# Doubt in parallel plate capacitor

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1. Nov 23, 2014

I was going through the capacitor lesson in my textbook. In it its written "...both positive and negative plates will produce electric field in the same direction, hence the electric field adds up....outside the plates, electric field is zero since the field due to negative and positive charges are in opposite direction..."
My doubt : i am confused at the 2nd statement. (i have attached two images)
• which electric fields are in opposite direction? how does it cancel out?
• Is this true only if the charge on the capacitor plates are same?
• Do electric fields extend to infinity?
• does the electric field outside capacitor for second image become zero?

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2. Nov 23, 2014

### Staff: Mentor

What do you think, and why? Then we can correct the specific misconceptions that you have.

3. Nov 23, 2014

this is what I think... let the positive plate be on the left and the negative on right. From gauss law, feild inside conductor should be zero. Maybe the positive field from left plate extends to right and the negetive from right plate also passes through plate 1 on .....I have no idea...

4. Nov 23, 2014

### sophiecentaur

Have you looked at images of the fields around a parallel plate capacitor? The situation between the plates is very distinct from the situation outside and at a distance.
This link shows what happens outside.

5. Nov 23, 2014

### Staff: Mentor

You're on the right track here. Do you know about the principle of superposition? Consider the field produced by each plate by itself, as if the other one were not there at all; then add the two fields together.

Last edited: Nov 23, 2014
6. Nov 24, 2014

you are going off topic

7. Nov 24, 2014

Ok.field of -Q charge from the inner surface of left plate passes through the metal plate. the electric field of +Q charge also passes through its own metal plate. Hence inside the metal plate the electric field cancels out. This also happens outside the plate. Hence there also field cancels out.

8. Nov 24, 2014

### Staff: Mentor

No... consider the directions of the two fields. The electric field produced by a positive charge points away from the charge, and the electric field produced by a negative charge points towards the charge.

9. Nov 24, 2014