Download 3 Using Heat

Name
CHAPTER 14
Class
Date
Heat and Temperature
SECTION
3 Using Heat
KEY IDEAS
As you read this section, keep these questions in mind:
• What is thermodynamics?
• What are the first and second laws of
thermodynamics?
• What is a heat engine?
What Is Thermodynamics?
People use many machines, such as air conditioners
and automobile engines, to move energy from place
to place. Scientists use the laws of thermodynamics to
describe how energy moves. Thermodynamics is the study
of how energy moves and changes in different situations.
READING TOOLBOX
Find Examples As you read,
make a list of examples of
the first and second laws of
thermodynamics.
THE FIRST LAW OF THERMODYNAMICS
The first law of thermodynamics states that the total
energy in any process always remains the same. Another
way to say this is that energy cannot be created or
destroyed.
Remember that work is the transfer of energy from
one object to another. In many cases, work increases the
kinetic energy of an object. For example, if you pick up
and throw a baseball, you have done work on the baseball. The figure below shows how chemical energy in
your body is transferred to kinetic energy in the baseball.
Processes, such as throwing a baseball, in which energy
is transferred as work are called mechanical processes.
READING CHECK
1. Describe What is the first
law of thermodynamics?
Your body stores energy from food
in chemical compounds.
Your muscles turn chemical energy into kinetic energy
as they contract to throw the baseball.
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Your muscles do work on the baseball,
causing the baseball to move.
2. Review Concepts If you
hold the baseball up but do
not throw it, what kind of
energy does the baseball
have?
The baseball’s kinetic energy increases when it moves.
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Heat and Temperature
Name
SECTION 3
Class
Date
Using Heat continued
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3. Apply Concepts If the
entropy of one part of the
universe decreases, what
must happen to the entropy
of the rest of the universe?
THE SECOND LAW OF THERMODYNAMICS
The second law of thermodynamics states that the
entropy of the universe is always increasing. Entropy is
a measure of how disordered a system is. The more wellordered a system is, the lower its entropy. For example,
look at the house of playing cards in the left-hand figure
below. The cards are very well-ordered. They are neatly
stacked and carefully balanced. The entropy of the system (the house of cards) is low.
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4. Describe What happens
to the entropy of the cards
when they are knocked over?
READING CHECK
5. Explain How can you
decrease the entropy of a
low-energy system?
This house of cards is a low-entropy
system.
When the cards fall into a random pile,
they are less well-ordered. The entropy
of the system is higher.
Now, look at the photo on the right. The house of
cards has been knocked down. The cards are more
disordered. Therefore, the entropy of the system has
increased.
In many cases, highly ordered systems are also high
in energy. Remember that you can transfer energy to a
system by doing work on it. Therefore, you can often do
work on a system to decrease its entropy. For example,
you can do work on the playing cards to create a house
of cards. However, the entropy of a larger system—such
as the universe—must increase when this happens.
What Is Usable Energy?
Energy that can be used to do work is called usable
energy. Many machines that people use transform one
form of usable energy into another. However, the amount
of usable energy after a transformation is always less
than it was before the transformation.
For example, air conditioners use electrical energy to
move heat out of a room. The total amount of energy in
the system stays the same. However, the amount of heat
energy an air conditioner moves is less than the amount
of electricity it uses.
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Heat and Temperature
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Using Heat continued
SECTION 3
What Is a Heat Engine?
A heat engine is a machine that uses heat to do work.
It changes chemical energy into kinetic energy. This happens by a process called combustion. During combustion,
fuel is burned to produce energy.
There are two main types of heat engines. If an engine
burns fuel outside the engine, it is called an externalcombustion engine. A steam engine is an example of an
external-combustion engine. If a heat engine burns fuel
inside the engine, it is an internal-combustion engine.
This is the type of engine found in most cars and trucks.
The figure below shows how an internal-combustion
engine works.
1
During the intake
stroke, the piston
moves downward. Air
and fuel move into the
cylinder.
2
During the compression stroke, the piston
moves upward. This
compresses the mixture
of fuel and air in the
cylinder.
Intake valve
Cylinder
Piston
1
2
3
Spark plug
4
Exhaust valve
Crankshaft
3
4
During the power
stroke, the spark plug
creates a spark, which
causes the fuel and air
to burn. This produces
pressure inside the
cylinder, causing
the piston to move
downward.
READING CHECK
6. Define What is a heat
engine?
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7. Identify What causes
the pistons in an internalcombustion engine to move?
8. Describe What happens
during the power stroke?
During the exhaust
stroke, the piston
moves up again. This
forces the waste gases
to move out of the cylinder. Then, the piston
moves down, and the
cycle starts over.
In an internal-combustion engine, not all of the
chemical potential energy in the fuel is changed into
kinetic energy. Much of the energy becomes heat, which
moves into the air. The heat cannot be used to do work.
Therefore, it is not usable energy.
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Heat and Temperature
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Section 3 Review
SECTION VOCABULARY
entropy a measure of the randomness or
disorder of a system
heat engine a machine that transforms heat into
mechanical energy, or work
1. Identify Give two examples of high-entropy systems and describe how you could
decrease the entropy of each system.
2. Define Write the second law of thermodynamics in your own words.
3. Explain How do heat engines change chemical energy into kinetic energy?
4. Compare In terms of using energy, how are your muscles and heat engines
similar?
5. Apply Concepts A heat engine has an energy input of 300 J. Its output of use-
ful energy is 150 J. What is the total output of non-useful energy from the heat
engine? Explain how you know.
6. Describe What happens during each of the four strokes in the cycle of an internal-
combustion engine?
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Interactive Reader
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Heat and Temperature