PressurePressureWeek 3

Definition of PressureDefinition of PressureDefined as force (F) acting

perpendicularly to a surface area (A)P = F/AIn the atmosphere the gas we

breathe (Nitrogen, Oxygen, Co2 and trace gases) are in constant motion, creating kinetic energy and FORCE applied against the surface of the earth. This is the atmospheric pressure

Atmospheric pressureAtmospheric pressureAtmospheric pressure is usually

measured by the height of a liquid in a closed, evacuated tube as shown on pages 178-179 in the text book.

You have already learned the normal values at sea level for one atmosphere in several different pressure scales.

At altitudes above sea level the atmospheric pressure is lower because there is less air pushing down on the surface.

At sea level, 1 ATM = 760 torr, In Denver, 1 mile above sea level, 1 ATM = 680 tor.

Atmospheric pressure Atmospheric pressure unitsunitsAt sea level:

◦760 mmHg◦760 torr◦1034 cmH2O◦33 ft H2O◦14.7 PSI◦29.9 inHg◦101.33 kPa

◦*in respiratory we will use mmhg/torr

Positive pressurePositive pressurePressure applied that is above

the current atmospheric pressureIn respiratory we do this all the

time with devices such as mechanical ventilators

We do this to overcomes a resistance or disruption in normal pressure gradients between the atmosphere and the patients lung

Negative pressureNegative pressure"Negative" pressure is a gauge pressure

that is below atmospheric pressure. Since atmospheric pressure measured in gauge pressure is 0, any gauge pressure that is below atmospheric pressure will be negative. It is very important to remember that if you are measuring pressure on an absolute scale, you can not have a pressure that is less than 0 or a negative number.

 Negative is also called vacuum pressureWe use Negative pressure when

suctioning patients. For example: a suction pressure of 60

mmHg is a vacuum, or 60 less that the current atmospheric pressure

The air is made up of molecules.

Gravity pulls the air molecules Gravity pulls the air molecules toward the earth, giving them toward the earth, giving them weight. The weight of the air weight. The weight of the air molecules all around us is molecules all around us is called the air pressure.called the air pressure.

Your weight is the result of gravity pulling your mass down on the bathroom scales. Note that weight has units of a force, such as pounds.

High altitudes = lower pressureHigh altitudes = lower pressure

Low altitudes = higher pressureLow altitudes = higher pressure

Atmospheric pressureAtmospheric pressureAir pressure can be thought of as

the column of air rising above us. As we go up in altitude, we get closer to the top of the column. Thus there are fewer molecules of air above us to be pulled down by gravity, so the air “weighs” less. Therefore, pressure always decreases as one goes up.

Atmospheric PressureAtmospheric Pressure

Gas pressure Gas pressure depends on both depends on both density and density and temperature.temperature.

Adding air Adding air molecules molecules increases the increases the pressure in a pressure in a balloon.balloon.

Heating the air Heating the air also increases also increases the pressure.the pressure.

Air pressure is Air pressure is equal in all equal in all directions.directions.

Pressure = force per unit areaPressure = force per unit area

Because air is a fluid, force applied in one direction is distributed equally in all directions. Thus the downward pull of gravity on air molecules produces air pressure in all directions.

As As elevation elevation goes upgoes up

Barometric Barometric pressure pressure

goes goes down.down.

This is an inverse relationship.This is an inverse relationship.

to to measure measure air air pressure.pressure.

A BarometerA Barometer

is is usedused

Torricelli’s barometer Torricelli’s barometer used a glass column used a glass column suspended in a bowl of suspended in a bowl of mercury. The pressure mercury. The pressure of the air molecules of the air molecules pushed the mercury up pushed the mercury up into the glass tube.into the glass tube.

The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.

As As atmospheric atmospheric pressure pressure increases…increases…

The mercury in The mercury in the tube rises.the tube rises.

The Mercury BarometerThe Mercury Barometer

Good:Good: Bad:Bad:

•Simple to constructSimple to construct

•Highly accurateHighly accurate

•Glass tube is fragileGlass tube is fragile

•Mercury is Mercury is very toxic!very toxic!

Although mercury has been used for hundreds of years, its toxic effects have only been fully realized in the last few decades. Students should NEVER handle mercury or broken mercury thermometers or barometers. Mercury should also never be thrown in the trash or washed down the drain, since it moves easily up the food chain from fish to humans.

The The AneroidAneroid BarometerBarometer

•No fragile tubes!No fragile tubes!

•No toxic chemicals!No toxic chemicals!

•No batteries!No batteries!

•Never needs winding!Never needs winding!

An aneroid barometer An aneroid barometer uses a cell which has uses a cell which has had most of the air had most of the air removed. removed.

As the air pressure As the air pressure around the cell around the cell increases, it presses increases, it presses on the cell, which on the cell, which causes the needle to causes the needle to move.move.

Television weather forecasters usually give barometric Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists pressure in inches of mercury. However, meteorologists measure atmospheric pressure in measure atmospheric pressure in millibars.millibars.

MILLIBARS

Most aneroid Most aneroid barometers have a barometers have a needle which can be needle which can be set to remember the set to remember the previous reading.previous reading.

Atmospheric PressureAtmospheric PressureAlthough pressure varies with altitude,

the concentration of gases do not change.

Meaning there is 21% O2 and 78% Nitrogen everywhere in the atmosphere.

The difference is the pressure. Less pressure= less pressure change from in the lung to the outside.

High altitudes = increased work of breathing

Changing PressureChanging Pressure

A A rising barometerrising barometer = increasing air pressure. = increasing air pressure.

This usually means:This usually means:

Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Changing PressureChanging Pressure

A A falling barometerfalling barometer = decreasing air pressure. = decreasing air pressure.

This usually means:This usually means:

Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

Air Movement and FlowAir Movement and Flow

Fluids (air and water) flow from areas of high pressure to areas of low pressure.

Change in pressure across a horizontal distance is a pressure gradient. Greater the difference in pressure and the shorter the distance between them, the steeper the pressure gradient and the high the velocity of flow

In the lung, flow travels to the alveoli based on the diameter of the airway and also the pressure gradient. We will learn about this in another lecture

6-1

Pressure gradient in lungPressure gradient in lung

speed, velocity, and flow. 

Speed is a measurement of an object’s movement in units of distance or length /time, (ie cm/sec, miles per hour).

Velocity is the speed of an object with its direction at a given instant, (ie 50 miles/hour South).

Fluid flow is a special kind of velocity expressed in units of volume/time, ie ml/sec, l/min. Remember that a fluid is a substance capable of flowing ---a gas or liquid.

Kinetic Theory of MatterThe Kinetic Theory of Matter is

the theory that all molecules are in constant motion resulting in kinetic energy. This theory applies to all three states of matter -- solids, liquids and gases.

Solids – have a high degree of internal order; their atoms have a strong mutual attractive force

Liquids – atoms exhibit less degree of mutual attraction compared with solids, they take the shape of their container, are difficult to compress, exhibit the phenomenon of flow

Gases – weak molecular attractive forces; gas molecules exhibit rapid, random motion with frequent collisions, gases are easily compressible, expand to fill their container, exhibit the phenomenon of flow

States of MatterStates of Matter

States of MatterStates of Matter

Internal energy of matter

•Potential energy (Position) The strong attractive forces between molecules that cause rigidity in solids

•Kinetic energy (Motion) Gases have weak attractive forces that allow the molecules to move about more freely, interacting with other objects that they come in contact with

Internal energy and temperature

•The two are closely related: internal energy can be increased by heating or by performing work on it.

•Absolute zero = no kinetic energy

All matter possesses energy. There are 2 types of internal energy:The energy of position, and the energy of motion.

Physical properties ofPhysical properties ofa solid: a solid:

Possess the least amount of KE

Mostly Potential Energy in intermolecular forces holding particles together

Can maintain their volume & shape

Physical properties ofPhysical properties ofa liquid: a liquid:

Intermolecular, cohesive forces are not as strong

They exhibit fluidity (particles sliding

They exhibit a buoyant force

Essentially incompressible

Assume the shape of their container

Physical properties ofPhysical properties ofa gas: a gas:

Extremely weak – if any – cohesive forces

Possess the greatest amount of KE & the least amount of Potential Energy

Motion of atoms & molecules is random Do not maintain their shapes & volumes

but expand to fill the available space Exhibit the phenomenon of flow Exhibits the least thermal conductivity Uses: Gas therapy (Oxygen, Heliox,

Nitrous oxide…HHN/SVN…)

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Change of StateChange of StateLiquid-solid phase changes (melting and freezing)

Melting = changeover from the solid to the liquid state

Melting point = the temperature at which melting occur.

Freezing = the opposite of melting

Freezing point = the temperature at which the substance freezes; same as its melting point

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Change of State (cont.)Change of State (cont.) Properties of liquids

◦ Pressure – depends on the height and weight density.

◦ Buoyancy – occurs because the pressure below a submerged object always exceeds the pressure above it

◦ Viscosity – the force opposing a fluid’s flow. The greater the viscosity of a fluid, the greater the resistance to flow. Blood has a viscosity five times greater than that of

water

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Change of State (cont.)Change of State (cont.) Heat transfer

◦ Conduction – transfers heat in solids

◦ Convection – transfers heat in liquids and gases

(Example: heating homes or infant incubators)

◦ Radiation – occurs without direct contact between two substances - example: microwave oven

◦ Evaporation/Condensation: requires heat energy to occur

◦ Sublimation - change from a solid to a gas without an intermediate change to a liquid - example dry ice turning into CO2 http://www.youtube.com/watch?v=lxPDAh4je54

Heat TransferHeat TransferConduction:

http://www.youtube.com/watch?v=9UxU0ELgYOA

Convectionhttp://www.youtube.com/watch?v=oFR

MvVNia00&feature=relmfuRadiationhttp://www.youtube.com/watch?v=RmFJ

OVOia_4&feature=relmfuCondensation/evaporationhttp://www.youtube.com/watch?v=QjSfI

DARTik&feature=related

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Change of State (cont.)Change of State (cont.)Pascal’s Principle. Liquid pressure depends only on the height and weight density of the liquid and not the shape of the vessel or total volume of a liquid.

Pascal's lawPascal's law Pascal's law states that pressure exerted

anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same.

Pascal’s Law states that when you apply pressure to confined fluids (contained in a flexible yet leak-proof enclosure so that it can’t flow out), the fluids will then transmit that same pressure in all directions within the container, at the same rate.

The simplest instance of this is stepping on a balloon; the balloon bulges out on all sides under the foot and not just on one side. This is precisely what Pascal’s Law is all about – the air which is the fluid in this case, was confined by the balloon, and you applied pressure with your foot causing it to get displaced uniformly.

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Change of State (cont.)Change of State (cont.) Cohesion and adhesion

◦ The attractive force between like molecules is cohesion.

◦ The attractive force between unlike molecules is adhesion.

The shape of the meniscus depends on the relative strengths of adhesion and cohesion.

H20: Adhesion > Cohesion Mercury: Cohesion > Adhesion

H20 Mercury

Cohesion and AdhesionCohesion and Adhesion

Cohesion: Water is attracted to waterAdhesion: Water is attracted to other substances

Adhesion and cohesion are water properties that affect every water molecule on earth and also the interaction of water molecules with molecules of other substances. Essentially, cohesion and adhesion are the "stickiness" that water molecules have for each other and for other substances.

The water drop is composed of water molecules that like to stick together, an example of the property of cohesion. The water drop is stuck to the end of the pine needles, which is an example of the property of adhesion. Notice I also threw in the all-important property of gravity, which is causing the water drops to roll along the pine needle, attempting to fall downwards. It is lucky for the drops that adhesion is holding them, at least for now, to the pine needle.

http://www.youtube.com/watch?v=VHnFMPxteGo

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Change of State (cont.)Change of State (cont.)Liquid to vapor phase changes

◦Boiling – heating a liquid to a temperature at which its vapor pressure equals atmospheric pressure.

◦Saturation – equilibrium condition in which a gas holds all the water vapor molecules that it can.

◦Dew point – temperature at which the water vapor in a gas begins to condense back into a liquid.

◦Evaporation – when water enters its gaseous state at a temperature below its boiling point.

The Gas LawThe Gas Law Ideal Gas follows kinetic molecular theory, made up of

large number of molecules that are in rapid random motion following perfect elastic collitions losing no momentum

How the Kinetic Molecular Theory Explains the Gas Laws

The pressure of a gas results from collisions between the gas particles and the walls of the container.

Each time a gas particle hits the wall, it exerts a force on the wall.

An increase in the number of gas particles in the container increases the frequency of collisions with the walls and therefore the pressure of the gas.

Avogadro's Hypothesis As the number of gas particles increases, the frequency of

collisions with the walls of the container must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible containers, such as a balloon, will expand until the

pressure of the gas inside the balloon once again balances the pressure of the gas outside.

Thus, the volume of the gas is proportional to the number of gas particles.

The Gas LawsThe Gas LawsCharles Law

◦ The volume of a gas increased with the temperature ◦ The volume of a given amount of dry ideal gas is

directly proportional to the Kelvin Temperature provided the amount of gas and the pressure remain fixed.

◦ When we plot the Volume of a gas against the Kelvin temperature it forms a straight line.

◦ V1 / T1 = V2 / T2

Boyle’s Law◦ the product of the pressure and volume are observed

to be nearly constant. ◦ The product of pressure and volume is exactly a

constant for an ideal gas. ◦ p * V = constant

WATER VAPOR

(9.8oC)