Review of surface integrals
Definition of flux of F through a tiny area (expressed as a vector dA): flux = F.dA
( dA points in direction of normal vector, for closed surfaces the convention is that the normal points outward)
Flux through a finite surface S is the double integral over S of F.dA
Physical explanation of flux: it tells you amount of stuff flowing through a surface, per unit time.
Examples of flux - fluid flow (J = rho*v), E field
Gauss' law, and an example with spherical symmetry
Reminder of definition of divergence, and lead in to connection between divergence and flux.
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