Flux and Source Strength, Part 1

Let us now imagine a rectangular box-shaped Gaussian surface that has been placed in a river. The water flows perpendicularly to a pair of side surfaces of the Gaussian enclosure, as shown:


Consider the following three situations:

(a) More water flows in than out.


(b) More water flows out than in.


(c) The same amount of water flows out as flows in.

If the amount of water flowing into the Gaussian surface--remember, a Gaussian surface is a closed surface!--is not equal to the amount of water flowing out of it, then one of two things must be true. There must either be a source, which is like a spring producing water, or a sink, which is like a dry well absorbing the water, within the Gaussian Surface. We will describe both the spring and the dry well as being different types of sources by saying

a positive source = a spring that produces water (source)

a negative source = a dry well that absorbs water (sink)

Make sure you understand this subject by trying out a few practice problems.


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