Atmospheric PressureAtmospheric Pressure

Atmospheric PressureAtmospheric Pressure

Earth Atmosphere

Surface of the Earth

Atmospheric pressure exerted on the surface of the Earth (or at the sea level) as well as objects on Earth.

Atmospheric PressureAtmospheric Pressure

The gas The gas molecules have molecules have weight.weight.

Atmospheric Atmospheric Pressure is Pressure is caused by the caused by the weight (force) of weight (force) of the thick layer of the thick layer of air above the air above the earth’s surface.earth’s surface.

HOW does a human body withstand the pressure on its surface?

Living cells maintain an internal pressure that is about the same as 1 atm, which is similar to the atmospheric pressure. So, we don’t feel high atmospheric pressure exerted to us.

Surface of bottleInternal pressure

External pressure

External pressure

Internal pressure

Internal Pressure = External pressure

Atmospheric Pressure acts equally in all Atmospheric Pressure acts equally in all direction.direction.

Characteristics of Atmospheric Characteristics of Atmospheric PressurePressure

It is not affected by the surface area.It is not affected by the surface area. The pressure exerted by the air molecules at sea level is 1.013 × 105 Pa. At sea level, PAt sea level, Patmatm = = 1.013 × 105 Pa (Nm-2 ) = 76 = 76

cmHgcmHg

= 10mH= 10mH22OO

Atmospheric Pressure decreases with altitude Atmospheric Pressure decreases with altitude because air is gets thinner as altitude increases.because air is gets thinner as altitude increases.

Peter goes to the top of mountain and feels breathing difficulty. The number of air molecules _________. The collision rate of air molecules _________ and he encounters ______ atmospheric pressure. So, he has to breath _____ when he goes up to mountain ( _____ altitude).

John enters to the underground mines and feels breathing difficulty. The number of air molecules _________. The collision rate of air molecules _________ and he encounters ______ atmospheric pressure. So, he has to breath _____ when he goes down mines ( ____ altitude).

decreasesdecreases

lower

morehigh

increases

increaseshigher

lesslow

Existence of Atmospheric Existence of Atmospheric PressurePressure

At normal condition, the rate of collision on inner wall is same with on outer wall. So, the air pressure inside and outside of the bottle is the same.

After the air has been pumped out, there are almost no more air molecule inside the bottle to balance the force of collision of air molecule outside the bottle. The external force due to atmospheric pressure crashes the bottle.

Existence of Atmospheric Existence of Atmospheric PressurePressure

Glass filled by water

Atmospheric pressure

Cardboard

Why the cardboard does not fall from the mouth of the inverted glass filled with water?

The atmospheric pressure that presses the cardboard against the glass produce a force that is strong enough to support the weight of the water in the glass.

Existence of Atmospheric Existence of Atmospheric PressurePressure

syringesyringe BarometerBarometer

Application of atmospheric pressure

Pouring the condensed milk from its can

A rubber suction cap on a smooth surface

Drinking with a straw

Removing dust with a vacuum cleaner

Putting plastic sticker onto the inner surface windscreen

Measuring blood pressure

The presence of a second hole is to enable air flow into the can. So, the air pressure inside the can same as atmospheric pressure. It will force the milk out of the lower hole.

Pouring the condensed milk from its can

Milk

Air flows into the can

Atmospheric pressure

A rubber suction cap on a smooth surface

When air is forced out of the suction cup, a partial vacuum is created in the space between the cup and the smooth surface. The surrounding atmospheric pressure forces the cup tightly against the smooth surface.

Removing dust with a vacuum cleaner

When the vacuum cleaner switches on, the motor will work and the fan blades turn, force the air forward towards the exhaust port.

The pressure level in the area behind the fan drops below the pressure level outside the vacuum cleaner.

This creates partial vacuum inside the vacuum cleaner.

The dust will flows into the vacuum cleaner through the intake port because the air pressure inside the vacuum cleaner is lower than the pressure outside.

Drinking with a straw

Straw

Atmospheric pressure forces the drink into the straw

Air being sucked up and creating a partial vacuum in the straw

When the air is being sucked from the straw, a partial vacuum is created in the straw. The surrounding atmospheric pressure will force the drink into the straw and enable it to be moved into the mouth.

Putting plastic sticker onto the inner surface windscreen

When the plastic is placed on the glass surface, air is being forced out of the space between the sticker and the glass surface, creating a partial vacuum. The surrounding atmospheric pressure will hold the sticker tightly on the windscreen.

Windscreen

Sticker

Pressure

Air is being force out

Instruments for measuring Instruments for measuring Atmospheric PressureAtmospheric Pressure

Mercury barometerMercury barometerFortin barometerFortin barometerAneroid barometer

Measuring atmospheric pressure

Simple Barometer

Made of a glass tube of about 100 cm filled with liquid (normally mercury is used).

For mercury, the liquid level will drop as it flows into a bowl to until a vertical height of above 76 cm from the surface of mercury in the bowl.

ρHg = 1.36 × 104 kg m-3

g = 9.8 N kg-1

h = 76 cm = 0.76 m

P = 1.36 × 104 × 9.8 × 0.76 = 1.103 × 105 Pa

QuestionLiquid A has been filled into the barometer to measure the atmospheric pressure at sea level. According to the figure, find the density of the liquid A.

SolutionAs we know, atmospheric pressure, P is equal to1.013 × 105 Pa at sea level.

So, P = 1.013 × 105 = hρg1.013 × 105 = 1.10 × ρA × 10ρA = 9209 kg m-3

Fortin BarometerFortin Barometer More accurate More accurate unit mmHgunit mmHg

Fortin barometers have to be set before each reading is taken.

Using the screw at the bottom adjust the mercury level, seen in the glass reservoir, until the surface just touches the tip of the pointer. The barometer is then ready for a reading to be taken.

Aneroid BarometerAneroid Barometer More handy and mobileMore handy and mobile

Aneroid BarometerAneroid Barometer

• the partially evacuated chamber expands and contracts in response to changes in atmospheric pressure

Aneroid Aneroid BarometerBarometer

Also an altimeterAlso an altimeter

Normally used by pilot to determine the atmospheric pressure in sky.

Sometimes, it is also used to measure the height above the sea level (altitude). So, it called altimeter.

Aneroid BarometerAneroid Barometer

Gas PressureGas Pressure

Kinetic Molecular Kinetic Molecular TheoryTheory

Basic assumptions:Basic assumptions: A gas consists of a collection of small particles which are

moving rapidly and randomly in straight-line motion and obeying Newton's Laws.

Gas molecules are constantly colliding with one another and the collisions is perfectly elastic (that is, no energy is gained or lost during the collision).

Average kinetic energy is equal to the temperature.Average kinetic energy is equal to the temperature.

Gas Molecule

Pressure in gases due to collision of molecules with the wall of a container.

Gas PressureGas Pressure

Gas molecules collide with Gas molecules collide with the wall of the container and the wall of the container and change velocity and change velocity and momentum,momentum,

the rate of change of the rate of change of momentum = Forcemomentum = Force

The force on the wall of the The force on the wall of the container creates gas container creates gas pressure. pressure.

Pressure

Rate of collisions

Number of particles

Speed of particles

Volume of container

When number of particle When number of particle INCREASESINCREASES

Distance between particles __________, ______ collisions occurs, pressure __________.

DECREASEMORE

INCREASE

When volume of container When volume of container INCREASEINCREASE

Particles have ______ space to move around, _____ collisions occurs, pressure __________.

MORE

LESS

DECREASE

When temperature inside of container When temperature inside of container INCREASEINCREASE

P

PP

P

PP

Particles gain ______ energy and move ________, average speed __________, ______ collisions occurs, pressure __________.

MORE

MOREINCREASE

INCREASE

FASTER

Instruments for measuring Instruments for measuring Gas pressureGas pressure

ManometerManometerBourdon gaugeBourdon gauge

Measuring gas pressureMeasuring gas pressure

Manometer

Manometer consists of a U-tube that is filled with a liquid, oil or mercury.

The figure shows a manometer is not connect of gas supply. The atmospheric pressure acts on both surfaces of the liquid at points A and B.

P0 – Atmospheric pressure

The figure shows one end of manometer is connected to the gas supply.

The gas would exert a pressure on the liquid at point A.

If the pressure greater than atmospheric pressure, liquid level at point A will be forced down.

Liquid in another end (point B) will be forced up in equilibrium.

P = Patm + hρg

QuestionA mercury manometer with one end attached to a gas supply measures a difference in the level of mercury of 32 cm as in figure.

Calculate the pressure of the gas supply in (a) cmHg (b) Pascal

[ Patm = 76 cmHg; g = 10 Nkg-1; ρmercury = 1.36 × 104 kgm-3 ]

Solution(a) Pressure = Atmospheric pressure +

pressure due to mercury column

= 76 cmHg + 32 cmHg = 108 cmHg

(b) Pressure of gas supply = hρg

= 108 × 10-2 × 1.36 × 104 × 10 = 1.46 × 105 Pa

QuestionA mercury manometer with one end attached to a gas supply measures a difference in the level of mercury of 10 cm as in figure.

Calculate the pressure of the gas supply in (a) cmHg (b) Pascal

[ Patm = 76 cmHg; g = 10 Nkg-1; ρmercury = 1.36 × 104 kgm-3 ]

Solution(a) Pgas = PHg + Patm

= 10 cmHg + 76 cmHg = 86 cmHg

(b) Pressure of gas supply = hρg

= 86 × 10-2 × 1.36 × 104 × 10 = 1.1696 × 105 Pa

Bourdon gaugeBourdon gauge

• More accurate• Measures in unit Pascal

Bourdon gaugeBourdon gaugeWhen gas supply is connected the pressure When gas supply is connected the pressure

in the gas acts to straighten the copper coilin the gas acts to straighten the copper coil

The movement The movement of the coil is of the coil is transferred to the transferred to the lever system lever system which actuates a which actuates a pointer to move pointer to move across a scale across a scale which has been which has been calibrated to give calibrated to give readings of readings of pressurpressur