Confused on determining an enclosed a postive/negative/no net charge

  • Thread starter mr_coffee
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Hello everyone, I'm stuck on a check point. The figure shows three situations in which a Gaussian cube sits in an electric feild. The arrows and hte values indicate the directions of th feild lines and the magnitudes of the flux through the six sides of each cube. (The ligther arrrows are for the hidden faces.) In which situation does the cube enclose (a) a positive net charge,l (b) a negative net charge, and (c) no net charge. The image attached is the diagram from the book. I understood the concept when i read it. It said, If the electric field is outward for all points on its surface, the flux of the electric field is + and so is the enclosed charge. It also said, if the elelctric field is inward then the flux is - and so is the enclosed charge. If the positive and negative charges have equal magnitudes (there are as many field lines leaving surface as entering it) then there is no net charge or flux. The answer is, cube 1, no net charge. (2) positive net charge. (3) negative net charge. To start off, why is C no net charge? Maybe I don't know how to interpret the magnitudes. I see 4 going in and 7 going out, meaning +2 flux. 3 in, 2 out = -1 flux. 7 in, 5 out = -2. If i add up all these results i get -1 flux, wouldn't this mean it would be a overal net charge of negative? Am i totally not reading these right? :bugeye: I had to touch up the picture because you couldn't see the arrows or magnitudes after i scanned it, sorry about the sloppiness, paint sucks! Okay i just viewed the image and it sucks even more then I realized. So i'm going to list the magnitudes going in and out just to make sure.

Cube 2: Magnitude of arrows going into cube: 4, 3, 6; Going out of cube: 10, 3, 5.

Cube 3: In: 6,5,7; Out: 8,2,5
 

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Answers and Replies

  • #2
Doc Al
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mr_coffee said:
To start off, why is C no net charge? Maybe I don't know how to interpret the magnitudes. I see 4 going in and 7 going out, meaning +2 flux. 3 in, 2 out = -1 flux. 7 in, 5 out = -2. If i add up all these results i get -1 flux, wouldn't this mean it would be a overal net charge of negative?
You made an arithmetic error. The net flux is 0.
 
  • #3
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ahhh, my bad, thanks! I get it now! :biggrin:
 

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