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The capacitive reactance reflects the capacity of the capacitor to block the AC current, which is inversely proportional to C and F. Under the same voltage, according to q = CUC formula, the larger C is, the greater the charge it holds, the smaller its blocking ability to current is, and the greater the current is; if f is higher, that is, the faster the charging and discharging process of capacitance is, the smaller the blocking ability of capacitance to current is, the greater the current is. Therefore, under the action of sinusoidal voltage with different frequency, the blocking effect of a capacitor on the current is different. The higher the frequency is, the smaller the XC is, and the easier the current is to pass through. If f = 0, XC = ¡Û, that is, the DC current cannot pass through the capacitor element. If there is capacitor in DC circuit, the circuit is equivalent to open circuit in steady state. It can be seen that in the circuit, capacitance has the function of isolating direct current and alternating current.
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The current and voltage waveforms in the capacitor circuit are shown in Figure B.
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Phase relationship between current and voltage
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The phase relationship between current and voltage is that current leads voltage by 90 ¢X or voltage lags current by 90 ¢X. In the form of phasor, the relationship between current and voltage is as follows:
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Phasor representation of current voltage relationship
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The upphasor represents not only the numerical relationship between voltage and current, but also their phase relationship. -Ji indicates that the voltage phasor rotates 90 ¢X clockwise based on the current phasor, and the phase difference angle between voltage and current is 90 ¢X. The current leads the voltage 90 ¢X in phase.
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Power relation
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Instantaneous power of capacitor circuit:
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Calculation formula of instantaneous power of capacitor circuit
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The waveform of instantaneous power PC is shown in Figure D. from the figure, it can be seen that the amplitude of PC is UI, and the frequency is 2 £s, that is, the frequency that is twice the voltage changes according to the sine law. When u and I are both positive or negative, as shown in the first and third 1 / 4 cycles of figure D, PC is positive, indicating that during this period, the capacitor absorbs electric energy from the power supply and stores it; when one of u and I is negative, as shown in the second and fourth 1 / 4 cycles of figure D, PC is negative, that is to say, during this period, the capacitor will send the stored electric energy back to the power supply. The area surrounded by the positive and negative half cycle curves of instantaneous power is equal, which means that the energy absorbed by the capacitor from the power source is equal to the energy returned to the power source, that is:
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Pure capacitance does not consume energy, it only exchanges energy with power supply continuously.
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The average power of the capacitor is:
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Calculation formula of average power of capacitor
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