The capacitor is an electronic device for storing charge. The simplest type is the parallel plate capacitor, illustrated in figure 17.1 . This consists of two conducting plates of area separated by distance .
Positive charge resides on one plate, while negative charge resides on the other.
Two views of a parallel plate capacitor. The electric field between the plates is , where the charge per unit area on the inside of the left plate in figure 17.1 is . The density on the right plate is just . All charge is assumed to reside on the inside surfaces and thus contribute to the electric field crossing the gap between the plates. The above formula for the electric field comes from applying Gauss's law to the sheet of charge on the positive plate.