Transcript Physics & Monitoring

Physics & Monitoring
Dr Rishi Mehra
28.7.03
Question
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Describe the laws governing the
behaviour of fluids (gases and liquids)
Daltons Law
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Daltons Law of partial pressures
John Dalton : 1766-1844
Pressure = Force per area = N / m
In a mixture of gases (e.g. air), each gas
exerts a pressure that it would if it occupied
the volume alone
This pressure = Partial pressure
Sum of partial pressures = total pressure
Daltons Law
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For example
- Total atmospheric pressure =
760mmHg
- Partial pressures
- pO2 = 760 x 21% = 159 mmHg
- pCO2 = 760 x 0.04% = 0.3 mmHg
- pN2 = 760 x 78% = 592 mmHg
- pArgon = 760 x 0.94% = 7 mmHg
Gas molecular theory
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Gas molecular theory:
- If sufficient heat applied to a solid,
increased kinetic energy of each
molecule
- Increased energy results in
breakdown of molecular lattice
- Liquid is formed
Gas molecular theory
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If energy added to a liquid, each
molecule gains more kinetic energy
Some molecules are able to overcome
Van der Waals forces and become
gaseous
At boiling point, all molecules begin to
transfer to the gaseous phase
Gas molecular theory
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At this time, they collide with each
other and walls of the container
Hence exert a force over an area
‘ ie pressure ’ – units = N/M
More commonly pressure referenced
to atmospheres or mmHg
Hence if temperature increased, more
kinetic energy and higher pressure
Gas molecular theory
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Likewise, decreased temperature and
lower kinetic energy
Hence drop in partial pressure
Henry’s Law
Applies to gaseous phase in contact
with a liquid in a closed environment:
Amount of gas
dissolved in liquid is
proportional to partial
pressure of gas
above liquid
Henry’s Law
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Changing pressure:
Doubling pressure
doubles
concentration
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Concentration
proportional to
partial pressure
Henry’s Law
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Constant = solubility coefficient
- Varies with individual gas
- Varies with temperature
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Solubility coefficient increases with
drop in temperature
Gas Law No 1: Boyle’s Law
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Robert Boyle: 1627 – 1691
Pressure x Volume = Constant
Hence inverse relationship between
pressure and volume of a perfect gas if
temperature kept constant
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E.g. halving volume causes doubling of
pressure
Gas Law No 2: Charles’ Law
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Jaques Charles 1746-1823
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Volume / Temperature = Constant
Describes relationship between volume and
temperature of a perfect gas at constant
pressure
Hence doubling temperature causes
doubling of volume (Pressure kept constant)
Gas Law No 3:
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Pressure / Temperature = Constant
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For a container maintained at constant
volume, absolute pressure of a gas varies
directly with absolute temperature
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Hence increase in temperature causes
increase in pressure
Avagadro’s hypothesis
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Count Amadeo Avagadro : 1776-1856
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Equal volumes of gases and the same
temperature and pressure contain
equal amounts of molecules
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One mole of any gas as STP occupies
22.4 litres
Laminar Flow
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Higher velocity of flow of a gas or
liquid in the centre of a cylinder
Laminar Flow
Hagen-Poiseuille Law
Q = flow
Delta P = pressure gradient
r = radius
n = viscosity
l = length
 .P.r
Q
8l
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Hagen-Poiseuille Law
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Valid for laminar flow
only
Predictor of whether
flow will be laminar =
Reynold’s Number
R=radius
V=mean velocity
ρ = density
η = viscosity
Re > 1500 = Nonlaminar flow
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Re 
2 r. v . 
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Non-laminar flow:
Q  P
Qr
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