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The charging and discharging principle of capacitor
A capacitor is a passive device that stores energy in the form of an electric field. Capacitors can release stored energy to the circuit when needed. A capacitor consists of two conductive parallel plates, with an insulating or dielectric substance filled between the plates. Figures 1 and 2 are the basic structure and symbols of capacitors respectively.
When a capacitor is connected to a circuit whose power source is direct current (DC), under certain circumstances, two processes occur, namely "charging" and "discharging" of the capacitor.
If the capacitor is connected to the DC power supply, as shown in Figure 3, there is current flowing in the circuit. The two plates will each receive an equal amount of opposite charges. At this time, the capacitor is charging, and the potential difference vc between its two ends gradually increases. Once the voltage vc across the capacitor increases to be equal to the power supply voltage V, vc = V, the capacitor is fully charged, no current flows in the circuit, and the charging process of the capacitor is completed.
Since after the capacitor charging process is completed, no current flows through the capacitor, so in a DC circuit, the capacitor can be equivalent to an open circuit or R = ∞, and the voltage vc on the capacitor cannot change suddenly.
When the connection between the capacitor and the power supply is cut off, the capacitor is discharged through the resistor RD, and the voltage between the two plates will gradually drop to zero, vc = 0
In Figure 3 and Figure 4, the resistance values of RC and RD affect the charging and discharging speed of the capacitor respectively.
The product of the resistance value R and the capacitance value C is called the time constant τ. This constant describes the charging and discharging speed of the capacitor, see Figure 5.
