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{ | ||
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"# Ideal gas law from first principles\n", | ||
"\n", | ||
"The partition function, $Q$ for $N$ particles in a 3-dimensional box is, \n", | ||
"\n", | ||
"$$\n", | ||
"Q = \\frac{V^N}{N!\\Lambda^{3N}},\n", | ||
"$$\n", | ||
"\n", | ||
"where, $V$ is the volume of the system and $\\Lambda$ is the de Broglie thermal wavelength, \n", | ||
"\n", | ||
"$$\n", | ||
"\\Lambda = \\bigg(\\frac{\\beta h^2}{2\\pi m}\\bigg)^{1/2}.\n", | ||
"$$\n", | ||
"\n", | ||
"The most useful way to work with the partition function is in its logarithmic form, \n", | ||
"\n", | ||
"$$\n", | ||
"\\ln Q = \\ln \\bigg(\\frac{V^N}{N!\\Lambda^{3N}}\\bigg) = \\ln V^N - \\ln N! - \\ln\\Lambda^{3N}.\n", | ||
"$$\n", | ||
"\n", | ||
"Using Stirling's approximation,\n", | ||
"\n", | ||
"$$\n", | ||
"\\ln Q = N\\ln V - (N\\ln N - N) - N\\ln\\Lambda^3,\n", | ||
"$$\n", | ||
"\n", | ||
"and substituting the de Broglie thermal wavelength,\n", | ||
"\n", | ||
"$$\n", | ||
"\\ln Q = N\\ln V - (N\\ln N - N) - \\frac{3}{2}N\\Bigg[\\ln \\beta + \\ln \\bigg(\\frac{\\beta h^2}{2\\pi m}\\bigg) \\Bigg].\n", | ||
"$$\n", | ||
"\n", | ||
"The partition function can give information about thermodynamics of the system, \n", | ||
"\n", | ||
"$$\n", | ||
"p = k_bT\\bigg(\\frac{\\partial\\ln Q}{\\partial V}\\bigg)_T,\n", | ||
"$$\n", | ||
"\n", | ||
"This means, using the version of the partition function outlined above we can obtain the ideal gas law, \n", | ||
"\n", | ||
"$$\n", | ||
"p = k_BT\\frac{N}{V} = \\frac{Nk_BT}{V}.\n", | ||
"$$" | ||
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"author": "Ideal gas law from first principles", | ||
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