Physics · Mechanical Properties Of Fluids · NEET
In a mercury barometer, the air outside pushes on the mercury in the trough. This pushes mercury up the closed tube. The mercury rises until the weight of the mercury column exactly balances the atmospheric push. At sea level this balance point is a column height of about 76 cm. So we say 1 atm = 76 cm of mercury. Using P = rho x g x h with mercury density rho = 13.6 x 10^3 kg/m^3, g = 9.8 m/s^2 and h = 0.76 m, you get P = 13.6 x 10^3 x 9.8 x 0.76 = 1.013 x 10^5 Pa, which matches 1 atm.
Take a long glass tube closed at one end. Fill it fully with mercury. Turn it upside down into a trough of mercury without letting air in. The mercury drops a little and settles at a height of about 76 cm above the trough surface. The empty space at the top (called the Torricellian vacuum) has only a tiny bit of mercury vapour, so its pressure is almost zero. The mercury stays up because atmospheric pressure on the trough surface holds it there. The column height directly tells you the atmospheric pressure.
No. The height of the mercury column does not depend on the width or cross-section area of the tube. From P = rho x g x h, the pressure depends only on the fluid density and the vertical height, not on area or shape. A thin tube and a thick tube both show the same 76 cm at sea level. Only the vertical height matters. This is a very common NEET trap.
It is (almost) a vacuum, not air. When you invert the filled tube, no air can enter from the closed top. The space that opens up above the mercury has only mercury vapour, whose pressure is so small it is taken as zero. So the pressure at the top of the mercury column is treated as zero, which is why the full atmospheric pressure supports the 76 cm column.
No. Atmospheric pressure (Pa) is the pressure of the air alone. Gauge pressure is the extra pressure above atmospheric, that is P minus Pa. Absolute pressure is the true total pressure P = Pa + rho x g x h. A tyre gauge reads gauge pressure, so a tyre reading 'zero' still contains air at atmospheric pressure. This links directly to the next concept, gauge vs absolute pressure.
Atmospheric pressure at a point equals the weight of the air column above that point. As you climb a hill or fly higher, there is less air left above you, so less weight pushes down, so pressure is lower. That is why the mercury column in a barometer is shorter at high altitude and why cooking is slower on mountains (water boils at a lower temperature when pressure is low).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1 atm = 1.013 x 10^5 Pa = 760 mm of mercury = 76 cm of mercury = 1.013 bar. Also 1 torr = 1 mm of Hg = 133 Pa, and 1 bar = 10^5 Pa.
Mercury is very dense (13.6 x 10^3 kg/m^3). Since P = rho x g x h, a dense liquid needs only a short column (about 76 cm) to balance atmospheric pressure. Water is about 13.6 times less dense, so a water barometer would need a column over 10 metres tall, which is not practical.
Set rho x g x h = 1.013 x 10^5 Pa with water density 1000 kg/m^3 and g = 9.8 m/s^2. Then h = 1.013 x 10^5 divided by (1000 x 9.8) = about 10.3 m. That is why mercury, not water, is used.
It is the nearly empty space above the mercury column in a barometer tube. It contains only a trace of mercury vapour, so its pressure is taken as zero. Torricelli was the first to create and explain it while measuring atmospheric pressure.
A falling barometer reading means atmospheric pressure is dropping. Low pressure regions often bring bad weather, so a drop of 10 mm or more in the mercury column is a sign that a storm may be coming.
