# $Q_{enc}$

Given a closed surface $S$ there is always an outward pointing normal, where by outward pointing we mean that if I travel along the normal direction I can keep going forever without intersecting $S$ again. We can use a similar definition of inside and outside of $S$. $Q_{enc}$ on the right hand side of Gauss' law is simply the total charge inside $S$.

We have a line of alternating charges on the $x$-axis at $x=...,-2,-1,0,1,2,....$ with a positive charge at the origin. What is the total charge inside a sphere of radius $3.5$, i.e. what is $Q_{enc}$?

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