A2L Item 214
- Description: Identify the correct statement(s) regarding flux.
- Goal: Hone the concept of flux
- Source: 283-400, Flux in and out of a balloon.
- Keywords: Electromagnetism, Electric Field, Flux
The question for students:
We construct a closed Gaussian surface in the shape of a sphericalWe construct a closed Gaussian surface in the shape of a spherical balloon. Assume that a small glass bead with total charge Q is in the vicinity of the balloon. Consider the following statements:
If the bead is inside the balloon, the electric flux over the balloon’s surface can never be 0.
If the bead is outside the balloon, the electric flux over the balloon surface must be 0.
Which of these statements is valid?
- Only A is valid.
- Only B is valid.
- Both A and B are valid.
- Neither one is valid.
Commentary for teachers:
Answer
(3) Students may accept statement A but still think that the value of the flux depends upon location of the bead in the sphere. Transition from just inside to just outside poses particular difficulty to some students. This usually derives from lack of experience with vectors and dot products. Having the student draw field lines does help, but only after they comprehend that the formal definition of flux is equivalent to counting the net number of lines of E crossing the surface.