In DC circuits, the Ceramic Capacitor is just like a breaker and makes an open circuit. Ceramic disc capacitor is one of the most common electronic components, and it is a component for storing charge. All these due to the structure of capacitor, even the simplest one will have two electrical conductors separated by a dielectric conductors (including air). So after electrifying, between the two electrodes of capacitor will form a potential difference (voltage,like High voltage resistor), and it can not be electric for its dielectric conductors in middle. But it can exist only on the condition of not reaching the breakdown voltage. We know, any matter is relatively insulated, when the voltage across the matter to a certain value, it can be conductive, and this voltage we call it breakdown voltage. Without exception, when capacitor is breakdown, it is not an insulator. However, in middle school, such voltage can not been seen in circuit, all the work doing within the breakdown voltage, so capacitor can be judged as insulator.
 　　
In AC circuit, however, because the current direction change functionally with the time, the charging and discharging of capacitor need time, then changeable electric field has formed between two electrodes, and this electric field also changed with the time in functional relation. In fact, the current pass the capacitor just by the way of electric field. In middle school, blocking direct current while allowing alternating current to pass expressed the same meaning.