A2L Item 127

• Description: Given configurations of Gaussian surfaces and electric field lines, identify the situations having non-zero flux.
• Goal: Link flux to field lines.
• Source: 283-415 nonzero flux?
• Keywords: Electromagnetism, Flux

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The question for students:

The circles in the picture below are Gaussian surfaces. All other lines are electric field lines. For which cases is the flux non-zero?

1. a
2. a, b, and f
3. a, b, e, and f
4. a, b, d, e, and h
5. a and b
6. All but g
7. All of them
8. None of the above
9. Cannot be determined

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Commentary for teachers:

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.

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