Atmospheric Pressure

Atmospheric pressure is caused by the weight of the column of air above a surface. Air molecules collide with surfaces and exert a force; the total force per unit area is the atmospheric pressure.

Key facts

• Atmospheric pressure at sea level is approximately $1.0 \times 10^5$ Pa (100 kPa).
• Atmospheric pressure decreases with altitude because there is less air above.
• Atmospheric pressure acts in all directions, not just downward.

Why atmospheric pressure decreases with altitude

At higher altitudes:

• There is less air above, so the weight of the air column is smaller.
• The air is less dense, so fewer molecules collide with surfaces per second.

Both effects reduce the pressure.

Measuring atmospheric pressure

A mercury barometer measures atmospheric pressure. A glass tube, closed at one end, is filled with mercury and inverted into a dish of mercury. The atmospheric pressure supports the mercury column. At sea level, the column height is approximately 760 mm of mercury.

$\text{atmospheric pressure} = \rho_{\text{Hg}} \times g \times h$

Using $\rho_{\text{Hg}} = 13\,600$ kg/m$^3$, $g = 9.8$ N/kg, $h = 0.760$ m:

$p = 13\,600 \times 9.8 \times 0.760 \approx 1.01 \times 10^5\text{ Pa}$

Applications

• Drinking through a straw: you reduce the air pressure inside the straw; atmospheric pressure on the liquid surface pushes the liquid up.
• Suction cups: removing air creates lower pressure inside; atmospheric pressure outside holds the cup in place.

Common errors and how to correct them

Saying “suction pulls the cup onto the surface.” There is no pulling force. The atmospheric pressure outside pushes the cup because the pressure inside is lower.

Stating atmospheric pressure is always exactly $1.0 \times 10^5$ Pa. This is an approximation at sea level. Atmospheric pressure varies with altitude, weather and temperature.

How to apply this in an exam

When explaining atmospheric pressure, state that it is caused by the weight of the air column above. When explaining applications (straws, suction cups), describe the pressure difference between inside and outside, not “suction.”