Atomoshperic pressure notes

7/29/2019 Atomoshperic pressure notes

1/15

What is Atmospheric Pressure?

According to the kinetic theory of gases, gases consist of molecules which are far apart

and in random motion at high speeds.

The gas molecules possess mass and experience the gravitational pull. The result is that

gases have weight.

The atmosphere is a thick layer of air that surrounds the Earth.

The atmosphere exerts a pressure called atmospheric pressure which is caused by the

weight of the thick layer of air above the Earth's surface.

Atmospheric pressure acts on every object on the surface of the earth.

Unit of atmospheric pressure

The units of atmospheric pressure used is:

- Pascal (Pa)

- centimetre/milimetre mercury (cmHg/mmHg)

- atmosphere (atm)-

Notes:

- Atmospheric Pressure at Sea Level = 1atm

- 1 atm = 76cmHg

Existence of Atmospheric Pressure


2/15

The cardboard does not fall and the water remains in the glass even

though its not supported by anything.

Explanation:

The force caused by the atmospheric pressure acts on the surface of

the cardboard is greater than the weight of the water in the glass.

Crushing Can


3/15

When a can filled with hot water is closed and is cooled down rapidly

by pouring cold water on it, it will crash instantly.

Explanation:

When the air inside the can is cooled, its pressure decreases. The

high atmospheric pressure outside exerts a great force on the can and

causes it crashes.

Magdeburg Hemisphere


4/15

Figure above shows a Magdeburg Hemisphere. When the air inside

the hemisphere is pumped out so that it becomes a vacuum, the

hemisphere cannot be separated even by a very great force.

Explanation:

The atmospheric pressure exerts a strong force on the outer surface

of the hemisphere, holding the hemisphere tightly together.

Atmospheric Pressure and Altitude


5/15

Atmospheric pressure decreases as the altitude increases.

Instrument Used to Measure Atmospheric Pressure

-Simple Barometer

Mercury Barometer

1. A mercury barometer consists of a thick-walled glass tube, which is closed at one end.

2. The tube is completely filled with mercury and inverted several times to remove air

bubbles.

The tube is then completely filled again with mercury.

3. After all air has been removed, the open end of the glass tube is inverted into a

container of mercury.

4. The mercury column drops until it reaches a height of about 76cm above the lower

surface. The space between the top of the mercury and the end of the tube should

contain no air; it is a complete vacuum.

5. The column of mercury in the tube is supported by the atmospheric pressure and its

height depends on the magnitude of the atmospheric pressure.

6. Since the atmospheric pressure at sea level can support a vertical column of mercury

76 cm or 760 mm high, we can, for convenience, express mm Hg as a unit of pressure. 1

Standard atmospheric pressure (1 P atm) = 76 cm Hg or 760 mm Hg (also known as one


6/15

atmosphere).

P atm = 76 cm Hg = 10 000 Pa.

Simple Barometer

Pa = 0cmHg

Pb = 26cmHg

Pe = 76cmHg

Pf= 84 cmHg


7/15

For a given liquid, the pressure will be the same at same level.

Therefore,

Pd = Pe = 76cmHg

The difference of gas pressure at different level can be ignored if the

difference of the height is not too large. Therefore,

Pc = Pd = 76cmHg

Figure above shows a simple barometer. Find the atmospheric pressure in the

unit of

a) cmHg

b) Pa

[Density of mercury = 13,600 kgm-3

]

Answer:

a) The atmospheric pressure,

P = 76cmHg


8/15

b) The atmospheric pressure,

P = hg

P = (0.76)(13,600)(10) = 103,360 Pa

Figure above shows a column of mercury in a glass tube. Some air is trapped

in the glass tube. If the atmospheric pressure is 76cmHg, what is the pressure

of the gas inside the tube?

Answer:

Patm = Pair+ Pmercury

(76) = Pair+ 50

Pair= 76 - 50 = 26cmHg

More About Simple Barometer


9/15

The height, h will remains unchanged when

i) the glass tube is lifted up from the dish

ii) the glass tube is lowered further into the dish

iii) the diameter of the glass tube increases

iv) the glass tube is tilted

v) the quantity of mercury in the dish is increased

Application of Atmospheric Pressure

Rubber Sucker


10/15

-When the sucker is pressed into place, the air inside is forced out.

-The sucker is held in position by the high atmospheric pressure on

the outside surface.

Syringe


11/15

When the piston is pulled up, the atmospheric pressure inside the

cylinder will decrease. The atmospheric pressure outside pushes the

liquid up into the syringe.

Straw

-When a person suck through the straw, the pressure in the straw

become low.

-The atmospheric pressure outside which is higher will force the water

into the straw and consequently into the mouth.

Vacuum Cleaner


12/15

When a vacuum cleaner is switched on, it sucks out the air inside the

cleaner, causes the pressure inside the cleaner become low.

The atmospheric pressure which is higher in magnitude, forces the air

and duct particles into the cleaner.

Manometer

Manometers are used to measure the pressure difference between two regions.

It consists of a tube of plastic or glass, bent into the U-shape and filled with a liquid that is often

oil.

If there is a pressure difference between the ends of the manometer, the liquid moves until the

pressure difference is balanced by the difference in heights of the ends of the liquid.

The greater the pressure the greater the difference in heights.

Oil is often used rather than water because water evaporates and also oil is less dense which

makes the manometer more sensitive.


13/15

Pgas = Patm + Pliquid

Pgas = Patm + hg

Pgas = Gas Pressure

Patm = Atmospheric Pressure

g = Gravitational Field Strength

The pressure of the gas is given by h (the difference in mercury levels) in mmHg.

Atmospheric pressure pushes on the mercury from one direction, and the gas in the

container pushes from the other direction. In a manometer, since the gas in the bulb is

pushing more than the atmospheric pressure, you add the atmospheric pressure to the

height difference:

Pgas > Patm Gas pressure = atmospheric pressure +h (height of the mercury)

Pgas =patm + gh


14/15

If the gas pressure is lower than the atmospheric pressure, it will drive down the liquid level in the right side

of the manometer. The pressure of the gas, Pgas can be calculated by:

Pgas < Patm Gas pressure = atmospheric pressure -h (height of the mercury)

Pgas =patm gh

Questions

Figure above shows the water levels in a water manometer used to measure the pressure

of a gas supply. Find the pressure of the gas in unit of


15/15

i) cm water

ii) Pa

[Atmospheric Pressure = 1000cm Water; Density of water = 1000kg/m3]

Answer:

i)

Gas Pressure,

P = Patm + Pwater

P = 1000 + (30-5) = 1025 cm Water

ii)

h = 1025cm = 10.25m

= 1000kgm3g = 10ms-2

Gas Pressure,

P = hg

P = (10.25)(1000)(10)

P = 102500 Pa

Documents

Atomoshperic pressure notes