哈里德物理学基础学习指导19TheKineticTheoryofGases.pdf

Chapter 19 The Kinetic Theory of Gases Gases consist of many molecules, often 1020 or more. Not all of these are travelling in the same direction, or even at the same speed. How can we deal with so many objects? When the numbers are that large, statistics works very, very well. Whatever a single molecule does, it typically travels only a short distance (micrometers at 25C and 1 atm) before colliding with another molecule, after which it has a new velocity. The average kinetic energy of an atom or molecule in a gas is: E^u -2r, 2 The total energy of a large number of atoms is: Ein -n (i*) -n(;) The difference between the two forms is that the first form uses the number of atoms (N) and the second uses the number of moles of atoms n. A mole is Avogadros number, Na : 6.022 x 1023 atoms, so n-NlNe, and R-Nok A mole is significant because one mole of atoms has a mass in grams equal to the mass number of the atom, so one mole of nitrogen, with mass number L4, has a mass of 14 grams. We can evaluate the sum of all atoms or molecules in a gas using the ideal gas law. pV -nRT:NkT Again it comes in two forms, one for atoms (or molecules) and one for moles of atoms. k - 1.3807x 10-2t JIK is the Boltzmann constant, and R - 8.3145 J/mol . K is the gas constant. Because T is a temperature and not a temperature difference, it must be in kelvin. Our goal is to calculate the change in energy AE, the heat flow Q, and the work done I4l. There are many specific processes that appear often, and that can aid us in our calculations. The chart on page I49 shows these important processes. An isothermal process is one that occurs at a constant temperature. Because the tem