ch15 - Chapter 15 Thermodynamics 15.1 Thermodynamic Systems...

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Chapter 15 Thermodynamics
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15.1 Thermodynamic Systems and Their Surroundings Thermodynamics is the branch of physics that is built upon the fundamental laws that heat and work obey. The collection of objects on which attention is being focused is called the system , while everything else in the environment is called the surroundings . Walls that permit heat flow are called diathermal walls , while walls that do not permit heat flow are called adiabatic walls . To understand thermodynamics, it is necessary to describe the state of a system .
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15.2 The Zeroth Law of Thermodynamics Two systems are said to be in thermal equilibrium if there is no heat flow between then when they are brought into contact. Temperature is the indicator of thermal equilibrium in the sense that there is no net flow of heat between two systems in thermal contact that have the same temperature.
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15.2 The Zeroth Law of Thermodynamics THE ZEROTH LAW OF THERMODYNAMICS Two systems individually in thermal equilibrium with a third system are in thermal equilibrium with each other.
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15.3 The First Law of Thermodynamics Suppose that a system gains heat Q and that is the only effect occurring. Consistent with the law of conservation of energy, the internal energy of the system changes: Q U U U i f = - = Heat is positive when the system gains heat and negative when the system loses heat.
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15.3 The First Law of Thermodynamics If a system does work W on its surroundings and there is no heat flow, conservation of energy indicates that the internal energy of the system will decrease: W U U U i f - = - = Work is positive when it is done by the system and negative when it is done on the system.
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15.3 The First Law of Thermodynamics THE FIRST LAW OF THERMODYNAMICS The internal energy of a system changes due to heat and work: W Q U U U i f - = - = Work is positive when it is done by the system and negative when it is done on the system. Heat is positive when the system gains heat and negative when the system loses heat.
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15.3 The First Law of Thermodynamics Example 1 Positive and Negative Work In part a of figure, the system gains 1500J of heat and 2200J of work is done by the system on its surroundings. In part b , the system also gains 1500J of heat, but 2200J of work is done on the system. In each case, determine the change in internal energy of the system.
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15.3 The First Law of Thermodynamics (a) (b) ( 29 ( 29 J 700 J 2200 J 1500 - = + - + = - = W Q U ( 29 ( 29 J 3700 J 2200 J 1500 + = - - + = - = W Q U
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15.3 The First Law of Thermodynamics Example 2 An Ideal Gas The temperature of three moles of a monatomic ideal gas is reduced from 540K to 350K as 5500J of heat flows into the gas. Find (a) the change in internal energy and (b) the work done by the gas. nRT U 2 3 = W Q U U U i f - = - =
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15.3 The First Law of Thermodynamics ( 29 ( 29 ( 29 ( 29 J 7100 K 540 K 350 K mol J 31 . 8 mol 0 . 3 2 3 2 3 2 3 - = - = - = i f nRT nRT U ( 29 J 12600 J 7100 J 5500 = - - = - = U Q W (a) (b)
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15.4 Thermal Processes A quasi-static process is one that occurs slowly enough that a uniform temperature and pressure exist throughout all regions of the system at all times.
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