A capacitor blocks direct current (DC) because when connected to a DC voltage source, it initially allows current to flow as it charges up. However, once fully charged, the voltage across the capacitor equals the supply voltage, and no additional current can flow. At this point, the capacitor acts like an open circuit to DC, effectively blocking DC current flow. In contrast, for alternating current (AC), the capacitor continuously charges and discharges due to the changing voltage polarity, allowing AC current to pass through.
How Capacitors Block DC
- A capacitor consists of two conductive plates separated by a dielectric insulator, which does not allow charge to flow directly across it.
- When DC voltage is applied, electrons accumulate on one plate, creating a negative charge, while the opposite plate becomes positively charged due to repelled electrons.
- This charging continues until the capacitor's voltage matches the DC supply voltage, stopping further current flow because the charge buildup opposes additional current.
- Mathematically, at DC (frequency f=0f=0f=0), the reactance XC=12πfCX_C=\frac{1}{2\pi fC}XC=2πfC1 becomes infinite, meaning no current can pass.
Why Capacitors Pass AC
- AC voltage constantly changes direction and magnitude, preventing the capacitor from reaching a steady charged state.
- The capacitor continually charges and discharges as the AC voltage oscillates, allowing current to flow through the circuit in the form of charge movement.
- The capacitive reactance for AC is finite, inversely proportional to the frequency and capacitance, enabling AC current flow.
Thus, capacitors behave as open circuits blocking steady DC current but act as reactive elements allowing AC current to pass through. This property is widely used in electronic circuits for filtering, coupling, and signal processing applications.
