A capacitor is a container that can store electrical charge. It is composed of two metal sheets that are close together, separated by an insulating material. According to different insulating materials, various capacitors can be made. Such as: mica, porcelain, paper, electrolytic capacitors, etc.

In structure, it is divided into fixed capacitors and variable capacitors. The capacitor has infinite resistance to DC, that is, the capacitor has a DC blocking effect. The resistance of a capacitor to alternating current is affected by the frequency of alternating current, that is, capacitors of the same capacity present different capacitive reactances to alternating currents of different frequencies. Why do these phenomena occur? This is because the capacitor relies on its charge and discharge function to work, when the power switch s is not closed.

When the switch S is closed, the free electrons on the positive plate of the capacitor are attracted by the power source and pushed onto the negative plate. Due to the insulating material between the two plates of the capacitor, the free electrons from the positive plate accumulate on the negative plate. The positive plate is positively charged due to the decrease of electrons, and the negative plate is negatively charged due to the gradual increase of electrons.

There is a potential difference between the two plates of the capacitor. When this potential difference is equal to the power supply voltage, the charging of the capacitor stops. If the power is cut off at this time, the capacitor can still maintain the charging voltage. For a charged capacitor, if we connect the two plates with a wire, due to the potential difference between the two plates, electrons will pass through the wire and return to the positive plate until the potential difference between the two plates is zero.

The capacitor returns to its neutral state without charge, and there is no current in the wire. The high frequency of the alternating current applied to the two plates of the capacitor increases the number of charging and discharging of the capacitor; the charging and discharging current also increases; that is to say, the obstructive effect of the capacitor on the high frequency alternating current is reduced, that is, the capacitive reactance is small, and vice versa Capacitors have large capacitive reactance to low-frequency alternating current. For alternating current of the same frequency. The larger the capacity of the container, the smaller the capacitive reactance, and the smaller the capacity, the greater the capacitive reactance.