Understanding Pressure: Definition, Units, and Applications | Exams Earth, Atmospheric, and Planetary Sciences

11.7

Atmospheric Pressure

Have you ever felt your ears “pop” in an airplane or when driving over a hilly road?

What you are experiencing is a change in air pressure. Like everything else near

Earth, the atmosphere is under the infl uence of gravity. Gravity pulls this enormous

mass toward the centre of Earth.

Pressure

Pressure is defi ned as the force per unit area. Like volume and temperature, pressure is a

physical property of a gas. As you will soon learn, there are many important applications

of gas pressure. Before discussing air pressure specifi cally, we will consider a more visible

example of pressure. Figure 1 illustrates some of the factors that determine pressure. Th e

gravitational pull of Earth exerts a downward force on everyone, including the person in

Figure 1. As a result, he exerts a downward force on the nails. However, to minimize pain,

this person would be wise to distribute this force over as large an area as possible. Th e

greater the area, the lower the pressure. He would still exert the same downward force if

he were to stand on one foot on the nails. However, the force would then be concentrated

into a smaller area. Th e smaller the area, the greater the pressure.

Mathematically, pressure, P, is expressed as

P5F

Th e SI unit for pressure is the pascal (1 Pa 5 1 N/m2). Pressure is directly related to the

size of force applied. Th e greater the force, the greater the pressure. Pressure is inversely

related to area. A large force applied to a small area will produce a large pressure. If the

same force is applied to a large area, the pressure will be less. For example, if a force 100 N

were applied to an area of 1.00 cm2 (0.000100 m2 or 1.00 3 1024 m2), the pressure would

be 1.00 3 106 Pa (Figure 2(a)). If the same 100 N force were applied to a much larger area

of 1.00 m2, the pressure would be only 100 Pa (Figure 2(b)).

Figure 1 The more nails there are, the

less painful this experience will be.

pistons

compressible gas

P =

100 N

0.000100 m2

= 1.00 � 106 Pa

P =

100 N

1 m2

= 100 Pa

A � 1.00 cm2

� 1.00 � 10�4 m2

A � 1.00 m2

F � 100 N

Figure 2 The smaller the surface area on which the mass is resting, the greater the pressure exerted.

(a) (b)

When a piston applies a pressure to a trapped sample of gas, as in Figure 2, the gas

exerts a pressure on the walls of its container. It is the force exerted by the gas mol-

ecules as they collide with the inner walls of the container that results in the observed

pressure. Th ese collisions are what keep bicycle tires hard when they are infl ated.

11.7 Atmospheric Pressure 541

NEL

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Atmospheric Pressure 11.

Have you ever felt your ears “pop” in an airplane or when driving over a hilly road?

What you are experiencing is a change in air pressure. Like everything else near

Earth, the atmosphere is under the influence of gravity. Gravity pulls this enormous

mass toward the centre of Earth.

Pressure

Pressure is defined as the force per unit area. Like volume and temperature, pressure is a

physical property of a gas. As you will soon learn, there are many important applications

of gas pressure. Before discussing air pressure specifically, we will consider a more visible

example of pressure. Figure 1 illustrates some of the factors that determine pressure. The

gravitational pull of Earth exerts a downward force on everyone, including the person in

Figure 1. As a result, he exerts a downward force on the nails. However, to minimize pain,

this person would be wise to distribute this force over as large an area as possible. The

greater the area, the lower the pressure. He would still exert the same downward force if

he were to stand on one foot on the nails. However, the force would then be concentrated

into a smaller area. The smaller the area, the greater the pressure.

Mathematically, pressure, P, is expressed as

P 5

F

A

The SI unit for pressure is the pascal (1 Pa 5 1 N/m^2 ). Pressure is directly related to the

size of force applied. The greater the force, the greater the pressure. Pressure is inversely

related to area. A large force applied to a small area will produce a large pressure. If the

same force is applied to a large area, the pressure will be less. For example, if a force 100 N

were applied to an area of 1.00 cm^2 (0.000100 m^2 or 1.00 3 1024 m^2 ), the pressure would

be 1.00 3 106 Pa ( Figure 2(a) ). If the same 100 N force were applied to a much larger area

of 1.00 m^2 , the pressure would be only 100 Pa ( Figure 2(b) ).

P = 100 N

P = 100 N

F � 100 N

F � 100 N

When a piston applies a pressure to a trapped sample of gas, as in Figure 2, the gas

exerts a pressure on the walls of its container. It is the force exerted by the gas mol-

ecules as they collide with the inner walls of the container that results in the observed

pressure. These collisions are what keep bicycle tires hard when they are inflated.

Atmospheric pressure is the force per unit area exerted by air on all objects. It is com-

monly reported in kilopascals, kPa. At sea level, the pressure exerted by a column of

air with a base of one square metre is equal to 101.325 kPa (often rounded to 101 kPa).

This pressure is known as standard pressure and is the basis for another unit of pressure:

the atmosphere. One atmosphere is equal to 101.325 kPa.

Traditionally, chemists defined the standard conditions for work with gases as

the temperature 0 °C and pressure 101.325 kPa. A gas sample at these conditions

is said to be at standard temperature and pressure (STP). However, since 0 °C is not

a convenient temperature at which to conduct laboratory investigations, scientists

have recently defined another set of standard conditions. These conditions are

called standard ambient temperature and pressure (SATP) and are defined as 25 °C and

100 kPa. The SATP standard is more convenient than STP because it more closely

represents the conditions in a laboratory.

Evangelista Torricelli (1608−1674) was the first person to devise a method of mea-

suring atmospheric pressure. He was trying to solve a problem. Pump makers in Tuscany

could not raise water more than 10 m using a suction pump. Torricelli used mercury,

which is denser than water, to investigate the vacuum and atmospheric pressure. He

prepared a glass tube similar to an extremely long test tube. He filled the tube with

mercury and carefully inverted it, submerging the open end into a dish containing

more mercury ( Figure 3 ). The mercury in the tube was pulled down by gravity. However,

the mercury did not all run out of the tube. Why not? Air pressure pushed on the mer-

cury in the dish, effectively pushing mercury into the tube. A vacuum formed at the top

of the tube. The vacuum exerted no downward pressure on the mercury inside the tube.

Torricelli noticed that the mercury level in the tube changed slightly from day to

day. The fluctuating mercury level was due to changes in air pressure. This device

for measuring atmospheric pressure became known as a barometer. At one time, the

standard pressure was defined as 760 mm Hg or 760 Torr in honour of Torricelli.

Scientists had been investigating gases for many years before there was a standard-

ized unit for pressure. Some scientists developed their own ways of measuring pressure.

This is one reason we now have so many units for pressure. Some of these units are used

in a specific situation. For example, medical professionals use mm Hg for measuring

blood pressure. In Canada we still commonly measure tire pressure in psi (pounds per

square inch), even though we use the metric system for many other quantities. Table 1

shows the conversion of several SI and non-SI pressure units.

The change in atmospheric pressure is the reason why your ears sometimes hurt when

you change altitude quickly. Your ear is a complicated organ designed to detect sound

waves. The middle ear is an air-filled chamber that is isolated from the outside air by the

eardrum and is connected to a channel called the Eustachian tube which vents into your

throat ( Figure 6(a) ).

When a plane takes off and climbs higher in the sky, the atmospheric pressure in

the cabin decreases. With less pressure on the eardrum, the volume of gas in the ear