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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. EHHDBG@<EHL>K 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. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 315 Heat and Temperature Name SECTION 3 Class Date Using Heat continued 8g^i^XVaI]^c`^c\ 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. EHHDBG@<EHL>K 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. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 316 Heat and Temperature Name Class Date 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? EHHDBG@<EHL>K 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. Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 317 Heat and Temperature Name Class Date 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? Copyright © by Holt, Rinehart and Winston. All rights reserved. Interactive Reader 318 Heat and Temperature