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UNIT VI STUDY GUIDE
Properties of Matter II
Course Learning Outcomes for Unit VI
Upon completion of this unit, students should be able to:
5. Identify the building blocks of matter to include their influence on physical properties.
5.1 Describe various fluid dynamics terminologies within certain applications.
5.2 Differentiate between atmospheric pressure and liquid pressure.
5.3 Describe ideal gas law for various practical applications.
Chapter 13: Liquids
Chapter 14: Gases
Unit Lesson
What Are Fluids?
Unlike solids, liquids and gases do not have a definite shape unless they are kept in a container. Something is
referred to as fluid when matter can flow without specific configuration. Both liquids and gases are examples
of fluids. Both water and air move from place to place if they are not confined in a vessel. Both of them show
a similar pattern of motion, but their density is very different because it depends on the property of matter.
Usually, gases have smaller densities than liquids have because the average distance between molecules is
greater in gases than in liquids. The density of gases is greatly dependent on pressure and temperature.
Pressure in Fluids
When a force (F) acts on an area (A) in a fluid, the pressure (P) can be expressed as P=F/A. Here, we only
consider the magnitude of F, so P is a scalar and its unit is the Pascal (N/m 2 =Pa). Atmospheric pressure at
sea level is about 101,300 Pa =1 atm.
You may experience greater pressure as you go deeper in a swimming pool or in an ocean. What is the
relation between pressure and depth? Let’s consider one column of water with height h below figure in a large
swimming pool. The area (A) of the top face is the same as that of the bottom face. The pressure (P t) on the
top face creates a downward force, or PtA. The pressure (Pb) creates an upward force, or PbA. Also, the
weight (mg) due to gravity points downward.
Pt A
mg
h
Pb A
PHS 1110, Principles of Classical Physical Science
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Water is at rest, and thus its acceleration is zero. That is, the column is in equilibrium. We can apply Newton’s
second law, and the summation of the vertical forces is zero: PbA-PtA-mg=0. Use m=ρV=ρAh. Then, Pb=Pt +
ρgh. You can clearly see that the pressure at a deeper level is greater than the pressure at a shallow level if
the density is not changing or if it is incompressible. In the case of gas, the density varies according to the
vertical distances or if it is compressible, the formula only works when h is very small. For instance, the
density of our atmosphere varies significantly from the earth’s surface to higher altitudes. The important thing
is that the pressure difference between lower and higher levels comes from the height, or the vertical
distance, not the horizontal distance within the fluid. See Figure 13.5 on p. 247 in the textbook.
Pascal’s and Archimedes’ Principles
Pascal’s principle states that any externally
applied pressure is transmitted undiminished to
everywhere in a completely enclosed fluid at
rest (Cutnell & Johnson, 2004). This is the same
analysis of the above equation: Pb=Pt + ρgh.
The bottom pressure is equal to the sum of top
pressure, which is the externally applied
pressure, and the static fluid pressure due to the
weight of the fluid. For instance, this is the case
with the mechanism of a hydraulic car lift when
the static fluid pressure is zero (See Figure
13.23 in p.256 in the textbook).
You may experience that it is very hard to push
a beach ball under the surface of the water. The
water, in fact, pushes back, and this upward
force is call the buoyant force; its cause is the
pressure of fluids, and its magnitude depends
Water towers use Pascal’s law to provide water to homes.
on depth. In the figure to the left, the net upward
force is called the buoyant force F= PbA-PtA=
ρghA=mg=weight. Notice that the buoyant force does not depend on the shape of the object. Archimedes
discovered this property more than 2,000 years ago. Archimedes’ principle states that a fluid exerts a buoyant
force to an immersed object. The magnitude of the buoyant force equals the weight of the displaced fluid.
Bernoulli’s Equation and Its Application
When fluids are in motion, they move in a variety of ways. You may have observed that water flows calmly in
a shallow stream and violently in a steep valley. The air blows sometimes very gently and sometimes
vigorously with great speed. In order to characterize the type of fluids, compressible/incompressible and
viscous/non-viscous categories are used. When the density of a fluid is almost constant, the fluid is said to be
incompressible. Luckily, most liquids are incompressible, but all gases are not.
When a fluid does not flow easily, like honey, it is a viscous fluid. On the other hand, water flows very easily
because its viscosity is very small. Incompressible and non-viscous fluids are called ideal fluids; this is great
to describe the motion of fluids with mathematical equations. For steady flow, Bernoulli studied the behavior
of ideal fluids. Its result is in his equation: P+1/2 ρv2 + ρgy=constant. In this equation, y is the elevation at any
point and v is the fluid speed. If the flow speed is not changing, the above equation goes back to the pressure
equation when water is at rest. If the flow is horizontal, that is, there is no elevation between two points, the
pressure is related with the speed. The higher fluid pressure makes the slow moving flow and vice versa. In
addition, when the volume flow rate Av=constant, if the cross-sectional area of a tube is large, the fluid speed
is small and vice versa.
With these fluid equations, we can describe the motion of liquids in a plumbing system, the speed changes of
oil in a pipe, and even the dynamics of an airplane wing. For more examples, see Figure 14.19 to Figure 14.
24 on pp. 274–275 in the textbook.
PHS 1110, Principles of Classical Physical Science
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Plasma
Most atoms are electrically neutral in nature; however, they can gain or
lose electrons through various interactions. When atoms have positive
or negative charges, they are called ions. Plasma is an ionized gas, and
it interacts easily with an electromagnetic field. Plasma has no definite
shape like gas, but forms shapes under the influence of a magnetic field.
Lightning, auroras, fluorescent lamps, neon street lights, and the sun are
examples of the plasma phenomena.
Ideal Gas Law
When a gas has a very low density (the average distance between
molecules of the gas is very large), it is called an ideal gas. The ideal
gas law is the relationship between pressure, volume, and temperature.
More exactly, the absolute pressure (P) of an ideal gas is proportional to
the temperature and is inversely proportional to the volume (V): P
=constant *T/V
Plasma gas ball
(Colin, 2013)
If the temperature is not changing, that is, T=constant, the pressure P is inversely proportional to the volume
V of the gas: PV=constant. This is called Boyle’s law. See Figure 14.13 on p. 270 in the textbook.
Similarly, if the pressure P is constant, the volume V is proportional to the temperature T: V/T=constant. This
is called Charles’ law.
References
https://commons.wikimedia.org/wiki/File:Plasma_globe_60th.jpg
Cutnell, J., & Johnson, K. (2004). Physics (6th ed.). Hoboken, NJ: Wiley.
Stern, D. P. (2001). #7H. Plasma physics – history. Retrieved from http://wwwspof.gsfc.nasa.gov/Education/whplasma.html
In this video, the speaker uses interactive experiments to demonstrate the properties of plasma. She also
explains its important uses in our everyday lives.
TEDx Talks. (2014,Nov. 3). The fourth state of matter – plasma [Video file]. Retrieved from

Nongraded Learning Activities are provided to aid students in their course of study. You do not have to submit
them. If you have questions, contact your instructor for further guidance and information.
To practice what you have learned in this unit, complete the following problems and questions from the
textbook. The answers to each problem can be found in the “Odd-numbered Answers” section in the back of
the textbook. The question number from the textbook is indicated in parentheses after each question.
1. The depth of water behind the Hoover Dam is 220 m. Show that the water pressure at the base of this
dam is 2200 kPa. (Textbook #35 on p. 260)
PHS 1110, Principles of Classical Physical Science
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2. A 12-kg piece of metal displaces 2 L of water when submerged. Show that its density is 6000 kg/m 3.
How does this compare with the density of water? (Textbook #39 on p. 260)
3. Why does your body get more rest when you’re lying down than when you’re sitting? Is blood
pressure in your legs greater? (Textbook #55 on p. 261)
4. A ship sailing from the ocean into a freshwater harbor sinks slightly deeper into the water. Does the
buoyant force on the ship change? If so, does it increase or decrease? (Textbook #86 on p. 262)
5. A small, dry paper clip can rest on the surface of still water. Why can’t a heavier paper clip do the
same without sinking? (Textbook #97 on p. 263)
6. What is the approximate mass of a column of air 1 cm 2 in area that extends from sea level to the
upper atmosphere? What is the weight of this amount of air? (Textbook #5 on p. 277)
7. Consider an airplane with a total wing surface of 100 square meters. At a certain speed the difference
in the air pressure below and above the wings is 4% of atmospheric pressure. Show that the lift of the
airplane is 400,000 N. (Textbook #43 on p. 279)
8. When an air bubble rises in water, what happens to its mass, volume, and density? (Textbook #55 on
p. 280)
9. What physics principle underlies these three observations? When passing an oncoming truck on the
highway, your car tends to sway toward the truck. The canvas roof of a convertible car bulges upward
when the car is traveling at high speeds. The windows of older trains sometimes break when a highspeed train passes by on the next track. (Textbook #91 on p. 281)
10. When boarding an airplane, you bring a bag of chips (or any other item packaged in an airtight foil
package) and, while you are in flight, you notice that the bag puffs up. Discuss why this happens.
(Textbook #95 on p. 281)
PHS 1110, Principles of Classical Physical Science
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