Gauss' Law for Infinite Sheets of Charge

1. Dec 16, 2013

kazukamikaze

1. The problem statement, all variables and given/known data
Three parallel, infinite, insulating planes (sheets) of charge are arranged as shown (see attached image). Note carefully the charge desnitties and distances given. From left to right the charge densities are -3σ, +σ, +σ. How does the magnitude of the electric field at point a compare to the magnitude of the field at point b?

2. Relevant equations
E = σ/2ε0

3. The attempt at a solution
From what I recall from my lecture, the E field of an infinite sheet of charge does not depend on distance. Second, I believe that the field lines look something like what I've drawn in the second attached image. For that reason, I argued the magnitude of A was larger than B as the charge density of the sheet is larger.

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Last edited: Dec 16, 2013
2. Dec 16, 2013

Dick

The field at A is going to be the sum of the fields from all three plates, isn't it?

3. Dec 16, 2013

kazukamikaze

Based off what one of my classmates told me, the field at the two points should be equal.

The only way I can rationalize these answer is by taking the sum of the three plates. I'm just confused as to why this is done.

4. Dec 16, 2013

Dick

Because the plates don't block the fields coming from the other plates, if that's what the confusion is. You could also directly use Gauss' theorem.