Gases and liquids are both fluids, but they have some important differences. Liquids are nearly incompressible; gases are highly compressible. The molecules in a liquid attract each other via molecular bonds; the molecules in a gas do not interact other than through occasional collisions. These differences affect how we think about pressure in gases and liquids.

Imagine that you have two sealed jars, each containing a small amount of mercury and nothing else. All the air has been removed from the jars. Suppose you take the two jars into orbit on the space station, where they are weightless. One jar you keep cool, so that the mercury is a liquid. The other you heat until the mercury becomes a gas. What can we say about the pressure in these two jars?

As FIGURE 14.5 shows, molecular bonds hold the liquid mercury together. It might quiver like Jello, but it remains a cohesive drop floating in the center of the jar. The liquid drop exerts no forces on the walls, so there’s no pressure in the jar containing the liquid. (If we actually did this experiment, a very small fraction of the mercury would be in the vapor phase and create what is called vapor pressure.)

FIGURE 14.5 A liquid and a gas in a weightless environment.

The gas is different. The gas molecules collide with the wall of the container, and each bounce exerts a force on the wall. The force from any one collision is incredibly small, but there are an extraordinarily large number of collisions every second. These collisions cause the gas to have a pressure. We will do the calculation in Chapter 20.

FIGURE 14.6 shows the jars back on earth. Because of gravity, the liquid now fills the bottom of the jar and exerts a force on the bottom and the sides. Liquid mercury is incompressible, so the volume of liquid in Figure 14.6 is the same as in Figure 14.5. There is still no pressure on the top of the jar (other than the very small vapor pressure).

FIGURE 14.6 Gravity affects the pressure of the fluids.

At first glance, the situation in the gas-filled jar seems unchanged from Figure 14.5. However, the earth’s gravitational pull causes the gas density to be slightly more at the bottom of the jar than at the top. Because the pressure due to collisions is proportional to the density, the pressure is slightly larger at the bottom of the jar than at the top.

Thus there appear to be two contributions to the pressure in a container of fluid:

1. A gravitational contribution that arises from gravity pulling down on the fluid. Because a fluid can flow, forces are exerted on both the bottom and sides of the container. The gravitational contribution depends on the strength of the gravitational force.

2. A thermal contribution due to the collisions of freely moving gas molecules with the walls. The thermal contribution depends on the absolute temperature of the gas.

A detailed analysis finds that these two contributions are not entirely independent of each other, but the distinction is useful for a basic understanding of pressure. Let’s see how these two contributions apply to different situations.