Gauss's Law

[Jess]

Can someone explain when, and why, you would need to integrate to find the charge enclosed? Is it only for non-conducting spheres or can it be for other shapes as well? 

[FizziksGuy]

Hi Jess,

     Gauss's Law is a law of physics, it's always true.  However, as far as usefulness goes, it's only really useful for finding the electric field when you have a charge distribution that exhibits spherical, cylindrical, or planar symmetry.  Some of the examples you'll find in the guide sheets (http://aplusphysics.com/courses/ap-c/tutorials/APC-EField.pdf) or tutorial video (http://aplusphysics.com/courses/ap-c/videos/APC-Gauss/APC-Gauss.html) include a sphere or shell of charge (spherical symmetry), a line of charge (cylindrical symmetry), and a plane of charge.  Hope that helps...  make it a great day!

[Jess]

When would it be necessary to integrate to find the charge enclosed? Here is the problem with my teacher's solution. Like I see that we are having to integrate, but I am not understanding why.

Gauss Law Test Solution.pdf

[FizziksGuy]

Aha.  I understand.  The integration your teacher is showing isn't the integration part of Gauss's Law, she's just using integration to find the charge enclosed by the Gaussian surface.  If she had just told you the charge enclosed was Q=Pi*a*R^3, you could have put that right into Gauss's Law.

 

So to answer your question, when do you have to integrate to find the charge enclosed?  When you're given some sort of charge density function instead of the total enclosed charge.  To use Gauss's Law, you have to know what Q_enclosed is.  If it's not given directly, you need to figure out some way to find it, and in many problems, that will involve some amount of integration.