Recall Flux

“Write down something you know about flux.”

This SWBQ is usually done as an introduction to the Concept of Flux Activity (there is a narrative that demonstrates how one might transition from one to the other). It is an opportunity for the instructor to find out what the students already know about the flux so as to focus the succeeding discussion appropriately.

The concept of flux is not only challenging, but multi-faceted. Therefore one should expect a wide variety of verbal, mathematical, and graphical responses (see narrative for a few specific examples). Giving the students a chance to comment on a few well chosen examples will often make your points for you.

• Many students confuse the dot product and the cross product, but do remember that the flux cares about the component perpendicular to the field.
  • This can be a good review of dot and cross products.
  • What kind of a beast is it? Is flux a vector or a scalar?
  • What is the direction of $\vec{da}$ for a specific surface (give an example)? You can also address the ambiguity in the sign of the direction.
• Those who write a mathematical expression will usually remember that there is an integral.
  • Is it a line or a surface integral? With a surface integral, there are 2 integrals
  • What are we chopping and adding (all perpendicular pieces)?
• Students often will use language such as the amount of stuff that “gets through” or “flows through” the surface It is important to emphasize that there is no time dependence (“points through” or “vectors on the surface” is more helpful language).
• Sometimes students will write something like “field/area” which can lead to a discussion of the difference between flux and flux density and the context where each is appropriate.

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