1. The question: This question is from purcell's E&M book (3.71). (a) The plates of a capacitor have are A and separation s (assumed to be small). The plates are isolated, so the charges on them remain constant, the charge densities are +-σ. A neutral conducting slab with the same area A but thickness s/2 is initially held outside the capacitor. The slab is released. What is its kinetic energy at the moment it is completely inside the capacitor? (b) Same question, but now let the plated be connected to a battery that maintaines constant potential difference. The charge densitied are initially +-σ. (Don't forget to include the work done by the battery). 2. Relevant equation: Energy stored in a capacitor: E = C * phi^2 / 2 3. The attempt at a solution I calculated the energy within the capacitor before the conductor was released: U_i= ε0 E^2 A S / 2 (where A is the surface of the plates). Then I calculated the enerdy within the capacitor after the conductor is fully inside. Now I can look at it as a thinner capacitor with only s/2 distance between the "plates". U_f = ε0 E^2 A S / 4 So my problem is - what does this energy actually mean? Is this the potential energy of the problem, that together with the kinetic energy becomes the total energy? Or is it something else? And in general, how do I translate this potential reduction into the work done on the slab? I'd really appriciate your help! Thank you.