Illustration Plate capacitor

A plate capacitor with rectangular capacitor plates, which have an area $$A$$. One plate is positively charged and carries the charge $$+ Q$$ and the other plate is charged with the same magnitude $$|Q|$$ of charge but with negative charge $$- Q$$. The distance between the two plates is $$d$$. There is also a dielectric (e.g. air, glass, etc.) between the plates, which can affect for example the capacitance of the capacitor. The dielectric is characterized by the dielectric constant $$\varepsilon_{\text r}$$.
Between the capacitor plates there is - because of the difference in charge - an electrical voltage $$U$$.