A capacitor is a fundamental electronic component designed to store electrical energy within an electric field. It accomplishes this by accumulating electric charge on two conductive plates that are separated by a non-conducting insulating material, known as a dielectric. When a capacitor is connected to an alternating current (AC) source, which is characterized by voltage that periodically reverses direction, it exhibits a unique behavior. This interaction allows the capacitor to appear as though it is passing current, even though the conductive plates are physically separated by an insulator.
Capacitor Structure and DC Operation
A capacitor is constructed from two parallel conductive plates, often metal foil, physically separated by a dielectric material such as ceramic or air. The dielectric is a non-conductive insulator that prevents the free flow of electrons through it. This physical separation is the key to the capacitor’s function.
Using Direct Current (DC), which is a constant voltage, electrons flow from the source onto one plate and are drawn away from the other, creating a charge imbalance. Current flows momentarily as the capacitor charges, building up an electric field in the dielectric.
Once the voltage across the capacitor plates equals the DC source voltage, the flow of charge stops completely. The charged capacitor acts like a break in the circuit, effectively becoming an open circuit because the insulator blocks any steady electron flow. This behavior is why capacitors are described as blocking DC current.
The Continuous Charge-Discharge Cycle in AC
In an AC circuit, the applied voltage constantly changes and reverses its polarity. This continuous change prevents the capacitor from fully charging and settling into the open-circuit state seen in DC operation. Instead, the capacitor is subjected to a constant cycle of charging and discharging.
As the AC voltage increases in one direction, the capacitor charges, pulling electrons onto one plate and pushing them off the other. When the AC voltage reverses its direction, the capacitor starts to discharge the stored energy back into the circuit. It then immediately begins to charge again with the opposite polarity as the source voltage continues its reversal.
The apparent flow of current in the external circuit is not electrons physically passing through the dielectric. It is the continuous movement of electrons onto and off the capacitor plates, driven by the changing AC voltage. This movement creates a “displacement current” inside the capacitor, which is the result of the time-varying electric field across the dielectric. This makes it seem as though the AC signal is passing through the component.
Capacitive Reactance and Frequency Dependence
The opposition a capacitor presents to the flow of alternating current is known as capacitive reactance, symbolized as Xc. Like resistance, capacitive reactance is measured in Ohms, but it is a distinct concept because it does not dissipate energy as heat. Instead, the capacitor absorbs and releases energy back into the circuit.
Capacitive reactance is dependent on the frequency of the AC signal. This opposition is inversely proportional to the frequency: a higher frequency results in a lower capacitive reactance. When the AC signal changes direction quickly, the capacitor has less time to build up a full charge before the voltage reverses.
This lack of time to fully charge means the opposition to the current flow remains low, allowing more current to pass. Conversely, at a very low frequency, the capacitor has a longer time to charge during each half-cycle. This leads to a large buildup of opposing voltage and a high capacitive reactance. At extremely low frequencies, the capacitor begins to act nearly like the open circuit it becomes in DC operation.
The Current-Voltage Phase Relationship
The behavior of a capacitor introduces a specific phase shift between the voltage across it and the current flowing through the circuit. In a purely capacitive AC circuit, the current reaches its maximum value a quarter-cycle, or 90 degrees, before the voltage reaches its maximum value. This relationship is summarized by stating that the current leads the voltage.
This phase difference occurs because the current flow is directly related to the rate of change of the voltage. The maximum current flows when the capacitor is uncharged and the voltage is changing the fastest, which happens when the voltage waveform crosses the zero-point. As the voltage approaches its maximum positive or negative peak, its rate of change momentarily slows to zero.
When the voltage is at its peak, the capacitor is fully charged, and the current flow momentarily stops, reaching zero. The current waveform peaks exactly when the voltage waveform is crossing zero, and the current is zero exactly when the voltage is peaking. This means the current’s cycle starts ahead of the voltage’s cycle by a quarter of a full wave.