work /electric potential question..

i know that work i the difference between the potential in the end
and the potential in the start.

but when i have this fds=qEds
and using the law of work
i dont know how to get to

dU=-qEds

how to get the minus
??

here we have the same work threw difference in potentials
but we dont have a minus

why??
https://i30.tinypic.com/f9h16a.jpg

why in one case we have minus
and in the other we dont?

The minus charge is one side of the cap and q is the charge

Think outside the square and square it