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Basic Gauss' law

Gauss's law is a very powerful method to determine the electric field due to a distribution of charges. The mathematical expression for Gauss's law is

SEdA=Qencϵ0, \int_{S} \vec{E} \cdot \vec{dA}=\frac{Q_{enc}}{\epsilon_0} ,

where SS is a surface, E\vec{E} is the electric field vector, dA\vec{dA} is the infinitesimal area element, QencQ_{enc} is the charge enclosed by S,S, and ϵ0\epsilon_0 is a constant.

In order to apply Gauss's law, we need to understand what each of the parts of this expression means. This set of problems will help you understand each of the components. Let's start with SS. You may be more familiar with integrals as the limit of a sum of a function over a line interval, which gives the "area under the curve." An integral over a surface of a function is just the sum of that function over all the points on the surface.

The surface in Gauss's law is a closed two-dimensional surface, such as the surface of a sphere or the surface of a cube. A closed surface is a surface that divides space into an inside and an outside, whereby dividing we mean there is no path that goes from inside to outside that does not penetrate the surface. Consider the surface SS of the objects below. For which of the objects is SS a closed surface?


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