Two Infinite Non-Conducting Planes

[blueskadoo42]

Homework Statement

The electric field equals 19j for points above plane 'r', -31j between the planes, and -19j below plane 's', where j is the unit vector in the +y direction and the fields are in V/m. Calculate σr, the suface charge density for plane 'r'.

positive y- straight up
positive x-to the right.

Homework Equations

E=Qenc/Epsilon0

The Attempt at a Solution

i tried adding the 19j + -31j then multiplying that by Epsilon 0. that didnt work. not sure how to add or apply the electric fields. any pointers on how to solve that?

 

[blueskadoo42]

bump!

 

[blueskadoo42]

anyone? i don't know which field to add or subtract. any help?

 

[turin]

You're using the wrong relationship between charge and E. While your relationship is strictly correct, what you need is

$$E=\frac{\sigma}{2\epsilon_0}$$

where $E>0$ means that it points away from the plane where $\sigma$ is (and $\sigma>0$), and $E<0$ means that it points toward the plane where $\sigma$ is (and $\sigma<0$). Then, you need to realize that the E in each of the three regions is the sum of the E's from the individual planes, and you have to be careful about the + and - signs. So, you can write 3 equations for the three regions in terms of the three E's and two $\sigma$'s.