Download Energy

Unit 2
Energy and Motion
• Chapter 3 ~ Energy
o Section 1 ~ What is Energy?
o Section 2 ~ Heat, Temperature, and Thermal
Energy
o Section 3 ~ The Transfer of Thermal Energy
Unit 2 covers the following framework standards: PS 13 and 14. Content was adapted the following:
McLaughlin, C. W., & Thompson, M. (1999). Physical science. Columbus, Ohio/USA:
Glencoe/McGraw-Hill.
16 Chapter 3: Energy
Section 3.1
What is Energy?
Terms:
• Energy
• Kinetic Energy
• Potential Energy
• Work
• Mechanical Energy
• Law of Conservation of Energy
You have seen several examples of energy being used today. Just about
everything you see or do involves energy. Energy is a bit mysterious—you
can’t smell and in most cases you can’t even see it. Light if one form of
energy you can see. You can’t see electricity, another form of energy, but
you can see its effects in a glowing light bulb and you can feel its effects in
the heat produced by the coils of a toaster. You can’t see energy in the
food you eat, but you can see and feel its effects when your muscles use
that energy to move.
Energy—An Agent of Change
If a baseball flies through the air and
shatters a window, it certainly changes the window! When an object is able
to change its environment, we say the object has energy. The baseball had
energy and did work on the window by causing the window to move. In
summary, the use of energy involves change.
Scientists have some difficulty defining energy because it exists in so
many different forms. Some of these forms include radiant, electrical,
chemical, thermal, and nuclear energy. Traditionally, energy has been
defined as the ability to do work—to cause something to move. But when
work is performed, there is always a change. This connection offers a
useful general definition. Energy is the ability to do work, or cause change.
Kinetic Energy: On the Move
Usually, when you think of energy,
you think of action—of some motion taking place. Kinetic energy is
energy in the form of motion. A spinning bicycle wheel, a flying Frisbee, or
cross-country runners all have kinetic energy. How much kinetic energy
something has depends on the mass and velocity of the moving object.
The greater the mass a moving object has, the more kinetic energy it has.
17 Similarly, the greater an object’s velocity, the more kinetic energy it has.
The truck traveling at 100km/hr in the figure below has more kinetic energy
than the motorcycle traveling at the same speed. However, the motorcycle
has more kinetic energy than an identical motorcycle traveling at 80 km/hr.
Potential Energy
Energy doesn’t have to involve
motion. Even motionless, any sample of matter may have stored energy
that gives it the potential to cause change if certain conditions are met.
Potential energy is stored energy. The amount of potential energy a
sample of matter has depends on its position or its condition. For example,
a flowerpot sitting on a second-floor windowsill has potential energy due to
its position. If something knocks the flowerpot off the windowsill, gravity will
cause it to fall toward the ground. As it falls, its potential energy changes to
kinetic energy.
There are two factors to keep in mind when deciding whether work is being
done: something has to move, and the motion must be in the same
direction as the applied force. If you pick up a pile of books from the floor,
work is done on the books. They move upward, in the direction of the
applied force. If you hold the books in your arms, no work is done on the
books. Some upward force is still being applied (to keep the books from
18 falling), but no movement is taking place. Even if you carry the books
across the floor at a constant speed, no work is done on the books. The
force being applied to the books is still upward, or vertical, but your motion
across the floor is sideward, or horizontal. In the following paragraphs, you
will examine the energy changes that result from the motion of a swing.
Conservation of Energy
Perhaps you have ridden on a playground
swing like the one in the picture. Try to
remember what it was like swinging back
and forth, high and low. Now think about
the energy changes involved in such a
ride, illustrated in the Figure right. The
ride starts with a push to get you
moving—to give you some kinetic energy.
As the swing rises, kinetic energy
changes to potential energy of position. At
the top of each swing, potential energy is
greatest. Then, as the swing moves
downward, potential energy changes to
kinetic energy. At the bottom of each swing, kinetic energy is greatest and
potential energy is at its minimum. As the swing continues to move back
and forth, energy is converted from kinetic to potential to kinetic, over and
over again. Taken together, the potential and kinetic energy of the swing
make up its mechanical energy. Mechanical energy is the total amount of
kinetic and potential energy in a system.
19 Conserving Energy—A Natural Law
Scientists have learned that in
any given situation, energy may change from one form to another, but the
total amount of energy remains constant. In other words, energy is
conserved. This fact is recognized as a law of nature. According to the law
of conservation of energy, energy may change form but it cannot be
created or destroyed under ordinary conditions. This law applies to closed
systems, in which energy cannot enter or leave the system.
Suppose the law of conservation of energy is applied to the swing. Would
you expect the swing to continue moving back and forth forever? You
know this doesn’t happen. The swing slows down and comes to a stop.
Where does the energy go? If you think about it, friction and air resistance
are always acting on the swing and rider. These unbalanced forces cause
some of the mechanical energy of the swing to change to thermal
energy—heat. With every pass of the swing, the temperatures of the
swing, the rider, and the air around them go up a little bit. So the
mechanical energy of the swing isn’t lost, it is converted to thermal energy.
Thermal energy is discussed in the next section.
Summary:
• Energy is the ability to do work, or cause change. Energy exists in
many different forms. Two of these forms are potential and
kinetic energy. There are countless examples of how energy is
converted from potential to kinetic, and vise versa.
• Kinetic energy is energy in the form of motion.
• Potential energy is stored energy. Potential energy depends on the
position of an object.
• Energy can change from form to other forms with no loss of total
energy.
• Work is the transfer of energy through motion. Work is done only
when force produces motion in the direction of the force.
• The Law of Conservation of Energy states that energy is not
created or destroyed when changing forms in a closed system.
20 Section 3.2
Temperature, Heat, and Thermal Energy
Terms:
• Temperature
• Thermal energy
• Heat
• Joules
• Work
Temperature
What do you know about temperature?
When the air temperature outside is high, you probably describe the
weather as hot. Ice cream, which has a lower temperature, feels cold. The
words hot and cold are commonly used to describe the temperature of a
material. Although not very scientific, these terms can be useful. Just about
everyone understands that hot indicates high temperature and cold
indicates low temperature.
When most people think of temperature, they automatically think of heat,
too. This association makes sense, because heat and temperature are
related. However, they are not the same. To understand the relationship
between heat and temperature, you need to know about matter.
Measuring Temperature
Temperature is one of the most important
factors affecting the weather. Air temperature is usually measured with
a thermometer. A thermometer is a thin glass tube with a bulb on the end
that contains a liquid, usually mercury or colored alcohol. Thermometers
work because liquids expand when they are heated and contract when
they are cooled. When the air temperature increases, the temperature of
the liquid in the bulb also increases. This causes the liquid to expand and
rise up the column.
Temperature Scales
Temperature is measured in units called
degrees. Two temperature scales are commonly used: the Celsius scale
and the Fahrenheit scale. Scientists use the Celsius scale. On the Celsius
scale, the freezing point of pure water is 0°C and the boiling point of pure
water at sea level is 100°C. Weather reports in the United States use the
Fahrenheit scale. On the Fahrenheit scale, the freezing point of water is
32°F and the boiling point is 212°F. A third scale that is referenced in
science is the Kelvin scale. Zero on the Kelvin scale represents absolute
zero—a point that matter does not move at all. No matter how close
scientists can get a particle to absolute zero, it is impossible! Remember,
21 every substance has particles that are
in motion.
Matter in Motion
All
matter is made up of particles so small
that you can’t see them, even with an
ordinary microscope. The particles that
make up any object are constantly
moving, even if the object itself
appears perfectly still. Everyone you
can think of—the book on your desk,
the shoe on your foot, even the foot
inside your shoe—is made up of
moving particles.
You know that moving things have
kinetic energy. Because the particles
that make up matter are in constant
motion, you can conclude that they
have kinetic energy. The faster the
particles move, the more kinetic energy
they have. Temperature is a measure
of the average kinetic energy of the
particles in a sample of matter. As the
particles in an object move faster and
their average kinetic energy becomes
greater, the temperature of the object
rises. Similarly, as the particles in an
object move more slowly, their average
kinetic energy decreases. Which
particles are moving faster, those in pot
A or those in pot B?
Thermal Energy
If
you
place an ice-cold teaspoon on top of a
scoop of ice cream, like the image left,
nothing will happen. But suppose you place a hot teaspoon on the ice
cream. Now the ice cream under the spoon stars to melt. The hot spoon
causes the ice cream to change
because it transfers energy to
the ice cream. Where does this
energy come from? The spoon
isn’t moving, and its position is
the same as that which the cold
22 spoon had. So the energy is not due to the motion or position of the spoon.
Instead, it is related to the temperature of the spoon.
The change in the ice cream is caused by the flow of thermal energy from
the spoon. Like temperature, thermal energy is related to the energy of the
particles that make up matter. Thermal energy is the total energy of the
particles in a material. This total includes both kinetic and potential energy.
The kinetic energy is due to the vibrations and movements within and
between particles. Forces that act within or between the particles
determine the potential energy. For example, suppose you stack two hot
teaspoons at the same temperature on a scoop of ice cream. What will
happen? The two spoons will melt more ice cream than the single spoon
did in the same amount of time. The stacked spoons have twice as much
mass, and therefore, twice as many moving particles. The more mas a
material has at the same temperature, the more thermal energy it has.
Different kinds of matter have different thermal energies, even when mass
and temperature are the same. For example, a 5-g sample of sand has a
different thermal energy than a 5-g sample of pudding at the same
temperature. This difference is due mainly to the ways in which the
particles of the materials are bound together. It is important to remember
that the thermal energy of a material depends on the total kinetic energy of
its particles. The kinetic energy of the object itself has no effect on its
thermal energy. For example, at 20°C, a golf ball has the same thermal
energy whether it’s sitting on the ground or speeding through the air.
Heat
What would happen if you pressed your left
hand against a cool tile wall? Your hand would feel cooler. Its temperature
would have decreased. If you then touched the same spot on the wall with
your right hand, the spot wouldn’t feel as cool as it did when you touched
in originally. The temperature of the spot would have increased when you
touched it with your left hand. Energy flowed from your warm left hand to
the cool tile. Heat is the transfer of thermal energy that flows from
something with a higher temperature to something with a lower
temperature. Remember that, in most cases, heat flows from warmer to
cooler materials—not from cooler to warmer.
The next time you listen to a weather report, think about the difference
between temperature and heat. Or when you drop an ice cube into your
drink, think about how the cooling occurs. Does the melting ice cause the
liquid to cool? Or does heat flowing form the liquid to the ice cool the drink
and cause the ice to melt?
23 Like work, heat is measured in joules and involves transfer of energy.
Heat is energy transferred between objects at different temperatures.
Work is energy transferred when a force acts over a distance.
Summary:
• Recognize that heat is a form of energy and that temperature
change results from adding or taking away heat from a system.
• Temperature is a measure of the average kinetic energy of the
particles in an object.
• Thermal energy is the total energy of the particles in a material.
This total includes both kinetic and potential energy.
• Heat is the transfer of thermal energy that flows from something
with a higher temperature to something with a lower
temperature.
• Heat is measured in joules and involves transfer of energy.
• Work is energy transferred when a force acts over a distance.
24 Section 3.3
The Transfer of Thermal Energy
Terms:
•
•
•
•
•
Conduction
Fluid
Conductor
Radiation
Convection
Thermal energy moves from place to place in three forms: conduction,
convection, and radiation.
Conduction is the transfer of energy through matter by direct contact of
particles. Recall that all matter is made up of tiny particles that are in
constant motion. The temperature of a material is a measure of the
average kinetic energy of its particles.
Transfer by Collisions
Energy is transferred when particles
moving at different speeds bump into each other. When faster-moving
particles collide with slower-moving particles, some of the momentum of
the faster-moving particles passes along to the slow-moving particles. The
faster particles slow down and the slower particles move up.
Heat may be transferred by conduction through a given material or from
one material to another. Think about what happens when one end of a
metal spoon is placed in boiling water. (Figure below.) Heat from the
water is transferred to the spoon. The end of the spoon in the water
becomes hotter than the other end of the spoon, but eventually the entire
spoon becomes hot.
Conduction takes place in
solids, liquids, and gases.
Because their particles
are
packed
closer
together, solids usually
conduct heat better than
liquids or gases. However,
some solids, such as the
pots in the image right,
conduct heat better than
others. Many metals have
loosely held electrons that
25 move around easily and transfer kinetic energy to nearby particles more
efficiently. Silver, copper, and aluminum are good heat conductors, while
woods, plastic, glass, and fiberglass are poor conductors of heat. Why do
you thin cooking pots are made of metal? What are the handles usually
made of?
Convection Liquids
and
gases differ from solids
because they flow. Any
material that flows is a fluid.
The most important way that
thermal energy is transferred
in fluids is by convection.
Convection is the transfer of
energy by the bulk movement
of matter. This differs from
conduction because in conduction, energy moves from particle to particle,
but the particles themselves remain approximately in place. Alternatively,
in convection, fluid particles move from one location to another, carrying
energy with them.
When heat is added to a fluid, the particles begin to move faster, just as
the particles of a solid do. However, the particles of a fluid have more
freedom to move. Therefore, they move farther apart, or expand. Consider
a hot-air balloon being blow up. Heat from the flame causes the air inside
the balloon to expand.
Transfer by Currents
Now think about what happens when a pot
of water is heated. The stove burner heats the bottom of the pot by
conduction. Water touching the bottom of the pot is also heated by
conduction. As this water is heated, it expands and becomes less dense.
Cooler, denser water at the top of the pot sinks and pushes the hot water
upward. As the hot water rises, it cools by conduction, becomes denser,
and sinks, forcing warmer water to rise. This movement creates convection
currents. These currents transfer thermal energy from warmer to cooler
parts of the fluid. Winds and some ocean currents are examples of
convection currents.
Radiation
There are 150 million kilometers of empty
space between the Earth and the sun. How does thermal energy reach
Earth? In order for conduction or convection to take place, matter must be
present. But there is almost no matter in outer space! There is a third type
of heat transfer that does not require matter—radiation. Radiation is the
transfer of energy in the form of waves. Energy that travels by radiation is
26 often called radiant energy. Once radiant energy from the sun reaches
Earth, some of it is absorbed. Only radiant energy that is absorbed
changes to thermal energy.
Different materials absorb radiant energy differently. Shiny metals reflect
radiant energy; dull materials absorb it. Dark-colored materials absorb
more radiant energy than light-colored materials. This explains why
summer clothing is usually made of lighter-colored materials, and winter
clothing is darker colored. Any object warmer than 0 K emits radiation. If
you hold your hand near a lighted electric bulb, your hand feels the heat.
The bulb’s radiation is converted to thermal energy as your hand absorbs
it.
Summary:
• Thermal energy moves from place to place in three forms:
conduction, convection, and radiation.
• Conduction involves direct contact between molecules.
• Convection involves fluids.
• Radiation is energy in the form of waves.
27