#thermodynamics
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No lens burns hotter than the sun: étendue and the second law
Give me a perfect lens of any size and I still cannot heat a target past about 5800 K with sunlight. The limit is not engineering, and it is not diffraction — it is Liouville's theorem wearing an optics costume, and it hands back the solar constant as a receipt.
Understanding the Virial Equation - A Systematic Taylor Expansion of the Ideal Gas Law
An exploration of how the virial equation emerges as a Taylor expansion of the ideal gas law, providing a systematic way to account for molecular interactions in real gases.