Electric Field in 3 Parallel Plates

[Broem]

Homework Statement

What is the electric field at point A in the unit of σ/ε0?

Homework Equations

E=σ/2ε

The Attempt at a Solution

I just want to make sure my thinking behind this is correct.
From the center we have +5σ. To find the A what I did is subtract the other two -σ from the 5: this gives me 5-1-1 = 3.
From here I just plug in: 3σ/2ε which in the units = 1.5 σ/ε

Is this the correct approach?

 

[TSny]

Yep.

 

[rude man]

Alternative sol'n: if you put a Gaussian box of area = 1m^2 and running from A to D, the inside net free charge is 3σ C and the total flux area is 2 m^2 (no flux out the other 4 sides if the plates are infinite in dimension).

That would make εE*2m^2 = 3σ so E = (3/2)σ/ε. Same answer!