Atmospheric Pressure

Card 1: What is Atmospheric Pressure

What is Atmospheric Pressure?

Atmospheric pressure is the pressure exerted on the surface of any objects by the atmospheric gas.

It is caused by the weight of the atmospheric gas above the earth surface.

Card 2: Unit of Atmospheric Pressure

Unit of atmospheric pressure

The units of atmospheric pressure used in the SPM syllabus include:

- Pascal (Pa)
- centimetre/milimetre mercury (cmHg/mmHg)
- atmosphere (atm)
- metre water

Notes:

- Atmospheric Pressure at Sea Level = 1atm
- 1 atm = 76cmHg

Card 3: Atmospheric Pressure and Altitude

Atmospheric Pressure and Altitude

Atmospheric pressure decreases as the altitude increases.

Card 4: Conversion of Pa and cmHg

Conversion of Pa and cmHg

h = height of mercury column
ρ = density of mercury
g = gravitational field strength

Example 1:

Given that the atmospheric pressure at Genting Highland is 72cmHg. Find the equivalent value of this pressure in the unit of Pascal (Pa). [density of mercury = 13600 kgm-3]

Answer:

h = 72cm = 0.72m
ρ = 13600 kgm-3
g = 10 ms-2

Pressure in the unit of Pascal,
P = hρg
P = (0.72)(13600)(10) = 97920 Pa

Card 5: Existence of atmospheric pressure 1

Existence of Atmospheric Pressure

The cardboard does not fall and the water remains in the glass even though it’s 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.

Card 6: Existence of atmospheric pressure 2 - Crushing Can

Crushing Can

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.

Card 7: Existence of atmospheric pressure 3 - Magdeburg Hemisphere

Magdeburg Hemisphere

magdeburg hemisphere

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.

Card 8: Instrument Used to Measure Atmospheric Pressure

Instrument Used to Measure Atmospheric Pressure

-Simple Barometer
-Fortin Barometer
-Anaroid Barometer

Card 9: Simple Barometer

Simple Barometer

Pa = 0cmHg
Pb = 26cmHg
Pe = 76cmHg
Pf = 84 cmHg

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

Example 2

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

b) The atmospheric pressure,
P = hρg
P = (0.76)(13,600)(10) = 103,360 Pa

Example 3

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

Card 10: More about simple barometer

More About Simple Barometer

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 

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