1.8 Pressure

• Relate (without calculation) pressure to force and area, using appropriate examples


Pressure is the amount of force acting on a certain area.
Pressure is measured in pascals. [Pa] = Newtons per square metre. [ N/m2]

Pressure of a bear question
What is the pressure on bear's feet when the mass of a bear is 150kg and size of a foot of the bear is 10cm2 ?
P = Force / Area
F= ma = 150kg x 10m/s2 = 1500N
A: A bear has 4 feet so the area the force acting on it is 10cm2 x 4 = 40 cm2
Therefore, the pressure = F/A = 1500N / 40 cm2 = 37.5 N/cm2
1 Pascal is the same value as 1N/m2
where 1 cm2 = 1 cm x 1 cm = 0.01m x 0.01m = 0.0001m2 as 1cm = 0.01m
So, 37.5 N/cm2 is equal to 37.5N / 0.0001 m2
The final pressure value on the bear's feet is 375000N/m2 which is the same as 375000Pa (375kPa).

• Describe the simple mercury barometer and its use in measuring atmospheric pressure

Mercury barometer


Mercury barometer is a long glass tube with one bottom end dipped in a dish of liquid mercury and sealed at the top.
It keeps a column of mercury up the tube showing Atmospheric pressure.
It is measured in millimetres of mercury [mm Hg].

The pressure at sea level is 760 mm Hg.



Image from Thomson Higher education
g_mercury_barometer.jpg

• Relate (without calculation) the pressure beneath a liquid surface to depth and to density, using appropriate examples

• Recall and use the equation p = F/A
Supplement


• Recall and use the equation p = hρg

Pressure on Dams.jpg
Image from Toya.net
Volume of liquid
V = A x depth of liquid (h).

Density of liquid
ρ = m/V, m = ρV =ρ A h

Pressure [P]
= Force/Area
= Weight of liquid / Area
= mg / A
= ρ g A h / A
= ρ g h

Calculate the pressure at depth from calctool
pressure in sea.png

• Use and describe the use of a manometer

Manometers are used to measure gas and liquid pressures. The height difference indicates the extra pressure that the gas supply has on top of atmospheric pressure. To find the actual pressure of the gas, we need to add on the value of atmospheric pressure.
(a) Pgas pressure in (a) < 760 mm Hg (It's below atmospheric pressure)

P = 760 mm Hg - The different value in height of between two columns.

(b) Pgas pressure in (b) > 760 mm Hg (It's above atmospheric pressure)

P = 760 mm Hg + The different value in height between two columns.
manometers.jpg
1. The open manometer in (a) is filled with mercury and connected to a container of gas. The mercury level is 90.0 mm higher in the arm of the tube connected to the gas
Atmospheric pressure is 760 mmHg.
What is the pressure of the hydrogen? Show your working.
Answer: 670 mmHg

2. The open manometer in (b) is filled with mercury and connected to a container of gas. The mercury level is 70.0 mm lower in the arm of the tube connected to the gas.
Atmospheric pressure is 760 mmHg.
What is the pressure of the hydrogen? Show your working
Answer: 830 mmHg

Image and questions from M.C.H.S Chemistry Page



Pressure

Gas law
Gas Properties
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