**Goal:** Link flux to field lines.

**Source:** 283-415 nonzero flux?

The circles in the picture below are Gaussian surfaces. All other lines

are electric field lines. For which cases is the flux *non*-zero?

- a
- a, b, and f
- a, b, e, and f
- a, b, d, e, and h
- a and b
- All but g
- All of them
- None of the above
- Cannot be determined

## Commentary:

## Answer

(2) As “follow-up” it is good to ask questions about comparisons

between two or more of the situations. For example, which is flux is

larger, a or f? Such questions help determine what students are focused

upon; are they keying on number of lines, do they understand that the

flux can be negative.