Capacitors are among the most useful components in electronics. One of their fascinating behaviors is that they block direct current (DC) but allow alternating current (AC) to pass through. Let’s understand why this happens.
1. Structure of a Capacitor
- A capacitor is made of two conductive plates separated by an insulating material (dielectric).
- Since the plates are not physically connected, no continuous path exists for current to flow.
2. Behavior with DC
- When a DC voltage is applied, electrons accumulate on one plate, creating an electric field across the dielectric.
- This continues until the capacitor is fully charged.
- At this point, the current stops flowing because the capacitor acts like an open circuit (no path for steady current).
- Hence, capacitors block DC after charging.
3. Behavior with AC
- With AC voltage, the polarity of the supply keeps changing continuously.
- This causes the capacitor to charge and discharge repeatedly.
- Even though electrons don’t physically cross the dielectric, the changing electric field allows a flow of displacement current.
- To the AC source, it appears as if current is flowing through the capacitor.
4. Frequency Effect
- The higher the AC frequency, the faster the capacitor charges and discharges.
- At high frequencies, a capacitor offers very little opposition (low reactance).
- At zero frequency (pure DC), the reactance becomes infinite, meaning it completely blocks the current.
Final Answer
A capacitor blocks DC because once charged, no further current can flow through the insulating layer. But it allows AC to pass since the alternating voltage continuously charges and discharges the capacitor, creating a flow of displacement current that mimics real current flow.