The pressure exerted by a 760 mm column of mercury at 0 °C is known as
- A.1 pascal
- B.1 atmosphere
- C.1 bar
- D.1 poise
Correct answer
Explanation
The correct answer is B, 1 atmosphere. Standard atmospheric pressure at sea level is defined as the pressure of a 760 mm column of mercury at 0 °C.
In 1643 Evangelista Torricelli showed that air pressure holds up a column of mercury about 760 mm (76 cm) tall in a glass tube, inventing the mercury barometer. This pressure became the unit called the atmosphere (atm). One atmosphere equals 101,325 pascals, about 1.013 bar, or 760 torr. Mercury is used because it is very dense, about 13.6 times as dense as water; a water barometer would need a tube about 10.3 metres tall.
A is wrong because one pascal, one newton per square metre, is a tiny pressure; 1 atmosphere is about 101,325 Pa. C is wrong because 1 bar is exactly 100,000 Pa, slightly less than 1 atmosphere. D is wrong because the poise is the CGS unit of viscosity, not of pressure.
Exam tip: 1 atm = 76 cm of Hg = 101,325 Pa ≈ 1.013 bar = 760 torr; poise measures viscosity.
Practice Questions
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