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Why does a body with $0\ K$ temperate does the internal energy NOT become $0\ J$? My lecturer said it had to do with some quantum mechanics, but I never really got any answers from him. I really want to understand this to better understand thermal physics.

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If the internal energy becomes zero, that means the particles constituting the body come to complete rest. This implies that the particles constituting the body have a definite position and momentum. According to Heisenberg's uncertainty principle

$\Delta x \Delta p \geq \frac{\hbar}{4\pi}$

So if a particle is at rest i.e has a definite position and zero momentum, this inequality isn't satisfied and hence it is not possible to have zero internal energy at absolute zero.

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  • $\begingroup$ Yes, "zero-point energy" if one wants to search for more details. $\endgroup$ – Pieter Oct 22 '18 at 8:31

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