# Electric field between two charged plates

1. Feb 5, 2009

### -EquinoX-

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

http://img11.imageshack.us/img11/667/efieldmm3.th.jpg [Broken]

Can someone please explain to me why the electric field inside is double of what's the outside based on the picture above? I don't understand what the green and red line is...

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: May 4, 2017
2. Feb 5, 2009

### Delphi51

The image can be seen full size at http://img11.imageshack.us/img11/667/efieldmm3.jpg [Broken]

Last edited by a moderator: May 4, 2017
3. Feb 5, 2009

### Staff: Mentor

I don't get it either. What's the context? Is there some text explanation that goes along with it? You don't generally have E-field outside a capacitor, except for some fringe E at the edges. Unless they are immersing a capacitor in some already-existing E-field?

Last edited by a moderator: May 4, 2017
4. Feb 5, 2009

### -EquinoX-

There's no explanation in the text and that's why I am asking...

Basically all I know is:

When two plate of different charge are placed near each other, the two E-fields between the plates add while the E-field outside the plate cancel.

But whyyyy? Which ones of which cancel out?

5. Feb 5, 2009

### Delphi51

Looks like the red E fields are due to the positive charge on the left plate and the green E fields are due to the negative charge on the other plate. They should have coloured the charges to make that clearer.

The diagram is saying that, on the left side of the parallel plates, there is an E away (left) from the positive charges and an E toward (right) the negative charges. These tend to cancel out. At a distance large compared to the separation of the plates, they would completely cancel out making the E field zero.

6. Feb 5, 2009

### Staff: Mentor

Oh, I think I understand now. Have you learned how to use Maxwell's equations yet? Electric flux density and Gauss' Law? What's the net flux through any closed surface when it doesn't contain a net charge? And when it does contain a net charge?

7. Feb 5, 2009

### -EquinoX-

When it does contain a net charge it's a non-zero and when it doesn't, it's zero right?

8. Feb 5, 2009

### -EquinoX-

As far as I see the diagram it the one on the left cancels out with the right side

9. Feb 6, 2009

### Staff: Mentor

Correct. So if you put a closed surface around the whole capacitor, the E-field outside the capacitor has to be what? And if you put a closed surface around just one of the plates.....