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:
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 negative source = a dry well that absorbs water (sink)
Make sure you understand this subject by trying out a few practice problems.