Ch 4 - Fluids

Question 1

What are fluids?

Answer 1

substances that have the ability to flow and conform to the shape of their containers

Question 2

What kind of forces can fluids exert?

Answer 2

perpendicular forces, cannot exert shear forces

Question 3

What phases of matter are fluids?

Answer 3

liquids and gases

Question 4

What characteristics of solids?

Answer 4

do not flow and they retain their shape regardless of their container

Question 5

What is density?

Answer 5

the mass per unit volume of a substance (fluid or solid)
- density = m/V
water = 1 g/cm^3 = 1000 kg/m^3

Question 6

What is pressure?

Answer 6

a measure of force per unit area; exerted by a fluid on the walls of its container and on objects placed on fluid
- P = F/A (Pa)
1 atm = 760 torr = 760 mmHg = 1.013 x 10^5 Pa

Question 7

What type of quantity of pressure?

Answer 7

scalar; value has magnitude only, no direction

Question 8

How does pressure relate to the walls of its container?

Answer 8

pressure exerted by a gas against the walls of its container will always be perpendicular (normal) to the container walls

Question 9

What is absolute pressure?

Answer 9

(or hydrostatic) the sum of all pressures at a certain point within a fluid
- equal to the pressure at the surface of the fluid (usually atmospheric pressure) plus the pressure due to the fluid itself
P = Po + pgz

Question 10

What is gauge pressure?

Answer 10

• name for the difference between absolute and atmospheric pressure
• in liquids, is caused by the weight of the liquid above the point of measurement
  Pgauge = P - Patm = (Po + pgz) - Patm

Question 11

What is Pascal’s principle?

Answer 11

a pressure applied to an incompressible fluid will be distributed undiminished throughout the entire volume of the fluid
P = F1/A1 = F2/A2
- the larger the area, the larger the force although exerted through smaller distance

Question 12

How do hydraulic machines operate?

Answer 12

based on the application of Pascal’s principle to generate mechanical advantage

Question 13

What is Archimedes’ principle?

Answer 13

• governs buoyant force
• when an object is placed in a fluid, the fluid generates a buoyant force against the object that is equal to the weight of the fluid displaced by the object
  Fbuoy = pfluid x Vfluiddisplaced x g = pfluid x Vsubmerged x g

Question 14

What direction is the buoyant force?

Answer 14

always opposite to the direction of gravity

Question 15

What happens if the maximum buoyant force is larger than the force of gravity?

Answer 15

the object will float, true if the object is less dense than the fluid it is in

Question 16

What happens if the maximum buoyant force is smaller than the force of gravity?

Answer 16

the object will sink, true if the object is more dense then the fluid it is in

Question 17

What is the difference between cohesive and adhesive forces?

Answer 17

• cohesive: attractive forces experienced by molecules of a fluid for one another (give rise to surface tension)
• adhesive: attractive forces experienced by molecules of a fluid for a different material (usually solid)
• if cohesive and adhesive equal the same, no meniscus would form.

Question 18

What are fluid dynamics?

Answer 18

set of principles regarding actively flowing fluids

Question 19

What is viscosity?

Answer 19

measurement of a fluid’s internal friction

- low viscosity fluids have low internal resistance to flow and behave like ideal fluids

Question 20

What is viscous drag?

Answer 20

a nonconservative force generated by viscosity

Question 21

What type of flow move fluids?

Answer 21

• laminar (smooth and orderly) or turbulent (rough and disorderly)
• rate of laminar flow determined by relationships in Poiseuille’s law
• incomprehensible fluids are assumed to have laminar flow and very low viscosity while flowing, allowing us to assume conservation of energy

Question 22

What is the continuity equation?

Answer 22

statement of conservation of mass as applied to fluid dynamics
Q = v1A1 = v2A2

Question 23

What is Bernoulli’s equation?

Answer 23

• an expression of conservation of energy for a flowing fluid
• states that the sum of the static pressure and the dynamic pressure will be constant between any 2 points in a closed system
  P1 + 1/2pv1^2 + pgh1 = 1/2pv2^2 + pgh2
• at equal heights, speed and pressure of a fluid are inversely related (venturi effect)

Question 24

What is Venturi effect?

Answer 24

• for a horizontal flow, there is an inverse relationship between pressure and speed, and in a closed system there is a direct relationship between cross sectional area and pressure exerted on the walls of the tube

Question 25

How does gauge pressure relate to the pressure exerted by a column of fluid?

Answer 25

• gauge pressure is equal to pressure exerted by a column of fluid plus ambient pressure above the fluid minus atmospheric pressure
• when atmospheric pressure is the only pressure above fluid column, gauge pressure is equal to fluid pressure

Question 26

What is the relationship between weight and density?

Answer 26

weight is equal to density times volume and acceleration due to gravity

Question 27

What is critical speed?

Answer 27

vc = (Nr x viscosity)/(density x diameter)

when turbulence begins

Question 28

What is the difference between dynamic and static pressure?

Answer 28

• dynamic: associated with flow = 1/2pv^2

- static: associated with position (sacrificed for dynamic during flow)

Question 29

What pitot tubes?

Answer 29

measure static pressure during flow to calculate speed

Question 30

How do Venturi effect, Bernoulli’s equation, and continuity equation relate to each other?

Answer 30

• continuity equation describes the relationship of flow and cross sectional area in a tube, while Bernoulli’s describes the relationship between height, pressure, and flow
• Venturi is the direct relationship between cross sectional area and pressure, and results from the combined relationships of the Bernoulli and continuity equations

Question 31

What effect would increasing each of the following have on flow rate: radius of tube, pressure gradient, viscosity, length of tube

Answer 31

Q = (pi x r^4 x deltaP)/(8 x viscosity x L)

• flow rate would increase when increasing either radius or pressure gradient
• flow rate would decrease with increasing viscosity or length of the tube

Question 32

Under what conditions could the continuity equation be applied to human circulation?

Answer 32

• the continuity equation cannot be applied to human circulation
• the presence of pulses, the elasticity of the vessels, and the nature of the pressure gradient precludes this type of analysis
• Poiseuille’s law should instead be used for isolated segments

Question 33

During exhalation, how does the total resistance of the encountered airways change as air leaves the alveoli to escape the nose and mouth?

Answer 33

• total resistance increases as the air exits the body despite the increase in the diameter of the airways
• because there are fewer airways in parallel with each other

Question 34

How does flow in the vena cava relate to flow in the main pulmonary artery?

Answer 34

• in theory, there should be equal flow in the vena cava and the main pulmonary trunk
• in reality, the flow in the vena cava is actually slightly less than in the pulmonary trunk because some of the blood entering the right side of the heart is actually from cardiac (coronary) circulation, not systemic circulation