Note: The analogy is good but can you detect an error in the uniform electric field diagram. The larger distance of the negative charge away from the negative plate will not give rise to more energy stored.
Work W=qV where q is the charge moved between 2 pts where the potential difference is V. If you move 1 C from a to b where the potential is 10V and 12 V . The work = (1C) x (12V-10V) = 2 J. Energy conversion : The KE of the charge reduce by 2 J and the EPE increase by 2 J.
If the same charge returns from b to a : The PE decrease by 2 J and the KE increase by 2 J .
For a negative 1C : a to b : PE decrease by 2 J and KE increase by 2J and vice versa for b to a.
Therefore : A positive charge at rest would move from high to low potential (just as water flows downhill). A negative charge at rest would move from low to high potential ( there is no analogy in gravitational field)
