Chapters 16, 17 Temperature, Heat, and the Thermal Behavior of Matter

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  • Chapters 16, 17 Temperature, Heat, and the Thermal Behavior of Matter
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  • Temperature Thermodynamics branch of physics studying thermal energy of systems Temperature ( T ), a scalar measure of the thermal (internal) energy of a system SI unit: K (Kelvin) Kelvin scale has a lower limit (absolute zero) and has no upper limit William Thomson (Lord Kelvin) (1824 - 1907)
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  • Kelvin scale Kelvin scale is defined by the temperature of the triple point of pure water Triple point set of pressure and temperature values at which solid, liquid, and gas phases can coexist International convention: T of the triple point of water is
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  • The zeroth law of thermodynamics If two (or more) bodies in contact dont change their internal energy with time, they are in thermal equilibrium 0th law of thermodynamics: if bodies are in thermal equilibrium, their temperatures are equal
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  • Measuring temperature Temperature measurement principle: if bodies A and B are each in thermal equilibrium with a third body C, then A and B are in thermal equilibrium with each other (and their temperatures are equal) The standard temperature for the Kelvin scale is measured by the constant-volume gas thermometer
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  • Constant-volume gas thermometer
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  • Celsius and Fahrenheit scales Celsius scale: Fahrenheit scale: Anders Cornelius Celsius (1701 - 1744) Gabriel Daniel Fahrenheit (1686 - 1736)
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  • Temperature and heat Heat ( Q ): energy transferred between a system and its environment because of a temperature difference that exists between them SI Unit: Joule Alternative unit: calorie (cal):
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  • Absorption of heat Specific heat ( c ): heat capacity per unit mass Common states (phases) of matter: solid, liquid, gas Latenet heat ( L ): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.) QQ
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  • Absorption of heat QQ
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  • Chapter 17 Problem 25 How much energy does it take to melt a 65-g ice cube?
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  • Heat transfer mechanisms Thermal conduction Conduction rate: Thermal resistance: Conduction through a composite rod: Thermal conductivity
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  • Absorption of heat
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  • Heat transfer mechanisms Thermal radiation Radiation rate: Stefan-Boltzmann constant: Absorption rate: Josef Stefan (1835-1893) Emissivity
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  • Heat transfer mechanisms Convection
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  • Heat transfer mechanisms
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  • Chapter 16 Problem 35 An oven loses energy at the rate of 14 W per C temperature difference between its interior and the 20C temperature of the kitchen. What average power must be supplied to maintain the oven at 180C?
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  • Avogadros number Mole amount of substance containing a number of atoms (molecules) equal to the number of atoms in a 12 g sample of 12 C This number is known as Avogadros number ( N A ): N A = 6.02 x 10 23 mol -1 The number of moles in a sample N total number of atoms (molecules) m total mass of a sample, m 0 mass of a single atom (molecule); M molar mass Amedeo Avogadro (1776 -1856)
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  • Ideal gases Ideal gas a gas obeying the ideal gas law: R gas constant R = 8.31 J/mol K k B Boltzmann constant k B = 1.38 x 10 23 J/K Ludwig Eduard Boltzmann (1844-1906)
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  • Ideal gases The gas under consideration is a pure substance All molecules are identical Macroscopic properties of a gas: P, V, T The number of molecules in the gas is large, and the average separation between the molecules is large compared with their dimensions the molecules occupy a negligible volume within the container The molecules obey Newtons laws of motion, but as a whole they move randomly (any molecule can move in any direction with any speed)
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  • Ideal gases The molecules interact only by short-range forces during elastic collisions The molecules make elastic collisions with the walls and these collisions lead to the macroscopic pressure on the walls of the container At low pressures the behavior of molecular gases approximate that of ideal gases quite well
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  • Ideal gases
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  • Root-mean-square (RMS) speed:
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  • Translational kinetic energy Average translational kinetic energy: At a given temperature, ideal gas molecules have the same average translational kinetic energy Temperature is proportional to the average translational kinetic energy of a gas
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  • Internal energy For the sample of n moles, the internal energy: Internal energy of an ideal gas is a function of gas temperature only
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  • James Clerk Maxwell (1831-1879) Distribution of molecular speeds Not all the molecules have the same speed Maxwells speed distribution law: N v dv fraction of molecules with speeds in the range from v to v + dv
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  • Distribution of molecular speeds Distribution function is normalized to 1: Average speed: RMS speed: Most probable speed:
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  • Thermal expansion Thermal expansion: increase in size with an increase of a temperature Linear expansion: Volume expansion:
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  • Thermal expansion
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  • Chapter 17 Problem 30 A copper wire is 20 m long on a winter day when the temperature is - 12C. By how much does its length increase on a 26C summer day?
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  • Questions?
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  • Answers to the even-numbered problems Chapter 16 Problem 22 2500 J/(kg K)
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  • Answers to the even-numbered problems Chapter 16 Problem 40 2.0 10 2 Pa/K
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  • Answers to the even-numbered problems Chapter 17 Problem 18 3.2 10 23
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  • Answers to the even-numbered problems Chapter 17 Problem 36 11 L


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