# Electric fields at points?

1. Jan 20, 2007

### abeltyukov

Hi,

1. The problem statement, all variables and given/known data

You've hung two very large sheets of plastic facing each other with distance d between them, as shown in Figure P26.50 ( http://i137.photobucket.com/albums/q208/infinitbelt/p26-50-1.gif [Broken]... ). By rubbing them with wool and silk, you've managed to give one sheet a uniform surface charge density n1 = -4(n0) and the other a uniform surface charge density n2 = 5(n0). What is the electric field vector at points 1, 2, and 3?

2. The attempt at a solution

I drew the force diagrams for the three points but that is where I think I am making my mistake. For example, for point 1, I have a force going to the left from the positive plate and a force going to the right from the negative plate. The difference I get is 1(n0), but that is wrong. Any ideas?

Thanks!

Last edited by a moderator: May 2, 2017
2. Jan 21, 2007

### abeltyukov

Any ideas?

Thanks!

3. Jan 21, 2007

### marcusl

Use the expression (probably in your book?) for the electric field from a uniformly charged sheet. The field at each point is a superposition (sum) of the fields from the two sheets.

For my own clarification; is "n0" a given surface charge density?

4. Jan 21, 2007

### abeltyukov

There is no numerical value given to "n0" in the problem. It is like -4x and 5x.

Thank you for the help!

5. Jan 22, 2007

### marcusl

Ok, then the answer will appear as a factor of the electric field from n0.

6. Jan 22, 2007

### teleport

What is the expression for the electric field due to the rectangular sheet?

In my book it is not present. I tried doing the derivation but the integral that I come up with when dividing the sheet into rods doesn't look nice to do. Could you do me the favor and show the expression? Thank you.

7. Jan 22, 2007

### marcusl

The field from an infinite sheet with a surface charge density
sigma is
$$E=\frac{\sigma}{2\epsilon_{0}}$$

EDIT: fix formula. Note, mks units are used.

Last edited: Jan 22, 2007