Uniform electric field paralell plates

[marmeduke]

hi, I've just been reading a bit of physics to get myself ready to do a degree after a year of not a lot and was looking at uniform fields so i tried to prove that infinite plates have a uniform field however i seem to have disproved it with coulombs law. here's my logic, please explain the fault.
looking at the a particle in the field separation of d from the plates (in the middle) the field due to each plate is 2kq/(d^2), now looking at it where the distance is say d/2,3d/2 now we get the field as 4kq/d^2+4kq/9d^2 which is obiviously much larger. the only thing which i can see i may have done wrong is treat the plates as point charges however i can't see how this is a problem because if you take the plates to be infinite then you integrate to find the force from the plates which is acting then it is a scalar multiple of the original force. i hope someone understands this and thank you for taking the time to read this :)

 

[Vailanor]

Your logic is correct in that if we look at the field at two different distances from an infinite plate, then the field will be different. However, this does not disprove the idea that the field is uniform, because the uniform field is referring to the field strength at a given distance from the plate. If you measure the field strength at a certain distance from the plate, it will be uniform no matter what shape or size the plate is.