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Physics Flashcards

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1
Q

What is the definition of pressure?

A

Force applied per unit area

N/m2, Pa, Bar, Atm, Psi, Torr, mmHg, cmH2O

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2
Q

What is the definition of a fluid?

A

A substance that continually deforms (or flows) under and applied shear stress)

(Can be used to describe different phases of matter: Gases, liquids, plasma, plastic solids)

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3
Q

What is fluid pressure?
Explain static fluid pressure vs dynamic fluid pressure?

A

The pressure at some point in a fluid

Static fluid pressure - fluid is not flowing
Dynamic fluid pressure - fluid is flowing

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4
Q

What is flow?
What is the equation?

A

Flow is the quantity of fluid (gas, vapour or liquid) passing a point per unit time

Flow = Quantity / Time (L/min)

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5
Q

What are the 2 types of flow?

A

Laminar
Turbulent

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6
Q

Describe laminar flow?

A

Molecules of a fluid pass smoothly and steadily, parallel to the walls of the vessel

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7
Q

What is turbulent flow?

A

Molecules of a fluid swirl into eddies with chaotic and unpredictable fluctuations

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8
Q

What is the definition of viscosity?

units?
greek letter?

A

The property of a fluid that causes it to resist flow

Pascal seconds (Pa.s)
greek letter ‘n’

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9
Q

Compare newtonian and non-newtonian fluids?

A

Newtonian - viscosity is constant (e.g water, air)
Non-newtonian - viscosity is not constant (e/g blood)

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10
Q

What is the definition of density?

Units?
Greek letter?

A

Density is the mass of a substance per unit volume

kg/m3
greek letter ‘p’

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11
Q

What is the definition of a pressure gradient?

A

The fall in pressure across the tube length or orfice

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11
Q

Compare a tube to an orfice?

A

Tube: Length > annular diameter

Orfice: Annular diameter > length

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12
Q

What equation can be used to describe laminar flow?

A

Hagen - Poiseuille Equation

Q = [pie x P x r(^4)] / 8 x n x l

Q = flow
P = Pressure
r = tube radius
n = fluid viscosity
l = tube length

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13
Q

What is the most important fluid factor for laminar flow?

A

radius

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14
Q

Explain why the tube radius is the most important factor with regards to flow?

A

Hagen - Poiseuille equation
Flow is proportional to radius to the power of 4

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15
Q

What is the most important fluid factor for turbulent flow?

A

Density

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16
Q

Which type of flow is always present through orfices?

A

Turbulent flow

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17
Q

Describe the relationship between flow and density?

A

Flow is inversely proportional to the square root of density

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18
Q

Why is heliox useful in upper airway obstruction but not useful in lower airway obstruction/bronchospasm?

A

Turbulent flow occurs in the upper airways

Heliox (79% helium, 21% oxygen)
Lower density than room air and so improves gas flow in turbulent conditions

Flow is largely laminar in the lower airways
Viscosity of heliox and air is similar

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19
Q

What is the purpose of Reynolds number? (Re)

A

Helps predict whether fluid flow is likely to be laminar or turbulent

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20
Q

What reynolds number (Re) suggests fluid flow is likely to be laminar vs turbulent?

A

Re <2000 = likely laminar
Re >2000 = likely turbulent

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21
Q

What is the equation for reynolds number?

Units?

Study These Flashcards
A

Re = [p.v.d]/n

p = fluid density
v = fluid velocity
d = tube diameter
n = fluid viscosity

dimensionless (no units)

22
Q

What is the formula for resistance during laminar flow?

Study These Flashcards
A

R = Delta P /Delta Q

R = [8nl]/pie x r(^4)

R = resistance
P = pressure
Q = Flow
n = viscositu
l = tube length
r,= radius

23
Q

What type of relationship is resistance during laminar flow?
Graph: (Resistance = Change in Pressure vs Change in Flow)

Study These Flashcards
A

Linear relationship

24
What is the formula for resistance during turbulent flow?
R = P/Q^2
25
What type of relationship is resistance during turbulent flow? Graph: (Resistance = Change in Pressure vs Change in Flow)
Exponential curve --> greater change in pressure is required to produce same change in flow
26
What is the bernoulli principle?
An increase of velocity in a fluid undergoing laminar flow (ie by introducing a constriction in the path) leads to a simultaneous reduction in pressure
27
What is the coanda effect?
The tendency of a stream of fluid which is flowing close to a convex surface, to follow the line of the surface rather than it original course
28
What is the venturi effect?
When a fluid flows through a constriction within a tube, the velocity increases and the pressure reduces as per the bernoulli principle. The reduction in pressure can be used to entrain a second fluid into the main flow.
29
How do we categorise flow meters?
Gas flow meters Liquid flow meters
30
What is a vitalograph?
A way of measuring gas flow Single expired breath act on bellows attached to a scribe which marks the volume on paper that moves at a constant speed
31
What is a bell spirometer?
A way of measuring gas flow Gas expired into an inverted bell that is immersed in water, which rises and falls on a pulley according to gas volume
32
What is a rotameter? (pressure vs orfice)
A way of measuring gas flow Dial controls a needle valve, precisely adjusts how much gas is allowed into the bottom of the tube. Gas pushes upwards against a bobbin, lifting it up the tube, Variable orifice constant pressure
33
What percentage accuracy do rotameters have?
accurate to within 2% of the flow rate
34
What is a pneumotachograph? (pressure vs orifice)
A way of measuring gas flow Creates a small resistance to gas flow, and the pressure drop across this resistance is directly proportional to flow rate (Delta Pressure = flow x resistance) Pressure difference measured by a differential pressure transducer Variable pressure Constant orifice
35
In a rotameter, what type of flow is seen at low vs high flows? Why?
Low flow - laminar flow High flows - turbulent flow Low flows - the bobbin is not very high up the tube, the gap between the bobbin and the wall of the tube is small Gap is small compared to the length of the bobbin --> acts a a tube, therefore laminar flow Higher flows - the bobbin is higher up and the gap between the bobbin and the wall is bigger Gap is bigger and so acts like an orifice allows the gas molecules to bounce around displaying turbulent flow
36
Which type of flow is more affected by viscosity of fluid? Which type of flow is more affected by density of fluid?
Laminar flow - more affected by viscosity Turbulent flow - more affected by density
37
Which gas should be added last via rotameters in the fresh gas flow for a anaesthetic machine? Why?
Oxygen last after vaporisers/nitrous In case of a leak, to avoid hypoxic mixture
38
Why is auxillary oxygen supplied via rotameters on an anaesthetic machine even in newer models?
Don't require electrical supply so still functional during an electricity blackout
39
Why are rotameters calibrated for specific gases?
Gases have different densities and viscosities, which affects flow
40
What are the 2 types of rotameter bobbin shapes? Where do you read them? What prevents sticking?
Conical - read from flat top Flutes along edge causes rotation around long axis to prevent sticking Spheres - read from centre Minimal surface area which comes into contact with wall of flowmeter
41
What is a vane meter (e.g wright spirometer?)
A way of measuring gas flow Gas flow moves a turbine linked to a dial Amount of spinning is proportional to flow rate
42
Compare pneumotachographs and vane meters in terms of direction of flow?
Pneumotachographs are bidirectional Vane meters are unidirectional
43
What are the pros (1) and cons (2) of vane meters for measuring gas flow
Pro: Require no power Cons: - Unidirectional can only measure exhaled fases - Inertia: continued spinning after flow has stopped due to such low resistance --> can overestimate
44
What is a hot-wire anemometer?
A way to measure gas flow Gas flows over heated wires and the degree of cooling is related to gas flow
45
What are ultrasonic flow meters?
A way to measure gas flow When gas flows in the same direction as an ultrasound signal, an increase in signal velocity occurs
46
What are the 8 ways tor measure gas flow/
1) Rotameters/flowmeters 2) Pneumotachographs 3) Vitalographs 4) Bell spirometer 5) Vane meters 6) Mechanical flow transducers 7) Hot wire anemometets 8) Ultrasonic flow meters
47
What are 2 commonly used methods to measure the flow of liquids?
Infusion pumps IV drip counters
48
Describe the tube of a rotameter? Why is this important?
Inverted conical tube with calibrated markings As the flow increases, the bobbin rises and allows more gas to pass
49
When adjusting the flow in a rotameter, what determines the level that the bobbin rises to?
Where the fore of the gas pushing it upwards is matched by the gravitational force pulling it down
50
What 2 factors can cause over-reading and therefore inaccuracy in the rotameter? Is this to do with different density or visocity?
1) Warmer gas lower density and lower viscosity may cause over-reading 2) Reduced atmospheric pressure lower density which may cause over-reading viscosity is unaffected by pressure.
51
What happens to the flow when the density of a fluid is lower? Which condition is this true for?
Lower density of fluid, greater flow under turbulent conditions.
52
Compare the relationship between flow and pressure vs flow and density in turbulent flow?
Flow is proportional to square root of pressure Flow is inversely proportional to the square root of density
53

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