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Lecture 6: Surface Integrals. Recall, area is a vector. Vector area of this surface is which has sensible magnitude and direction Or, by projection . z. (0,1,1). (1,0,1). y. (0,1,0). x. (1,0,0). z. z. x. y. y. (1,0). x. Surface Integrals.

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Lecture 6: Surface Integrals

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Lecture 6 surface integrals l.jpg

Lecture 6: Surface Integrals

  • Recall, area is a vector

  • Vector area of this surface is

    which has sensible magnitude and direction

  • Or, by projection












Surface integrals l.jpg




Surface Integrals

  • Example from Lecture 3 of a scalar field integrated over a small (differential) vector surface element in plane z=0

  • Example


Do x first

Then do y

  • But answer is a vector ; get positive sign by applying right-hand rule to an assumed path around the edge (z>0 is “outside” of surface)

Vector surface integrals l.jpg

Vector Surface Integrals

Example: volume flow of fluid, with velocity field , through a surface element yielding a scale quantity

Consider an incompressible fluid

“What goes into surface S through S1 comes out through surface S2”

S1 and S2 end on same rim

and depends only on rim not surface

Examples l.jpg


  • Consider a simple fluid velocity field

  • Through the triangular surface

  • More complicated fluid flow requires an integral





(see second sheet of this Lecture)

Evaluation of surface integrals by projection l.jpg

Evaluation of Surface Integrals by Projection

want to calculate





Note S need not be planar!

Note also, project onto easiest plane

Example l.jpg


  • Surface Area of some general shape

Another example l.jpg

Another Example

  • and S is the closed surface given by the paraboloid S1

    and the disk S2

  • On S1 :

Exercises for student (in polars over the unit circle, i.e. projection of both S1 and S2 onto xz plane)

  • On S2 :

  • So total “flux through closed surface” (S1+S2) is zero

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