, and Related Processes
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  14. 13 Temperature, Kinetic Theory, and the Gas Laws
    1. Connection for AP® Courses
    2. 13.1 Temperature
    3. 13.2 Thermal Expansion of Solids and Liquids
    4. 13.3 The Ideal Gas Law
    5. 13.4 Kinetic Theory: Atomic and Molecular Explanation of Pressure and Temperature
    6. 13.5 Phase Changes
    7. 13.6 Humidity, Evaporation, and Boiling
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  15. 14 Heat and Heat Transfer Methods
    1. Connection for AP® Courses
    2. 14.1 Heat
    3. 14.2 Temperature Change and Heat Capacity
    4. 14.3 Phase Change and Latent Heat
    5. 14.4 Heat Transfer Methods
    6. 14.5 Conduction
    7. 14.6 Convection
    8. 14.7 Radiation
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  16. 15 Thermodynamics
    1. Connection for AP® Courses
    2. 15.1 The First Law of Thermodynamics
    3. 15.2 The First Law of Thermodynamics and Some Simple Processes
    4. 15.3 Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency
    5. 15.4 Carnot’s Perfect Heat Engine: The Second Law of Thermodynamics Restated
    6. 15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators
    7. 15.6 Entropy and the Second Law of Thermodynamics: Disorder and the Unavailability of Energy
    8. 15.7 Statistical Interpretation of Entropy and the Second Law of Thermodynamics: The Underlying Explanation
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  17. 16 Oscillatory Motion and Waves
    1. Connection for AP® Courses
    2. 16.1 Hooke’s Law: Stress and Strain Revisited
    3. 16.2 Period and Frequency in Oscillations
    4. 16.3 Simple Harmonic Motion: A Special Periodic Motion
    5. 16.4 The Simple Pendulum
    6. 16.5 Energy and the Simple Harmonic Oscillator
    7. 16.6 Uniform Circular Motion and Simple Harmonic Motion
    8. 16.7 Damped Harmonic Motion
    9. 16.8 Forced Oscillations and Resonance
    10. 16.9 Waves
    11. 16.10 Superposition and Interference
    12. 16.11 Energy in Waves: Intensity
    13. Glossary
    14. Section Summary
    15. Conceptual Questions
    16. Problems & Exercises
    17. Test Prep for AP® Courses
  18. 17 Physics of Hearing
    1. Connection for AP® Courses
    2. 17.1 Sound
    3. 17.2 Speed of Sound, Frequency, and Wavelength
    4. 17.3 Sound Intensity and Sound Level
    5. 17.4 Doppler Effect and Sonic Booms
    6. 17.5 Sound Interference and Resonance: Standing Waves in Air Columns
    7. 17.6 Hearing
    8. 17.7 Ultrasound
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  19. 18 Electric Charge and Electric Field
    1. Connection for AP® Courses
    2. 18.1 Static Electricity and Charge: Conservation of Charge
    3. 18.2 Conductors and Insulators
    4. 18.3 Conductors and Electric Fields in Static Equilibrium
    5. 18.4 Coulomb’s Law
    6. 18.5 Electric Field: Concept of a Field Revisited
    7. 18.6 Electric Field Lines: Multiple Charges
    8. 18.7 Electric Forces in Biology
    9. 18.8 Applications of Electrostatics
    10. Glossary
    11. Section Summary
    12. Conceptual Questions
    13. Problems & Exercises
    14. Test Prep for AP® Courses
  20. 19 Electric Potential and Electric Field
    1. Connection for AP® Courses
    2. 19.1 Electric Potential Energy: Potential Difference
    3. 19.2 Electric Potential in a Uniform Electric Field
    4. 19.3 Electrical Potential Due to a Point Charge
    5. 19.4 Equipotential Lines
    6. 19.5 Capacitors and Dielectrics
    7. 19.6 Capacitors in Series and Parallel
    8. 19.7 Energy Stored in Capacitors
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  21. 20 Electric Current, Resistance, and Ohm's Law
    1. Connection for AP® Courses
    2. 20.1 Current
    3. 20.2 Ohm’s Law: Resistance and Simple Circuits
    4. 20.3 Resistance and Resistivity
    5. 20.4 Electric Power and Energy
    6. 20.5 Alternating Current versus Direct Current
    7. 20.6 Electric Hazards and the Human Body
    8. 20.7 Nerve Conduction–Electrocardiograms
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  22. 21 Circuits, Bioelectricity, and DC Instruments
    1. Connection for AP® Courses
    2. 21.1 Resistors in Series and Parallel
    3. 21.2 Electromotive Force: Terminal Voltage
    4. 21.3 Kirchhoff’s Rules
    5. 21.4 DC Voltmeters and Ammeters
    6. 21.5 Null Measurements
    7. 21.6 DC Circuits Containing Resistors and Capacitors
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  23. 22 Magnetism
    1. Connection for AP® Courses
    2. 22.1 Magnets
    3. 22.2 Ferromagnets and Electromagnets
    4. 22.3 Magnetic Fields and Magnetic Field Lines
    5. 22.4 Magnetic Field Strength: Force on a Moving Charge in a Magnetic Field
    6. 22.5 Force on a Moving Charge in a Magnetic Field: Examples and Applications
    7. 22.6 The Hall Effect
    8. 22.7 Magnetic Force on a Current-Carrying Conductor
    9. 22.8 Torque on a Current Loop: Motors and Meters
    10. 22.9 Magnetic Fields Produced by Currents: Ampere’s Law
    11. 22.10 Magnetic Force between Two Parallel Conductors
    12. 22.11 More Applications of Magnetism
    13. Glossary
    14. Section Summary
    15. Conceptual Questions
    16. Problems & Exercises
    17. Test Prep for AP® Courses
  24. 23 Electromagnetic Induction, AC Circuits, and Electrical Technologies
    1. Connection for AP® Courses
    2. 23.1 Induced Emf and Magnetic Flux
    3. 23.2 Faraday’s Law of Induction: Lenz’s Law
    4. 23.3 Motional Emf
    5. 23.4 Eddy Currents and Magnetic Damping
    6. 23.5 Electric Generators
    7. 23.6 Back Emf
    8. 23.7 Transformers
    9. 23.8 Electrical Safety: Systems and Devices
    10. 23.9 Inductance
    11. 23.10 RL Circuits
    12. 23.11 Reactance, Inductive and Capacitive
    13. 23.12 RLC Series AC Circuits
    14. Glossary
    15. Section Summary
    16. Conceptual Questions
    17. Problems & Exercises
    18. Test Prep for AP® Courses
  25. 24 Electromagnetic Waves
    1. Connection for AP® Courses
    2. 24.1 Maxwell’s Equations: Electromagnetic Waves Predicted and Observed
    3. 24.2 Production of Electromagnetic Waves
    4. 24.3 The Electromagnetic Spectrum
    5. 24.4 Energy in Electromagnetic Waves
    6. Glossary
    7. Section Summary
    8. Conceptual Questions
    9. Problems & Exercises
    10. Test Prep for AP® Courses
  26. 25 Geometric Optics
    1. Connection for AP® Courses
    2. 25.1 The Ray Aspect of Light
    3. 25.2 The Law of Reflection
    4. 25.3 The Law of Refraction
    5. 25.4 Total Internal Reflection
    6. 25.5 Dispersion: The Rainbow and Prisms
    7. 25.6 Image Formation by Lenses
    8. 25.7 Image Formation by Mirrors
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  27. 26 Vision and Optical Instruments
    1. Connection for AP® Courses
    2. 26.1 Physics of the Eye
    3. 26.2 Vision Correction
    4. 26.3 Color and Color Vision
    5. 26.4 Microscopes
    6. 26.5 Telescopes
    7. 26.6 Aberrations
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  28. 27 Wave Optics
    1. Connection for AP® Courses
    2. 27.1 The Wave Aspect of Light: Interference
    3. 27.2 Huygens's Principle: Diffraction
    4. 27.3 Young’s Double Slit Experiment
    5. 27.4 Multiple Slit Diffraction
    6. 27.5 Single Slit Diffraction
    7. 27.6 Limits of Resolution: The Rayleigh Criterion
    8. 27.7 Thin Film Interference
    9. 27.8 Polarization
    10. 27.9 *Extended Topic* Microscopy Enhanced by the Wave Characteristics of Light
    11. Glossary
    12. Section Summary
    13. Conceptual Questions
    14. Problems & Exercises
    15. Test Prep for AP® Courses
  29. 28 Special Relativity
    1. Connection for AP® Courses
    2. 28.1 Einstein’s Postulates
    3. 28.2 Simultaneity And Time Dilation
    4. 28.3 Length Contraction
    5. 28.4 Relativistic Addition of Velocities
    6. 28.5 Relativistic Momentum
    7. 28.6 Relativistic Energy
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  30. 29 Introduction to Quantum Physics
    1. Connection for AP® Courses
    2. 29.1 Quantization of Energy
    3. 29.2 The Photoelectric Effect
    4. 29.3 Photon Energies and the Electromagnetic Spectrum
    5. 29.4 Photon Momentum
    6. 29.5 The Particle-Wave Duality
    7. 29.6 The Wave Nature of Matter
    8. 29.7 Probability: The Heisenberg Uncertainty Principle
    9. 29.8 The Particle-Wave Duality Reviewed
    10. Glossary
    11. Section Summary
    12. Conceptual Questions
    13. Problems & Exercises
    14. Test Prep for AP® Courses
  31. 30 Atomic Physics
    1. Connection for AP® Courses
    2. 30.1 Discovery of the Atom
    3. 30.2 Discovery of the Parts of the Atom: Electrons and Nuclei
    4. 30.3 Bohr’s Theory of the Hydrogen Atom
    5. 30.4 X Rays: Atomic Origins and Applications
    6. 30.5 Applications of Atomic Excitations and De-Excitations
    7. 30.6 The Wave Nature of Matter Causes Quantization
    8. 30.7 Patterns in Spectra Reveal More Quantization
    9. 30.8 Quantum Numbers and Rules
    10. 30.9 The Pauli Exclusion Principle
    11. Glossary
    12. Section Summary
    13. Conceptual Questions
    14. Problems & Exercises
    15. Test Prep for AP® Courses
  32. 31 Radioactivity and Nuclear Physics
    1. Connection for AP® Courses
    2. 31.1 Nuclear Radioactivity
    3. 31.2 Radiation Detection and Detectors
    4. 31.3 Substructure of the Nucleus
    5. 31.4 Nuclear Decay and Conservation Laws
    6. 31.5 Half-Life and Activity
    7. 31.6 Binding Energy
    8. 31.7 Tunneling
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  33. 32 Medical Applications of Nuclear Physics
    1. Connection for AP® Courses
    2. 32.1 Medical Imaging and Diagnostics
    3. 32.2 Biological Effects of Ionizing Radiation
    4. 32.3 Therapeutic Uses of Ionizing Radiation
    5. 32.4 Food Irradiation
    6. 32.5 Fusion
    7. 32.6 Fission
    8. 32.7 Nuclear Weapons
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
    13. Test Prep for AP® Courses
  34. 33 Particle Physics
    1. Connection for AP® Courses
    2. 33.1 The Yukawa Particle and the Heisenberg Uncertainty Principle Revisited
    3. 33.2 The Four Basic Forces
    4. 33.3 Accelerators Create Matter from Energy
    5. 33.4 Particles, Patterns, and Conservation Laws
    6. 33.5 Quarks: Is That All There Is?
    7. 33.6 GUTs: The Unification of Forces
    8. Glossary
    9. Section Summary
    10. Conceptual Questions
    11. Problems & Exercises
    12. Test Prep for AP® Courses
  35. 34 Frontiers of Physics
    1. Connection for AP® Courses
    2. 34.1 Cosmology and Particle Physics
    3. 34.2 General Relativity and Quantum Gravity
    4. 34.3 Superstrings
    5. 34.4 Dark Matter and Closure
    6. 34.5 Complexity and Chaos
    7. 34.6 High-Temperature Superconductors
    8. 34.7 Some Questions We Know to Ask
    9. Glossary
    10. Section Summary
    11. Conceptual Questions
    12. Problems & Exercises
  36. A | Atomic Masses
  37. B | Selected Radioactive Isotopes
  38. C | Useful Information
  39. D | Glossary of Key Symbols and Notation
  40. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
    12. Chapter 12
    13. Chapter 13
    14. Chapter 14
    15. Chapter 15
    16. Chapter 16
    17. Chapter 17
    18. Chapter 18
    19. Chapter 19
    20. Chapter 20
    21. Chapter 21
    22. Chapter 22
    23. Chapter 23
    24. Chapter 24
    25. Chapter 25
    26. Chapter 26
    27. Chapter 27
    28. Chapter 28
    29. Chapter 29
    30. Chapter 30
    31. Chapter 31
    32. Chapter 32
    33. Chapter 33
    34. Chapter 34
  41. Index

Learning Objectives

By the end of this section, you will be able to:

  • Calculate flow rate.
  • Define units of volume.
  • Describe incompressible fluids.
  • Explain the consequences of the equation of continuity.

The information presented in this section supports the following AP® learning objectives and science practices:

  • 5.F.1.1 The student is able to make calculations of quantities related to flow of a fluid, using mass conservation principles (the continuity equation). (S.P. 6.4, 7.2)

Flow rate QQ size 12{Q} {} is defined to be the volume of fluid passing by some location through an area during a period of time, as seen in Figure 12.2. In symbols, this can be written as

Q=Vt,Q=Vt, size 12{Q= { {V} over {t} } } {}
12.1

where VV size 12{V} {} is the volume and tt size 12{t} {} is the elapsed time.

The SI unit for flow rate is m3/sm3/s size 12{m rSup { size 8{3} } "/s"} {}, but a number of other units for QQ size 12{Q} {} are in common use. For example, the heart of a resting adult pumps blood at a rate of 5.00 liters per minute (L/min). Note that a liter (L) is 1/1000 of a cubic meter or 1000 cubic centimeters (103m3103m3 size 12{"10" rSup { size 8{ - 3} } `m rSup { size 8{3} } } {} or 103cm3103cm3 size 12{"10" rSup { size 8{3} } `"cm" rSup { size 8{3} } } {}). In this text we shall use whatever metric units are most convenient for a given situation.

The figure shows a fluid flowing through a cylindrical pipe open at both ends. A portion of the cylindrical pipe with the fluid is shaded for a length d. The velocity of the fluid in the shaded region is shown by v toward the right. The cross sections of the shaded cylinder are marked as A. This cylinder of fluid flows past a point P on the cylindrical pipe. The velocity v is equal to d over t.
Figure 12.2 Flow rate is the volume of fluid per unit time flowing past a point through the area AA size 12{A} {}. Here the shaded cylinder of fluid flows past point PP size 12{P} {} in a uniform pipe in time tt size 12{t} {}. The volume of the cylinder is AdAd size 12{ ital "Ad"} {} and the average velocity is v ¯ =d/t v ¯ =d/t size 12{ {overline {v}} =d/t} {} so that the flow rate is Q=Ad/t=A v ¯ Q=Ad/t=A v ¯ size 12{Q= ital "Ad"/t=A {overline {v}} } {}.

Example 12.1 Calculating Volume from Flow Rate: The Heart Pumps a Lot of Blood in a Lifetime

How many cubic meters of blood does the heart pump in a 75-year lifetime, assuming the average flow rate is 5.00 L/min?

Strategy

Time and flow rate QQ size 12{Q} {} are given, and so the volume VV size 12{V} {} can be calculated from the definition of flow rate.

Solution

Solving Q=V/tQ=V/t size 12{Q=V/t} {} for volume gives

V=Qt.V=Qt. size 12{V= ital "Qt"} {}
12.2

Substituting known values yields

V = 5.00L1 min(75y)1m3103L5.26×105miny = 2.0×105 m3. V = 5.00L1 min(75y)1m3103L5.26×105miny = 2.0×105 m3. alignl { stack { size 12{V= left ( { {5 "." "00"" L"} over {"1 min"} } right ) \( "75"" y" \) left ( { {1" m" rSup { size 8{3} } } over {"10" rSup { size 8{3} } " L"} } right ) left (5 "." "26" times "10" rSup { size 8{5} } { {"min"} over {y} } right )} {} # " "=2 "." 0 times "10" rSup { size 8{5} } " m" rSup { size 8{3} } {} } } {}
12.3

Discussion

This amount is about 200,000 tons of blood. For comparison, this value is equivalent to about 200 times the volume of water contained in a 6-lane 50-m lap pool.

Flow rate and velocity are related, but quite different, physical quantities. To make the distinction clear, think about the flow rate of a river. The greater the velocity of the water, the greater the flow rate of the river. But flow rate also depends on the size of the river. A rapid mountain stream carries far less water than the Amazon River in Brazil, for example. The precise relationship between flow rate QQ size 12{Q} {} and velocity v ¯ v ¯ size 12{ {overline {v}} } {} is

Q = A v ¯ , Q = A v ¯ , size 12{Q=A {overline {v}} } {}
12.4

where AA size 12{A} {} is the cross-sectional area and v ¯ v ¯ size 12{ {overline {v}} } {} is the average velocity. This equation seems logical enough. The relationship tells us that flow rate is directly proportional to both the magnitude of the average velocity (hereafter referred to as the speed) and the size of a river, pipe, or other conduit. The larger the conduit, the greater its cross-sectional area. Figure 12.2 illustrates how this relationship is obtained. The shaded cylinder has a volume

V=Ad,V=Ad, size 12{V= ital "Ad"} {}
12.5

which flows past the point PP size 12{P} {} in a time tt size 12{t} {}. Dividing both sides of this relationship by tt size 12{t} {} gives

Vt=Adt.Vt=Adt. size 12{ { {V} over {t} } = { { ital "Ad"} over {t} } } {}
12.6

We note that Q=V/tQ=V/t size 12{Q=V/t} {} and the average speed is v ¯ =d/t v ¯ =d/t size 12{ {overline {v}} =d/t} {}. Thus the equation becomes Q=A v ¯ Q=A v ¯ size 12{Q=A {overline {v}} } {}.

Figure 12.3 shows an incompressible fluid flowing along a pipe of decreasing radius. Because the fluid is incompressible, the same amount of fluid must flow past any point in the tube in a given time to ensure continuity of flow. In this case, because the cross-sectional area of the pipe decreases, the velocity must necessarily increase. This logic can be extended to say that the flow rate must be the same at all points along the pipe. In particular, for points 1 and 2,

Q 1 = Q 2 A 1 v ¯ 1 = A 2 v ¯ 2 } . Q 1 = Q 2 A 1 v ¯ 1 = A 2 v ¯ 2 } . size 12{ left none matrix { Q rSub { size 8{1} } =Q rSub { size 8{2} } {} ## A rSub { size 8{1} } {overline {v rSub { size 8{1} } }} =A rSub { size 8{2} } {overline {v rSub { size 8{2} } }} } right rbrace "." } {}
12.7

This is called the equation of continuity and is valid for any incompressible fluid. The consequences of the equation of continuity can be observed when water flows from a hose into a narrow spray nozzle: it emerges with a large speed—that is the purpose of the nozzle. Conversely, when a river empties into one end of a reservoir, the water slows considerably, perhaps picking up speed again when it leaves the other end of the reservoir. In other words, speed increases when cross-sectional area decreases, and speed decreases when cross-sectional area increases.

The figure shows a cylindrical tube broad at the left and narrow at the right. The fluid is shown to flow through the cylindrical tube toward right along the axis of the tube. A shaded area is marked on the broader cylinder on the left. A cross section is marked on it as A one. A point one is marked on this cross section. The velocity of the fluid through the shaded area on narrow tube is marked by v one as an arrow toward right. Another shaded area is marked on the narrow cylindrical on the right. The shaded area on narrow tube is longer than the one on broader tube to show that when a tube narrows, the same volume occupies a greater length. A cross section is marked on the narrow cylindrical tube as A two. A point two is marked on this cross section. The velocity of fluid through the shaded area on narrow tube is marked v two toward right. The arrow depicting v two is longer than for v one showing v two to be greater in value than v one.
Figure 12.3 When a tube narrows, the same volume occupies a greater length. For the same volume to pass points 1 and 2 in a given time, the speed must be greater at point 2. The process is exactly reversible. If the fluid flows in the opposite direction, its speed will decrease when the tube widens. (Note that the relative volumes of the two cylinders and the corresponding velocity vector arrows are not drawn to scale.)

Since liquids are essentially incompressible, the equation of continuity is valid for all liquids. However, gases are compressible, and so the equation must be applied with caution to gases if they are subjected to compression or expansion.

Making Connections: Incompressible Fluid

The continuity equation tells us that the flow rate must be the same throughout an incompressible fluid. The flow rate, Q, has units of volume per unit time (m3/s). Another way to think about it would be as a conservation principle, that the volume of fluid flowing past any point in a given amount of time must be conserved throughout the fluid.

For incompressible fluids, we can also say that the mass flowing past any point in a given amount of time must also be conserved. That is because the mass of a given volume of fluid is just the density of the fluid multiplied by the volume:

m=ρVm=ρV
12.8

When we say a fluid is incompressible, we mean that the density of the fluid does not change. Every cubic meter of fluid has the same number of particles. There is no room to add more particles, nor is the fluid allowed to expand so that the particles will spread out. Since the density is constant, we can express the conservation principle as follows for any two regions of fluid flow, starting with the continuity equation:

V 1 t 1 =  V 2 t 2 V 1 t 1 =  V 2 t 2
12.9
ρ V 1 t 1 =  ρ V 2 t 2 ρ V 1 t 1 =  ρ V 2 t 2
12.10
m 1 t 1 =  m 2 t 2 m 1 t 1 =  m 2 t 2
12.11

More generally, we say that the mass flow rate (ΔmΔt)(ΔmΔt) is conserved.

Example 12.2 Calculating Fluid Speed: Speed Increases When a Tube Narrows

A nozzle with a radius of 0.250 cm is attached to a garden hose with a radius of 0.900 cm. The flow rate through hose and nozzle is 0.500 L/s. Calculate the speed of the water (a) in the hose and (b) in the nozzle.

Strategy

We can use the relationship between flow rate and speed to find both velocities. We will use the subscript 1 for the hose and 2 for the nozzle.

Solution for (a)

First, we solve Q=A v ¯ Q=A v ¯ size 12{Q=A {overline {v}} } {} for v1v1 size 12{v rSub { size 8{1} } } {} and note that the cross-sectional area is A=πr2A=πr2 size 12{A=πr rSup { size 8{2} } } {}, yielding

v ¯ 1=QA1=Q πr 1 2 . v ¯ 1=QA1=Q πr 1 2 . size 12{ {overline {v rSub { size 8{1} } }} = { {Q} over {A rSub { size 8{1} } } } = { {Q} over {πr rSub { size 8{1} rSup { size 8{2} } } } } } {}
12.12

Substituting known values and making appropriate unit conversions yields

v ¯ 1=(0.500L/s)(103m3/L)π(9.00×103m)2=1.96m/s. v ¯ 1=(0.500L/s)(103m3/L)π(9.00×103m)2=1.96m/s. size 12{ {overline {v rSub { size 8{1} } }} = { { \( 0 "." "500"" L/s" \) \( "10" rSup { size 8{ - 3} } " m" rSup { size 8{3} } /L \) } over {π \( 9 "." "00" times "10" rSup { size 8{ - 3} } " m" \) rSup { size 8{2} } } } =1 "." "96"" m/s"} {}
12.13

Solution for (b)

We could repeat this calculation to find the speed in the nozzle v ¯ 2 v ¯ 2 size 12{ {overline {v rSub { size 8{2} } }} } {}, but we will use the equation of continuity to give a somewhat different insight. Using the equation which states

A1 v ¯ 1=A2 v ¯ 2,A1 v ¯ 1=A2 v ¯ 2, size 12{A rSub { size 8{1} } {overline {v rSub { size 8{1} } }} =A rSub { size 8{2} } {overline {v rSub { size 8{2} } }} } {}
12.14

solving for v ¯ 2 v ¯ 2 size 12{ {overline {v rSub { size 8{2} } }} } {} and substituting πr2πr2 size 12{πr rSup { size 8{2} } } {} for the cross-sectional area yields

v ¯ 2=A1A2 v ¯ 1= πr 1 2 πr 2 2 v ¯ 1=r12r22 v ¯ 1. v ¯ 2=A1A2 v ¯ 1= πr 1 2 πr 2 2 v ¯ 1=r12r22 v ¯ 1. size 12{ {overline {v rSub { size 8{2} } }} = { {A rSub { size 8{1} } } over {A rSub { size 8{2} } } } {overline {v rSub { size 8{1} } }} = { {πr rSub { size 8{1} rSup { size 8{2} } } } over {πr rSub { size 8{2} rSup { size 8{2} } } } } {overline {v rSub { size 8{1} } }} = { {r rSub { size 8{1} rSup { size 8{2} } } } over {r rSub { size 8{2} rSup { size 8{2} } } } } {overline {v rSub { size 8{1} } }} } {}
12.15

Substituting known values,

v ¯ 2=(0.900cm)2(0.250cm)21.96m/s=25.5 m/s. v ¯ 2=(0.900cm)2(0.250cm)21.96m/s=25.5 m/s. size 12{ {overline {v rSub { size 8{2} } }} = { { \( 0 "." "900"" cm" \) rSup { size 8{2} } } over { \( 0 "." "250"" cm" \) rSup { size 8{2} } } } 1 "." "96"" m/s"="25" "." "5 m/s"} {}
12.16

Discussion

A speed of 1.96 m/s is about right for water emerging from a nozzleless hose. The nozzle produces a considerably faster stream merely by constricting the flow to a narrower tube.

Making Connections: Different-Sized Pipes

For incompressible fluids, the density of the fluid remains constant throughout, no matter the flow rate or the size of the opening through which the fluid flows. We say that, to ensure continuity of flow, the amount of fluid that flows past any point is constant. That amount can be measured by either volume or mass.

Flow rate has units of volume/time (m3/s or L/s). Mass flow rate ( Δm Δt ) ( Δm Δt ) has units of mass/time (kg/s) and can be calculated from the flow rate by using the density:

m= ρV m= ρV
12.17

The average mass flow rate can be found from the flow rate:

Δm Δt = m t =  ρV t =  ˙ ρQ=ρAv Δm Δt = m t =  ρV t =  ˙ ρQ=ρAv
12.18

Suppose that crude oil with a density of 880 kg/m3 is flowing through a pipe with a diameter of 55 cm and a speed of 1.8 m/s. Calculate the new speed of the crude oil when the pipe narrows to a new diameter of 31 cm, and calculate the mass flow rate in both sections of the pipe, assuming the density of the oil is constant throughout the pipe.

Solution: To calculate the new speed, we simply use the continuity equation.

Since the cross section of a pipe is a circle, the area of each cross section can be found as follows:

For the larger pipe:

A 1 = π ( d 1 2 ) 2 = π ( 0.275 ) 2 =0.238 m 2 A 1 = π ( d 1 2 ) 2 = π ( 0.275 ) 2 =0.238 m 2
12.19

For the smaller pipe:

A 2 =π (0.155) 2 = 0.0755 m 2 A 2 =π (0.155) 2 = 0.0755 m 2
12.20

So the larger part of the pipe (A1) has a cross-sectional area of 0.238 m2, and the smaller part of the pipe (A2) has a cross-sectional area of 0.0755 m2. The continuity equation tells us that the oil will flow faster through the portion of the pipe with the smaller cross-sectional area. Using the continuity equation, we get

A 1 v 1 =  A 2 v 2 A 1 v 1 =  A 2 v 2
12.21
v 2 = ( A 1 A 2 ) v 1 = ( 0.238 0.0755 )( 1.8 )= 5.7 m/s v 2 = ( A 1 A 2 ) v 1 = ( 0.238 0.0755 )( 1.8 )= 5.7 m/s
12.22

So we find that the oil is flowing at a speed of 1.8 m/s through the larger section of the pipe (A1), and it is flowing much faster (5.7 m/s) through the smaller section (A2).

The mass flow rate in both sections should be the same.

For the larger portion of the pipe:

( Δm Δt ) 1 = ρ A 1 v 1 =( 880 )( 0.238 )( 1.8 )= 380 kg/s  ( Δm Δt ) 1 = ρ A 1 v 1 =( 880 )( 0.238 )( 1.8 )= 380 kg/s 
12.23

For the smaller portion of the pipe:

( Δm Δt ) 2 = ρ A 2 v 2 =( 880 )( 0.75538 )( 5.7 )= 380 kg/s ( Δm Δt ) 2 = ρ A 2 v 2 =( 880 )( 0.75538 )( 5.7 )= 380 kg/s
12.24

And so mass is conserved throughout the pipe. Every second, 380 kg of oil flows out of the larger portion of the pipe, and 380 kg of oil flows into the smaller portion of the pipe.

The solution to the last part of the example shows that speed is inversely proportional to the square of the radius of the tube, making for large effects when radius varies. We can blow out a candle at quite a distance, for example, by pursing our lips, whereas blowing on a candle with our mouth wide open is quite ineffective.

In many situations, including in the cardiovascular system, branching of the flow occurs. The blood is pumped from the heart into arteries that subdivide into smaller arteries (arterioles) which branch into very fine vessels called capillaries. In this situation, continuity of flow is maintained but it is the sum of the flow rates in each of the branches in any portion along the tube that is maintained. The equation of continuity in a more general form becomes

n1A1 v ¯ 1=n2A2 v ¯ 2,n1A1 v ¯ 1=n2A2 v ¯ 2, size 12{n rSub { size 8{1} } A rSub { size 8{1} } {overline {v rSub { size 8{1} } }} =n rSub { size 8{2} } A rSub { size 8{2} } {overline {v rSub { size 8{2} } }} } {}
12.25

where n1n1 size 12{n rSub { size 8{1} } } {} and n2n2 size 12{n rSub { size 8{2} } } {} are the number of branches in each of the sections along the tube.

Example 12.3 Calculating Flow Speed and Vessel Diameter: Branching in the Cardiovascular System

The aorta is the principal blood vessel through which blood leaves the heart in order to circulate around the body. (a) Calculate the average speed of the blood in the aorta if the flow rate is 5.0 L/min. The aorta has a radius of 10 mm. (b) Blood also flows through smaller blood vessels known as capillaries. When the rate of blood flow in the aorta is 5.0 L/min, the speed of blood in the capillaries is about 0.33 mm/s. Given that the average diameter of a capillary is 8.0μm8.0μm, calculate the number of capillaries in the blood circulatory system.

Strategy

We can use Q=A v ¯ Q=A v ¯ size 12{Q=A {overline {v}} } {} to calculate the speed of flow in the aorta and then use the general form of the equation of continuity to calculate the number of capillaries as all of the other variables are known.

Solution for (a)

The flow rate is given by Q=A v ¯ Q=A v ¯ size 12{Q=A {overline {v}} } {} or v ¯ =Qπr2 v ¯ =Qπr2 size 12{ {overline {v}} = { {Q} over {πr rSup { size 8{2} } } } } {} for a cylindrical vessel.

Substituting the known values (converted to units of meters and seconds) gives

v ¯ = 5.0 L/min 10 3 m 3 /L 1 min/ 60 s π 0 . 010 m 2 = 0 . 27 m/s . v ¯ = 5.0 L/min 10 3 m 3 /L 1 min/ 60 s π 0 . 010 m 2 = 0 . 27 m/s . size 12{ { bar {v}}= { { left (5 "." 0`"L/min" right ) left ("10" rSup { size 8{ - 3} } `m rSup { size 8{3} } "/L" right ) left (1`"min/""60"`s right )} over {π left (0 "." "010 m" right ) rSup { size 8{2} } } } =0 "." "27"`"m/s"} {}
12.26

Solution for (b)

Using n1A1 v ¯ 1=n2A2 v ¯ 1n1A1 v ¯ 1=n2A2 v ¯ 1 size 12{n rSub { size 8{1} } A rSub { size 8{1} } {overline {v rSub { size 8{1} } }} =n rSub { size 8{2} } A rSub { size 8{2} } {overline {v rSub { size 8{2} } }} } {}, assigning the subscript 1 to the aorta and 2 to the capillaries, and solving for n2n2 size 12{n rSub { size 8{2} } } {} (the number of capillaries) gives n2= n 1 A 1 v ¯ 1 A 2 v ¯ 2 n2= n 1 A 1 v ¯ 1 A 2 v ¯ 2 . Converting all quantities to units of meters and seconds and substituting into the equation above gives

n 2 = 1 π 10 × 10 3 m 2 0.27 m/s π 4.0 × 10 6 m 2 0.33 × 10 3 m/s = 5.0 × 10 9 capillaries . n 2 = 1 π 10 × 10 3 m 2 0.27 m/s π 4.0 × 10 6 m 2 0.33 × 10 3 m/s = 5.0 × 10 9 capillaries . size 12{n rSub { size 8{2} } = { { left (1 right ) left (π right ) left ("10" times "10" rSup { size 8{ - 3} } " m" right ) rSup { size 8{2} } left (0 "." "27"" m/s" right )} over { left (π right ) left (4 "." 0 times "10" rSup { size 8{ - 6} } " m" right ) rSup { size 8{2} } left (0 "." "33" times "10" rSup { size 8{ - 3} } " m/s" right )} } =5 "." 0 times "10" rSup { size 8{9} } " capillaries"} {}
12.27

Discussion

Note that the speed of flow in the capillaries is considerably reduced relative to the speed in the aorta due to the significant increase in the total cross-sectional area at the capillaries. This low speed is to allow sufficient time for effective exchange to occur although it is equally important for the flow not to become stationary in order to avoid the possibility of clotting. Does this large number of capillaries in the body seem reasonable? In active muscle, one finds about 200 capillaries per mm3mm3 size 12{"mm" rSup { size 8{3} } } {}, or about 200×106200×106 size 12{"200" times "10" rSup { size 8{6} } } {} per 1 kg of muscle. For 20 kg of muscle, this amounts to about 4×1094×109 size 12{4 times "10" rSup { size 8{9} } } {} capillaries.

Making Connections: Syringes

A horizontally oriented hypodermic syringe has a barrel diameter of 1.2 cm and a needle diameter of 2.4 mm. A plunger pushes liquid in the barrel at a rate of 4.0 mm/s. Calculate the flow rate of liquid in both parts of the syringe (in mL/s) and the velocity of the liquid emerging from the needle.

Solution:

First, calculate the area of both parts of the syringe:

A 1 = π ( d 1 2 ) 2 = π (0.006) 2 = 1.13 ×  10 4 m 2 A 1 = π ( d 1 2 ) 2 = π (0.006) 2 = 1.13 ×  10 4 m 2
12.28
A 2 = π ( d 2 2 ) 2 = π (0.0012) 2 = 4.52 ×  10 6 m 2 A 2 = π ( d 2 2 ) 2 = π (0.0012) 2 = 4.52 ×  10 6 m 2
12.29

Next, we can use the continuity equation to find the velocity of the liquid in the smaller part of the barrel (v2):

A 1 v 1 =  A 2 v 2 A 1 v 1 =  A 2 v 2
12.30
v 2 = ( A 1 A 2 ) v 1 v 2 = ( A 1 A 2 ) v 1
12.31
v 2 = ( 1.13× 10 4 4.52× 10 6 )( 0.004 )= 0.10 m/s v 2 = ( 1.13× 10 4 4.52× 10 6 )( 0.004 )= 0.10 m/s
12.32

Double-check the numbers to be sure that the flow rate in both parts of the syringe is the same:

Q 1 =  A 1 v 1 =( 1.13× 10 4 )( 0.004 )= 4.52× 10 7   m 3 /s Q 1 =  A 1 v 1 =( 1.13× 10 4 )( 0.004 )= 4.52× 10 7   m 3 /s
12.33
Q 2 =  A 2 v 2 =( 4.52× 10 6 )( 0.10 )= 4.52× 10 7   m 3 /s Q 2 =  A 2 v 2 =( 4.52× 10 6 )( 0.10 )= 4.52× 10 7   m 3 /s
12.34

Finally, by converting to mL/s:

( 4.52× 10 7   m 3 1 s )( 10 6  mL 1  m 3 )=0.452 mL/s ( 4.52× 10 7   m 3 1 s )( 10 6  mL 1  m 3 )=0.452 mL/s
12.35
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