A contribution deals with a parallel-plate capacitor with a dielectric slab between plates. The slab can move without friction. Two cases are considered: 1) A charged capacitor with the slab is closed system, where a law of conservation of energy is valid. Here is an electric charge Q = const.. 2) The capacitor is connected to a battery, the system is open, and a voltage U = const.. A role of the fringing field around the ends of the capacitor as well as role of the charges in the corners inside the capacitor is explained.