1. The problem statement, all variables and given/known data Consider a parallel-plate capacitor with plates of area A and with separation d. Find F(V), the magnitude of the force each plate experiences due to the other plate as a function of V, the potential drop across the capacitor. Hint i used If the plate separation were changed while the voltage was kept constant, the stored energy would change. The force between the plates would be the quantitiy that would be multiplied by the change in the plate separation to obtain the change in energy. In other words, F= -dU/dd Im not sure if im fully understanding this hint. Is the formula they've provided simply Work = force x distance? Besides im not actually supposed to move the plates, so what would i have to input for the the dd (change in distance) part? 2. Relevant equations 3. The attempt at a solution U = (ϵ0AV2)/(2d) The formula they told me to work out for energy stored in a capacitor. So im also feeling quite confused about whether work should be positive or negative. Because the two plates are oppositely charged, it means that they experience a force of attraction - therefore moving the plates further apart should mean positive work being done right? However in the hint for this question im working on, the work is deemed to be negative. From my experiences with work being done, i've always felt that doing positive work means that some external force has to act upon the object to cause that particular change in position. Therefore for negative work, it is like releasing an object and letting it move by itself without you having to put any extra effort in. If there is any easy way that people can determine whether work is positive or negative in the context of electric charge then please do tell.