continuity equation

continuity principle

 or continuity equation

Principle of fluid mechanics. Stated simply, what flows into a defined volume in a defined time, minus what flows out of that volume in that time, must accumulate in that volume. If the sign of the accumulation is negative, then the material in that volume is being depleted. The principle is a consequence of the law of conservation of mass. The behaviour of fluids in motion is fully described by this equation, plus a second equation, based on the second of Newton's laws of motion, and a third equation, based on the conservation of energy.

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continuity equation [‚känt·ən′ü·əd·ē i′kwā·zhən]

(physics)

An equation obeyed by any conserved, indestructible quantity such as mass, electric charge, thermal energy, electrical energy, or quantum-mechanical probability, which is essentially a statement that the rate of increase of the quantity in any region equals the total current flowing into the region. Also known as equation of continuity.