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 Summary
 It is about why ## \frac{ d\rho} {dt} =  \nabla \cdot \vec J ## is called continuity equation.
I understand that from local conservation of charge, we get eqn. 8.4. I don't get why it is called continuity eqn. What is continuous in it?
Conservation of momentum gives us equation, ## \frac {d\vec p }{dt} = \vec F ##. This equation is not called continuity equation. Can we get a continuity equation from every conservation law?
The images are taken from Griffith's Electrodynamics, 4ed.
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