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Atmospheric Pressure
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
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.
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.
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.
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
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
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
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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