Hey goiz, I got a quick question,

Why is the electric field between two parallel charged plates uniform? I mean I get that their field lines are parallel, but why are they even parallel?

Also why does the field line curve outwards at the ends of the plates?

Hey there. It's actually not. The electric field between two INFINITE parallel plates (in both the x and y dirextions) is uniform. However, real plates have finite area, so you'll find the field isn't constant (near the middle is certainly different to being near the edge of the plates).

In the infinite case, there are a few reasons why the field is constant.

1. There is no sense of distance. Your plates are infinitely sized. Getting closer to a plate is like zooming in on the plate...which, being infinitely sized, looks the same (try zooming in on an infinitely sized square. It will look the same). Hence, the field strength can't change.

2. If the plates are infinite and oriented horizontally, then if you move across horizontally, you find that the plates look the same. This means the field lines have to look the same as you move between the plates.

3. If you rotate the infinite plates about an axis perpendicular to both plates, you find the plates still look the same. Thus, the shape of the field lines can't be affected by such a rotation, and they must ALL be perpendicular from one plate to another (think about it)

This is an introduction to how symmetry can greatly simplify a physics problem. Ask if you're still stuck! This explanation isn't needed for VCE, don't worry.

Note: none of the above holds for finite-sized parallel plates. Infinite plates are an approximation and a model only.