Multivariable Calculus · free preview

Flux Integrals and Orientation

Orientable surfaces and consistent normal choices, the flux integral as a vector-surface integral, and the efficient cross-product formula.

1. Two-sided surfaces and orientation

Two-sided surfaces and orientation

2. The cross product carries both jobs

The cross product carries both jobs

3. Flux out of the unit sphere

Flux out of the unit sphere

4. Open surfaces and lids

Open surfaces and lids

5. What orientation adds

A disk in the xyxy-plane sits in the field F=(0,0,3)\mathbf{F} = (0, 0, 3), flowing upward. What is the flux through the disk oriented upward?

6. The folded cross-product formula

Computing flux via ∬DF(X)⋅(Xu×Xv) du dv\iint_D \mathbf{F}(\mathbf{X}) \cdot (\mathbf{X}_u \times \mathbf{X}_v)\, du\, dv, a student divides by ∣Xu×Xv∣|\mathbf{X}_u \times \mathbf{X}_v| first. What happens?

7. Reading radial flux

Why is the flux of F=(x,y,z)\mathbf{F} = (x, y, z) out of the unit sphere computable without any parametrized integral?

8. Downward orientation sign

A field has upward flux 77 through a surface. Give the flux through the same surface with the opposite orientation, as a plain number.

9. Flux through a graph

For F=(0,0,1)\mathbf{F} = (0, 0, 1) through the graph of gg over a region DD of area 66, oriented upward, give the flux, as a plain number.

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