Understand the sign of work in electricity

Imagine the plates of a capacitor has a potential difference of 10V. How much work must be done in order to move the electron from the negative plate to positive plate?

Given that electric field goes from positive plate to negative plate. If you want to move the electron from negative to positive plate, you need to invest some energy to make that happen (positive work) because you are going up against the electric field.

The equation to solve that is W= qV. I was very confused about the sign of work due to the sign of V and q. However, I realize that no matter what you do, your work must be POSITIVE! Therefore, you can do whatever you want to q and V to make work positive :)

Understand the sign of work

At some point, we are all confused by the meaning of the sign of work; at least I did. 

Here is how I interpret the sign of work.

We all know that W=P times change of V.

Let's think of a situation in the perspective of a system (cylinder with a piston on top of it).

When the final volume is less than the initial volume, we have negative work. What does this mean?

It means that the system is not doing work! It is not investing any energy (negative work). So, work is being done on the system!

What if final volume is greater? that means the system is doing work! The system is investing energy to do work (positive). We describe this as work is being done by the system!