Tensors and Relativity: Chapter 4
Conservation of particles
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It may also happen, that in a fluid flow, the total number of particles does not change. This conservation law is derivable in the same way as the conservation of energy and momentum [ Assignment 4 ]:
or
We will confine ourselves to discussing fluids which obey this conservation law. This is hardly any restriction since we can always take n to be the number density of baryons.