14.1 Fluids, Density, and Pressure

Characteristics of Solids

Solids are rigid and have specific shapes and definite volumes. The atoms or molecules in a solid are in close proximity to each other, and there is a significant force between these molecules. Solids will take a form determined by the nature of these forces between the molecules. Although true solids are not incompressible, it nevertheless requires a large force to change the shape of a solid. In some cases, the force between molecules can cause the molecules to organize into a lattice as shown in Figure. The structure of this three-dimensional lattice is represented as molecules connected by rigid bonds (modeled as stiff springs), which allow limited freedom for movement. Even a large force produces only small displacements in the atoms or molecules of the lattice, and the solid maintains its shape. Solids also resist shearing forces. (Shearing forces are forces applied tangentially to a surface, as described in Static Equilibrium and Elasticity.)

Characteristics of Fluids

Liquids and gases are considered to be fluids because they yield to shearing forces, whereas solids resist them. Like solids, the molecules in a liquid are bonded to neighboring molecules, but possess many fewer of these bonds. The molecules in a liquid are not locked in place and can move with respect to each other. The distance between molecules is similar to the distances in a solid, and so liquids have definite volumes, but the shape of a liquid changes, depending on the shape of its container. Gases are not bonded to neighboring atoms and can have large separations between molecules. Gases have neither specific shapes nor definite volumes, since their molecules move to fill the container in which they are held (Figure).

Liquids deform easily when stressed and do not spring back to their original shape once a force is removed. This occurs because the atoms or molecules in a liquid are free to slide about and change neighbors. That is, liquids flow (so they are a type of fluid), with the molecules held together by mutual attraction. When a liquid is placed in a container with no lid, it remains in the container. Because the atoms are closely packed, liquids, like solids, resist compression; an extremely large force is necessary to change the volume of a liquid.

In contrast, atoms in gases are separated by large distances, and the forces between atoms in a gas are therefore very weak, except when the atoms collide with one another. This makes gases relatively easy to compress and allows them to flow (which makes them fluids). When placed in an open container, gases, unlike liquids, will escape.

In this chapter, we generally refer to both gases and liquids simply as fluids, making a distinction between them only when they behave differently. There exists one other phase of matter, plasma, which exists at very high temperatures. At high temperatures, molecules may disassociate into atoms, and atoms disassociate into electrons (with negative charges) and protons (with positive charges), forming a plasma. Plasma will not be discussed in depth in this chapter because plasma has very different properties from the three other common phases of matter, discussed in this chapter, due to the strong electrical forces between the charges.

Density

Suppose a block of brass and a block of wood have exactly the same mass. If both blocks are dropped in a tank of water, why does the wood float and the brass sink (Figure)? This occurs because the brass has a greater density than water, whereas the wood has a lower density than water.

Density is an important characteristic of substances. It is crucial, for example, in determining whether an object sinks or floats in a fluid.

The SI unit of density is [latex]{\text{kg/m}}^{\text{3}}[/latex]. Figure lists some representative values. The cgs unit of density is the gram per cubic centimeter, [latex]{\text{g/cm}}^{3}[/latex], where

The metric system was originally devised so that water would have a density of [latex]1\,{\text{g/cm}}^{3}[/latex], equivalent to [latex]{10}^{3\,}\,{\text{kg/m}}^{3}[/latex]. Thus, the basic mass unit, the kilogram, was first devised to be the mass of 1000 mL of water, which has a volume of [latex]1000\,{\text{cm}}^{3}[/latex].

As you can see by examining Figure, the density of an object may help identify its composition. The density of gold, for example, is about 2.5 times the density of iron, which is about 2.5 times the density of aluminum. Density also reveals something about the phase of the matter and its substructure. Notice that the densities of liquids and solids are roughly comparable, consistent with the fact that their atoms are in close contact. The densities of gases are much less than those of liquids and solids, because the atoms in gases are separated by large amounts of empty space. The gases are displayed for a standard temperature of [latex]0.0^\circ\text{C}[/latex] and a standard pressure of 101.3 kPa, and there is a strong dependence of the densities on temperature and pressure. The densities of the solids and liquids displayed are given for the standard temperature of [latex]0.0^\circ\text{C}[/latex] and the densities of solids and liquids depend on the temperature. The density of solids and liquids normally increase with decreasing temperature.

Figure shows the density of water in various phases and temperature. The density of water increases with decreasing temperature, reaching a maximum at [latex]4.0^\circ\text{C,}[/latex] and then decreases as the temperature falls below [latex]4.0^\circ\text{C}[/latex]. This behavior of the density of water explains why ice forms at the top of a body of water.

The density of a substance is not necessarily constant throughout the volume of a substance. If the density is constant throughout a substance, the substance is said to be a homogeneous substance. A solid iron bar is an example of a homogeneous substance. The density is constant throughout, and the density of any sample of the substance is the same as its average density. If the density of a substance were not constant, the substance is said to be a heterogeneous substance. A chunk of Swiss cheese is an example of a heterogeneous material containing both the solid cheese and gas-filled voids. The density at a specific location within a heterogeneous material is called local density, and is given as a function of location, [latex]\rho =\rho (x,y,z)[/latex] (Figure).

Local density can be obtained by a limiting process, based on the average density in a small volume around the point in question, taking the limit where the size of the volume approaches zero,

where [latex]\rho[/latex] is the density, m is the mass, and V is the volume.

Since gases are free to expand and contract, the densities of the gases vary considerably with temperature, whereas the densities of liquids vary little with temperature. Therefore, the densities of liquids are often treated as constant, with the density equal to the average density.

Density is a dimensional property; therefore, when comparing the densities of two substances, the units must be taken into consideration. For this reason, a more convenient, dimensionless quantity called the specific gravity is often used to compare densities. Specific gravity is defined as the ratio of the density of the material to the density of water at [latex]4.0\,{}^\circ \text{C}[/latex] and one atmosphere of pressure, which is [latex]1000\,{\text{kg/m}}^{3}[/latex]:

The comparison uses water because the density of water is [latex]1\,{\text{g/cm}}^{3}[/latex], which was originally used to define the kilogram. Specific gravity, being dimensionless, provides a ready comparison among materials without having to worry about the unit of density. For instance, the density of aluminum is 2.7 in [latex]{\text{g/cm}}^{3}[/latex] (2700 in [latex]{\text{kg/m}}^{3}[/latex]), but its specific gravity is 2.7, regardless of the unit of density. Specific gravity is a particularly useful quantity with regard to buoyancy, which we will discuss later in this chapter.

Pressure

You have no doubt heard the word ‘pressure’ used in relation to blood (high or low blood pressure) and in relation to weather (high- and low-pressure weather systems). These are only two of many examples of pressure in fluids. (Recall that we introduced the idea of pressure in Static Equilibrium and Elasticity, in the context of bulk stress and strain.)

A given force can have a significantly different effect, depending on the area over which the force is exerted. For instance, a force applied to an area of [latex]1\,{\text{mm}}^{2}[/latex] has a pressure that is 100 times as great as the same force applied to an area of [latex]1\,{\text{cm}}^{2}.[/latex] That is why a sharp needle is able to poke through skin when a small force is exerted, but applying the same force with a finger does not puncture the skin (Figure).

Note that although force is a vector, pressure is a scalar. Pressure is a scalar quantity because it is defined to be proportional to the magnitude of the force acting perpendicular to the surface area. The SI unit for pressure is the pascal (Pa), named after the French mathematician and physicist Blaise Pascal (1623–1662), where

Several other units are used for pressure, which we discuss later in the chapter.

Variation of pressure with depth in a fluid of constant density

Pressure is defined for all states of matter, but it is particularly important when discussing fluids. An important characteristic of fluids is that there is no significant resistance to the component of a force applied parallel to the surface of a fluid. The molecules of the fluid simply flow to accommodate the horizontal force. A force applied perpendicular to the surface compresses or expands the fluid. If you try to compress a fluid, you find that a reaction force develops at each point inside the fluid in the outward direction, balancing the force applied on the molecules at the boundary.

Consider a fluid of constant density as shown in Figure. The pressure at the bottom of the container is due to the pressure of the atmosphere [latex]({p}_{0})[/latex] plus the pressure due to the weight of the fluid. The pressure due to the fluid is equal to the weight of the fluid divided by the area. The weight of the fluid is equal to its mass times the acceleration due to gravity.

Since the density is constant, the weight can be calculated using the density:

[latex]w=mg=\rho Vg=\rho Ahg.[/latex]

The pressure at the bottom of the container is therefore equal to atmospheric pressure added to the weight of the fluid divided by the area:

[latex]p={p}_{0}+\frac{\rho Ahg}{A}={p}_{0}+\rho hg.[/latex]

This equation is only good for pressure at a depth for a fluid of constant density.

Pressure at a Depth for a Fluid of Constant Density

The pressure at a depth in a fluid of constant density is equal to the pressure of the atmosphere plus the pressure due to the weight of the fluid, or

[latex]p={p}_{0}+\rho hg,[/latex]

Where p is the pressure at a particular depth, [latex]{p}_{0}[/latex] is the pressure of the atmosphere, [latex]\rho[/latex] is the density of the fluid, g is the acceleration due to gravity, and h is the depth.

Example

What Force Must a Dam Withstand?

Consider the pressure and force acting on the dam retaining a reservoir of water (Figure). Suppose the dam is 500-m wide and the water is 80.0-m deep at the dam, as illustrated below. (a) What is the average pressure on the dam due to the water? (b) Calculate the force exerted against the dam.

The average pressure p due to the weight of the water is the pressure at the average depth h of 40.0 m, since pressure increases linearly with depth. The force exerted on the dam by the water is the average pressure times the area of contact, [latex]F=pA.[/latex]

solution

1. The average pressure due to the weight of a fluid is
   [latex]p=h\rho g.[/latex]

   Entering the density of water from Figure and taking h to be the average depth of 40.0 m, we obtain

   [latex]\begin{array}{cc}\hfill p& =(40.0\,\text{m})({10}^{3}\,\frac{\text{kg}}{{\text{m}}^{3}})(9.80\frac{\text{m}}{{\text{s}}^{2}})\hfill \\ & =3.92\times {10}^{5}\,\frac{\text{N}}{{\text{m}}^{2}}=392\,\text{kPa}\text{.}\hfill \end{array}[/latex]
2. We have already found the value for p. The area of the dam is
   [latex]A=80.0\,\text{m}\times 500\,\text{m}=4.00\times {10}^{4}\,{\text{m}}^{2},[/latex]

   so that

   [latex]\begin{array}{}\\ \hfill F& =(3.92\times {10}^{5}\,{\text{N/m}}^{2})(4.00\times {10}^{4}\,{\text{m}}^{2})\hfill \\ & =1.57\times {10}^{10}\,\text{N}\text{.}\hfill \end{array}[/latex]

Significance

Although this force seems large, it is small compared with the [latex]1.96\times {10}^{13}\text{N}[/latex] weight of the water in the reservoir. In fact, it is only 0.0800% of the weight.

Check Your Understanding

If the reservoir in Figure covered twice the area, but was kept to the same depth, would the dam need to be redesigned?

Show Solution

The pressure found in part (a) of the example is completely independent of the width and length of the lake; it depends only on its average depth at the dam. Thus, the force depends only on the water’s average depth and the dimensions of the dam, not on the horizontal extent of the reservoir. In the diagram, note that the thickness of the dam increases with depth to balance the increasing force due to the increasing pressure.

Pressure in a static fluid in a uniform gravitational field

A static fluid is a fluid that is not in motion. At any point within a static fluid, the pressure on all sides must be equal—otherwise, the fluid at that point would react to a net force and accelerate.

The pressure at any point in a static fluid depends only on the depth at that point. As discussed, pressure in a fluid near Earth varies with depth due to the weight of fluid above a particular level. In the above examples, we assumed density to be constant and the average density of the fluid to be a good representation of the density. This is a reasonable approximation for liquids like water, where large forces are required to compress the liquid or change the volume. In a swimming pool, for example, the density is approximately constant, and the water at the bottom is compressed very little by the weight of the water on top. Traveling up in the atmosphere is quite a different situation, however. The density of the air begins to change significantly just a short distance above Earth’s surface.

To derive a formula for the variation of pressure with depth in a tank containing a fluid of density ρ on the surface of Earth, we must start with the assumption that the density of the fluid is not constant. Fluid located at deeper levels is subjected to more force than fluid nearer to the surface due to the weight of the fluid above it. Therefore, the pressure calculated at a given depth is different than the pressure calculated using a constant density.

Imagine a thin element of fluid at a depth h, as shown in Figure. Let the element have a cross-sectional area A and height [latex]\Delta y[/latex]. The forces acting upon the element are due to the pressures p(y) above and [latex]p(y+\Delta y)[/latex] below it. The weight of the element itself is also shown in the free-body diagram.

Since the element of fluid between y and [latex]y+\Delta y[/latex] is not accelerating, the forces are balanced. Using a Cartesian y-axis oriented up, we find the following equation for the y-component:

[latex]p(y+\Delta y)A-p(y)A-g\Delta m=0(\Delta y \gt 0).[/latex]

Note that if the element had a non-zero y-component of acceleration, the right-hand side would not be zero but would instead be the mass times the y-acceleration. The mass of the element can be written in terms of the density of the fluid and the volume of the elements:

[latex]\Delta m=|\rho A\Delta y|=\text{−}\rho A\Delta y\text{ }(\Delta y \gt 0).[/latex]

Putting this expression for [latex]\Delta m[/latex] into Figure and then dividing both sides by [latex]A\Delta y[/latex], we find

[latex]\frac{p(y+\Delta y)-p(y)}{\Delta y}=\text{−}\rho g.[/latex]

Taking the limit of the infinitesimally thin element [latex]\Delta y\to 0[/latex], we obtain the following differential equation, which gives the variation of pressure in a fluid:

[latex]\frac{dp}{dy}=\text{−}\rho g.[/latex]

This equation tells us that the rate of change of pressure in a fluid is proportional to the density of the fluid. The solution of this equation depends upon whether the density ρ is constant or changes with depth; that is, the function ρ(y).

If the range of the depth being analyzed is not too great, we can assume the density to be constant. But if the range of depth is large enough for the density to vary appreciably, such as in the case of the atmosphere, there is significant change in density with depth. In that case, we cannot use the approximation of a constant density.

Pressure in a fluid with a constant density

Let’s use Figure to work out a formula for the pressure at a depth h from the surface in a tank of a liquid such as water, where the density of the liquid can be taken to be constant.

We need to integrate Figure from [latex]y=0,[/latex] where the pressure is atmospheric pressure [latex]({p}_{0}),[/latex] to [latex]y=\text{−}h,[/latex] the y-coordinate of the depth:

[latex]\begin{array}{ccc}\hfill {\int }_{{p}_{0}}^{p}dp& =\hfill & \text{−}{\int }_{0}^{\text{−}h}\rho gdy\hfill \\ \hfill p-{p}_{0}& =\hfill & \rho gh\hfill \\ \hfill p& =\hfill & {p}_{0}+\rho gh.\hfill \end{array}[/latex]

Hence, pressure at a depth of fluid on the surface of Earth is equal to the atmospheric pressure plus ρgh if the density of the fluid is constant over the height, as we found previously.

Note that the pressure in a fluid depends only on the depth from the surface and not on the shape of the container. Thus, in a container where a fluid can freely move in various parts, the liquid stays at the same level in every part, regardless of the shape, as shown in Figure.

Direction of pressure in a fluid

Fluid pressure has no direction, being a scalar quantity, whereas the forces due to pressure have well-defined directions: They are always exerted perpendicular to any surface. The reason is that fluids cannot withstand or exert shearing forces. Thus, in a static fluid enclosed in a tank, the force exerted on the walls of the tank is exerted perpendicular to the inside surface. Likewise, pressure is exerted perpendicular to the surfaces of any object within the fluid. Figure illustrates the pressure exerted by air on the walls of a tire and by water on the body of a swimmer.