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electric field lines
electric field lines  

I am having difficulty with these questions. Any help would be greatly appreciated.

1)   What is the net charge shown in this figure?

2)   What is the electric flux through each of the surfaces in the figure? Give each answer as a multiple of q/(epsilon_0)

1) Since the number of flux lines LEAVING the surface is 8 and the number of flux lines ENTERING the surface is 8 the net flux through the surface is 0. Therefore, the net charge contained within the surface must be 0.
2) The net charge contained in figure a is Qnet=+3q-2q=q, the net flux through the surface must be:
   For figure b the net charge contained is Qnet=+q-2q=-q and so the net flux is:
  For figure the net charge contained is Qnet=0 and so the net flux will also be zero. The charges outside the closed surface do not matter at all since any flux line generated by charges located outside the figure both enters and exits the surface.


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James J. Kovalcin


I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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