A capacitor is composed of two end plates and an insulating dielectric (including air) at the center. After being electrified, the electrode plate is charged, forming a voltage (potential difference), but due to the insulating material in the center, the entire capacitor is non-conductive. However, this situation occurs without exceeding the critical voltage (breakdown voltage) of the capacitor. We know that any substance is relatively insulated. When the voltage at both ends of the substance increases to a certain extent, the substance can conduct electricity. We call this voltage the breakdown voltage. Capacitors are not an exception. After a capacitor is broken down, it is no longer an insulator.

However, in middle school, such voltages are not visible in circuits, so they are all operated below the breakdown voltage and can be seen as insulators. However, in AC circuits, the direction of the current is constantly changing in a certain functional relationship. The process of capacitor charging and discharging is timed, at which a changing electric field is formed between the electrode plates, and this electric field is also a function of time. In fact, current flows through capacitors through a field.