Confusion to the solution of a MIT ocw problem

[JYangQi00]

Homework Statement

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

Homework Equations

$E_{sheet}$=$\frac{σ}{2ε_{0}}$
$E_{slab}$=$\frac{ρD}{2ε_{0}}$
$\frac{Q}{ε_{0}}$=$\oint$EdA

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?

 

[majormaaz]

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]

Homework Statement

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

Homework Equations

$E_{sheet}$=$\frac{σ}{2ε_{0}}$
$E_{slab}$=$\frac{ρD}{2ε_{0}}$
$\frac{Q}{ε_{0}}$=$\oint$EdA

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:

 

[rude man]

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

 

[JYangQi00]

Welcome to PF !

Here is an image of the problem:

Thanks! My snipping tool makes it so that the screen turns gray and I snip black images >_<