Electric field between parallel plates

1. Nov 16, 2011

quark001

I don't understand why the electric field intensity is uniform between parallel plates. No explanation in my textbook...

Surely as a charge moves up/down between the plates, parallel to the lines of the field, the electric force that it experiences would change (using Coulomb's law)?

2. Nov 16, 2011

Harrisonized

Last edited by a moderator: Apr 26, 2017
3. Nov 16, 2011

quark001

Thanks for the link. I get it when using E = V/d, because it's easy to see that the sum of the distance between a charge and each plate is constant no matter where the charge is situated.

But I'm still getting confused with Coulomb's law. Say there are two equal but opposite 1C charges 5m from each other - like two charged plates. Say I place another +1C charge between them. If I move this charge up and down within the 5m radius, the net force does not seem to be uniform.

4. Nov 16, 2011

Staff: Mentor

It is a property of electric field lines that they leave (or impinge upon) a conducting surface at right angles to that surface. For a lone conducting sphere that means that the field lines emerge radially. The field lines also "like" to have as much room to themselves as possible, so they diverge uniformly with distance from the sphere. This graphically accounts for the inverse square law for a point charge or a charged sphere. The force felt by a charge located in an electric field depends upon the field strength, which is proportional to the field line density at its location.

Now consider a uniformly charged, very large flat conducting plate, in a region somewhere near its center. Once again the field lines emerge directed perpendicular to the surface so they are all parallel to begin with. For any given field line the density of other field lines surrounding it is uniform in all directions, so there is no motivation to deviate direction and nowhere it can go to spread itself thinner. So the field lines remain parallel. Note that near the edges of the plate the situation is different, because then there will definitely be a decrease in field lines edge-ward, so the closer to the edges you get the more the lines will splay outwards and away from each other.

For a parallel plate capacitor image two such plates, oppositely charged and parallel to each other. The field lines of the facing plates will almost all begin and end on the plate surfaces, emerging from the + plate and ending on the - plate. There will still be the bowing-out of the field near the edges, but it will be a small effect. Inwards of the edges the field lines are of uniform density and are parallel, so the field is constant.

5. Nov 16, 2011

quark001

Oh - now I see! Thanks a lot.

6. Nov 16, 2011

Liquidxlax

Last edited by a moderator: Apr 26, 2017
7. Nov 16, 2011

quark001

Yup, that's high school science for you! Lovely isn't it?!

8. Nov 17, 2011

Liquidxlax

well fringe fielding didn't occur till second year of university for myself. You're lucky you're even doing e&m in highschool.