# Choosing a Gaussian Surface

Choosing a Gaussian surface is not very hard, but it is fairly subtle.
Unlike the complicated surface shown in the introduction, we are going to
choose simple surfaces for which it is easy to calculate the flux. There
are two parts to the flux integral. First, the magnitude of the field can
vary. To simplify this aspect, we want to choose a surface for which the
field has constant magnitude.

Next, the dot product can vary if the angle between the normal to the
surface and field vector changes for different area elements dA. Remember
the dot product gives EdA cos(theta). We want to choose a surface for which
the angle remains constant. The simplest angles are those for which E and
dA are parallel and those for which E and dA are perpendicular. We will
often use these.

## Rules:

### 1. Choose the surface perpendicular to the field so that E and dA are
parallel.

### 2. Choose the points on the surface to be a constant distance from the
charge so that the magnitude of E does not vary.

### 3. If it is not possible to satisfy points (1) and (2), then choose
the surface parallel to E so that E and dA are perpendicular and the dot
product gives zero.

Next go to Gauss' Sphere1.

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