The continuity equation is a fundamental principle in fluid dynamics and other fields that describes the transport of some quantity (such as mass, energy, or charge) in a system. It expresses the idea that, in a closed system, the rate at which a quantity enters a volume must equal the rate at which it leaves that volume, plus any accumulation of that quantity within the volume.
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The continuity equation expresses local conservation as time change plus flux divergence equal to zero.