Today we will study the different effects of capacitors on direct current and alternating current. The two circuits, the small bulb and the capacitor are exactly the same, and the effective value of the DC voltage and the AC voltage are equal. When the switch is closed, the small light bulb in the DC circuit does not emit light, and the small light bulb in the AC circuit normally emits light. This means that there is no current in the DC circuit, but there is current in the AC circuit. It seems that DC can’t pass through the capacitor, AC, electricity can pass through the capacitor, this is why first study the DC circuit before closing the switch, there is no electrical closing switch on the capacitor plates, and then, because the capacitor is connected to the power supply, the positive charge will hit the right half , Accumulation, negative charge will accumulate to the left electrode plate, voltage will gradually appear between the two electrode plates, knowing that the voltage is equal to the power supply voltage, this is the charging process of the capacitor.
In this process, whether it is a positive charge moving from right to left or a negative charge moving from left to right, a current from right to left will be formed. Therefore, there is a charging current in the circuit just after the switch is closed, and the light bulb may light up briefly, but as the capacitor charging is completed quickly, the charging current disappears, and the light bulb will not turn on. If represented by an image, the voltage on the capacitor gradually increases from 0 to the power supply voltage, and the current gradually decreases from the maximum value to 0. After analyzing the DC circuit, let us look at the AC circuit is closed and the AC power is connected to both ends of the capacitor. Because the size and direction of the AC voltage are periodically changing, the voltage across the capacitor will also change periodically, which makes the capacitor Non-stop charging and discharging.
Just knowing that the capacitor is constantly charging, the discharge is not enough. How exactly does it charge and discharge? Then we have to specifically analyze what happens to the capacitor in the next cycle? Assuming that the voltage of the AC power supply changes sinusoidally, the voltage is positive, indicating the direction. From the right to the left, the right substrate of the capacitor is positively charged in the first 1/4 cycle, and the voltage becomes larger, so the positive charge strikes the right half and then A negative The charge is to the left, and the bonds accumulate, forming a charging current to the left. In the second 1/4 cycle, the capacitor voltage begins to decrease, and the plate is discharging positive charge, and the left plate is discharging negative charge, forming a discharge current to the right. In the third 1/4 cycle, the left electrode plate of the capacitor is positively charged and the voltage becomes larger. The positive charge is trapped to the left and the negative charge is accumulated to the right-click plate to form the right charging current.
In the last 1/4 cycle, the capacitor voltage U decreases, the left electrode plate discharges positive charge and the electrode plate discharges negative charge, forming a discharge current to the left. In short, due to the periodic changes in the voltage of the capacitor and the two-electrode plate, the power of the two-electrode plate changes periodically, so that the capacitor is continuously charged and discharged to form a current. This is the reason why alternating current can pass through capacitors. We call alternating current through capacitors, but the characteristic that constant current cannot pass through is called passing alternating current across the flow. Not only that, if the AC frequency is high, the higher the frequency of charging and discharging on the capacitor, which requires the charging and discharging process to be completed in a short time, so the current is relatively large.
On the contrary, if the frequency of the alternating current is low, the frequency of charging and discharging on the capacitor is also low, and the charging and discharging process is completed only in a relatively long time, so the current is relatively small. That is to say, when the effective value of the voltage is the same, the current when the high-frequency alternating current passes through the capacitor is large, and the first-frequency alternating current and the current when passing through the capacitor are small.