Confusion to the solution of a MIT ocw problem (1 Viewer)

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1. The problem statement, all variables and given/known data
My snipping tool is totally broken. So i'm going to leave a link :x
It's problem 1.6
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/assignments/ps1a.pdf

2. Relevant equations

[itex]E_{sheet}[/itex]=[itex]\frac{σ}{2ε_{0}}[/itex]
[itex]E_{slab}[/itex]=[itex]\frac{ρD}{2ε_{0}}[/itex]
[itex]\frac{Q}{ε_{0}}[/itex]=[itex]\oint[/itex]EdA

3. The attempt at a solution

I got everything on the solutions page (http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/assignments/pss1.pdf) But I can't seem to understand why region I of Esheet is positive and Regions II and III are negative. If σ is a negative charge, shouldn't the signs be switched around?
 
It's interesting that you have Gauss's Law as one of your equations. When taking a look at the plane, have you thought about it from a flux perspective?
 

SammyS

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1. The problem statement, all variables and given/known data
My snipping tool is totally broken. So i'm going to leave a link :x
It's problem 1.6
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/assignments/ps1a.pdf

2. Relevant equations

[itex]E_{sheet}[/itex]=[itex]\frac{σ}{2ε_{0}}[/itex]
[itex]E_{slab}[/itex]=[itex]\frac{ρD}{2ε_{0}}[/itex]
[itex]\frac{Q}{ε_{0}}[/itex]=[itex]\oint[/itex]EdA

3. The attempt at a solution

I got everything on the solutions page (http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/assignments/pss1.pdf) But I can't seem to understand why region I of Esheet is positive and Regions II and III are negative. If σ is a negative charge, shouldn't the signs be switched around?
Welcome to PF !

Here is an image of the problem:

attachment.php?attachmentid=67685&stc=1&d=1394920480.png
 

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rude man

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Post #2 is a good hint, follow it!
3 equations, 3 unknowns, 3 gaussian surfaces.
Hint: make sure two of them line up correctly!
 
Welcome to PF !

Here is an image of the problem:

attachment.php?attachmentid=67685&stc=1&d=1394920480.png
Thanks! My snipping tool makes it so that the screen turns gray and I snip black images >_<
 

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