Atmospheric pressure is the pressure exerted by the weight of the thick layer of air surrounding the Earth acting on its surface. Every object on Earth experiences atmospheric pressure, which can be measured using instruments like the mercury barometer, Fortin barometer, and Aneroid barometer.
Atmospheric pressure acting on the open pool of mercury supports a vertical column of mercury of height h = 760 mm against the vacuum at the top.
Atmospheric pressure pushes downward on the exposed mercury surface in the bowl. This pressure supports the weight of the mercury column inside the sealed tube. The space above the mercury column is a Torricellian vacuum.
At sea level under standard conditions, the mercury column height is 760 mm, so atmospheric pressure is 760 mm Hg.
Using \(P = h\rho g\), this equals approximately 101 396 Pa.
Atmospheric pressure is the pressure due to the weight of the layer of air acting on the surface of the Earth. The Earth is surrounded by a thick layer of gases that has mass and weight, exerting downward pressure on all objects on the Earth's surface.
Large rubber suckers used to lift heavy glass sheets stick firmly because atmospheric pressure acting from the outside presses them against the glass surface when air inside is expelled.
A 1-metre-long glass tube is completely filled with mercury, covered with a lid, inverted, and immersed into a bowl of mercury. When the lid is removed, the mercury column falls until it reaches a specific height and remains stationary.
Evangelista Torricelli conducted this experiment in 1643 and concluded that the mercury column is supported by atmospheric pressure.
Pascal (Pa) is the S.I. unit of pressure, where 1 Pa = 1 N m⁻². mm Hg is a manometric unit used in medicine, meteorology, aviation, and science. m H₂O is used for water pressure in seas, reservoirs, and dams. Millibar (mbar) is used extensively in meteorology, where 1 mbar = 1 hPa = 100 Pa.
Standard atmospheric pressure is 760 mm Hg, which corresponds to approximately 101 396 Pa or 1 013 mbar.
Answer in your mind, then press “Check Answer”.
What creates atmospheric pressure on the surface of the Earth?
Calculate the pressure in Pa exerted by a mercury column of height 756 mm if the density of mercury is 13 600 kg m⁻³ and g = 9.81 m s⁻².
Why must deep-sea divers ascend to the surface at a slow, controlled rate?
The mercury in a barometer stays up because the vacuum at the top “sucks” the mercury upward.
A vacuum cannot exert a pull. The mercury is held up because atmospheric pressure pushes downward on the exposed surface of the mercury in the bowl, forcing it up the tube until the weight of the mercury column equals the atmospheric force.
Pressure decreases as you dive deeper underwater because water gets colder.
Water pressure increases with depth solely because the weight of the water column above the diver increases with depth, \(P = h\rho g\).
Objective: Test your understanding of atmospheric pressure, barometers, pressure units, and adaptations at extreme altitudes and depths.
1 Which statement correctly explains why a mercury column remains at a height of approximately 760 mm inside a simple barometer at sea level?
2 What mechanical feature allows an Aneroid barometer to detect changes in atmospheric pressure?
3 The unit of pressure commonly used in meteorology that is equivalent to 1 hectopascal (1 hPa) is the .
4 If a diver ascends too rapidly from deep water, dissolved nitrogen gas will form dangerous in body tissues and blood vessels.
Match each pressure unit or instrument to its correct textbook description or application. If using a phone, tap the answer first, then tap the matching box.
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