TỤ ĐIỆN

Capacitors allow AC to pass through while blocking DC because they charge and discharge with AC, creating a continuous current flow. In contrast, with DC, after initial charging, the capacitor becomes fully charged and stops current flow. Thus, capacitors facilitate oscillating charges with AC but restrict steady current with DC. 

In both DC (direct current) and AC (alternating current) circuits, capacitors function in similar ways, but the behavior of capacitors differs depending on whether they are used in a DC or AC circuit.

In a DC circuit:

1. Charging and discharging: When a capacitor is connected to a DC voltage source, it charges up to the same voltage as the source. The capacitor stores electrical energy by creating an electric field between its plates. When disconnected from the voltage source, the charged capacitor can then discharge its stored energy through a load.

2. Blocking DC: A fully charged capacitor blocks the flow of direct current once it reaches its maximum charge, acting as an open circuit in steady-state conditions.

In an AC circuit:

1. Reactance: In an AC circuit, capacitors exhibit reactance due to their ability to store and release electrical energy over time. The reactance of a capacitor depends on its capacitance value and the frequency of the AC signal.

2. Phase shift: Capacitors introduce a phase shift between voltage and current in an AC circuit due to their ability to store and release charge at different points in each cycle of the alternating waveform.

Capacitance has the property of delaying changes in voltage as described in Module 4.3. That is, the applied voltage reaches steady state only after a time dictated by the time constant. In AC circuits voltage and current are changing continuously, and in a purely capacitive AC circuit the peak value of the voltage waveform occurs a quarter of a cycle after the peak value of the current. Therefore a phase shift is occurring in the capacitor, the amount of phase shift between voltage and current is +90° for a purely capacitive circuit, with the current LEADING the voltage. The opposite phase shift to an inductive circuit.

3. Filtering: Capacitors are commonly used for filtering out specific frequencies in an AC signal or for coupling signals from one part of a circuit to another.

Overall, while capacitors operate similarly in both DC and AC circuits with respect to storing and releasing electric charge, their behavior varies due to factors such as reactance and phase shifts when dealing with alternating currents. 

APPLICATIONS

Capacitors are widely used in various applications across different fields. Some common uses of capacitors include:

1. Filtering and Decoupling: Capacitors are often used in electronic circuits to filter out noise or high-frequency signals from power supplies and communication lines. They can also be used for decoupling to stabilize voltage levels and reduce power supply interference.

2. Energy Storage: Capacitors are utilized for storing electrical energy, especially in applications where rapid energy discharge is required, such as camera flashes, defibrillators, and pulse-forming networks.

3. Timing Circuits: Capacitors can be integrated into timing circuits to control the rate of charge or discharge, leading to specific time delays or frequency generation.

4. Power Factor Correction: In AC circuits, capacitors are employed to improve the power factor by compensating for reactive power and reducing line losses.

5. Signal Coupling: Capacitors enable the coupling of alternating current signals between different stages of amplifiers without compromising DC biasing.

6. Motor Starters: Capacitor-start motors use capacitors during the starting phase to provide an additional phase shift that helps initiate the motor's rotation.

7. Tuning Circuits: In radio frequency (RF) circuits and oscillators, variable capacitors play a critical role in tuning frequencies by adjusting capacitance values.

These are just a few examples of where capacitors find application; their versatility makes them essential components across numerous electronic 

In an AC circuit, the factors of a capacitor that are important to consider include:

1. Capacitance (C): The measure of a capacitor's ability to store charge, which determines its ability to store and release energy in an AC circuit.

2. Impedance (Zc): This is the opposition that a capacitor presents to the flow of alternating current, and it varies with frequency according to the formula Zc = 1 / (2πfC), where f is the frequency and C is the capacitance.

3. Voltage Rating: The maximum voltage that can be applied across the capacitor without causing damage or breakdown in an AC circuit.

4. Power Factor: This measures how effectively a capacitor can store energy when subjected to an alternating current.

5. Frequency Response: The behavior of a capacitor at different frequencies, as its impedance changes with frequency in an AC circuit.

These factors play a crucial role in determining how capacitors perform within AC circuits and are essential considerations for proper selection and application of capacitors in such circuits.

In a DC circuit, the factors of a capacitor that are important to consider include:

1. Capacitance (C): The measure of a capacitor's ability to store charge, which determines its ability to store and release energy in a DC circuit.

2. Voltage Rating: The maximum voltage that can be applied across the capacitor without causing damage or breakdown in a DC circuit.

3. Leakage Current: This is the small amount of current that flows through a dielectric material in the capacitor, even when it is fully charged. It's an important factor to consider for long-term stability and reliability in DC circuits.

4. ESR (Equivalent Series Resistance): In a DC circuit, ESR measures the resistance inherent within the components of the capacitor itself and affects its performance.

5. Temperature Stability: Capacitors can be sensitive to temperature changes, so considering how they perform over different temperatures is crucial for proper functioning within DC circuits.

These factors are essential considerations for selecting and applying capacitors in DC circuits to ensure their proper operation and longevity.