# Ranking 4 numbered points according to net electric field

Hello everyone. I'm having problems figuring out this check point. I have attached an image that has the diagram. Incase you can't read the picture it says: The figure shows two large, parallel, noconducting sheets with identical positive uniform surface charge densities, and a sphere with a uniform (positive) volume charge density. Rank the four numbered points according to the magnitude of the net electric field there, greatest first. The answers are: 3 and 4 tie, then 2, 1. Why would 3 and 4 be the greatest? i'm confused! Do the two non conducting plates not play a role in this problem because they are equal?

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Doc Al
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mr_coffee said:
Do the two non conducting plates not play a role in this problem because they are equal?
What's the field between two large parallel sheets of identical charge (not counting any other charge that might be present)?

isn't it, for a nonconducting sheet:

E = $$\delta$$/(2Eo);
Thats or a sheet of charge. So two sheets of charge would it be E = $$\delta$$/Eo ?

Doc Al
Mentor
You have the correct formula for the field from a single sheet of charge. But what direction does the field have?

The direction would be to the left for the left plate and right for the right plate. Because like charges repel eachother and There is a positive charge inbetween both of them. So really, we'll have e forces going out of the point charge in the middle. So that explains why 3 and 4 are equal. It then goes 2 and 1...but why is 1 the smallest? Is it because its at an angle, so you would have to break the e field up into components which are weaker? am i getting that right?

Doc Al
Mentor
The field from a sheet of positive charge points away from the sheet. In between the two sheets, the field from one sheet cancels the other since they point in opposite directions: the net field is zero. Bottom line: You can ignore the charged sheets.

Now the problem is just the field from the charged sphere, which should be easy. The points closest to the sphere will have the stronger field.

Awesome, thanks! its like pulling teeth hah