MSP/Materials/model-intro/Conservation of extensities.md ..
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1. [Continuity equations](https://en.wikipedia.org/wiki/Continuity_equation) and general [**conservation laws**](https://en.wikipedia.org/wiki/Conservation_law).
2. Conservation principles for extensities: [**invariants**](https://en.wikipedia.org/wiki/Invariant_(physics)) / [**symmetries**](https://en.wikipedia.org/wiki/Symmetry_(physics)).
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From the [Wikipedia](Continuity equations) page:
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From the [Wikipedia](https://en.wikipedia.org/wiki/Continuity_equation) page:
>Continuity equations are a stronger, local form of conservation laws. For example, a weak version of the law of conservation of energy states that energy can neither be created nor destroyed—i.e., the total amount of energy in the universe is fixed. This statement does not rule out the possibility that a quantity of energy could disappear from one point while simultaneously appearing at another point. A stronger statement is that energy is locally conserved: energy can neither be created nor destroyed, nor can it "teleport" from one place to another—it can only move by a continuous flow. A continuity equation is the mathematical way to express this kind of statement. For example, the continuity equation for electric charge states that the amount of electric charge in any volume of space can only change by the amount of electric current flowing into or out of that volume through its boundaries.
