A direct current cannot pass through a parallel plate capacitor because

Analyze the question: We want to know why a steady (DC) current cannot flow through a parallel plate capacitor.

Concept: A parallel plate capacitor has two metal plates separated by an insulating material called a dielectric (air, paper, mica, etc.). The plates themselves are conductors. Charge flows onto the plates during charging, but no charge can cross the gap between them.

Solution:

• A. The plates are non-conducting. Wrong. Capacitor plates are made of metal, so they are good conductors.
• B. The dielectric between the plates is non-conducting. Correct. The insulating dielectric blocks any flow of charge across the gap. When a DC source is connected, current flows only briefly while the plates charge up. Once the capacitor's p.d. equals the supply voltage, the flow stops. No steady current passes through the capacitor.
• C. There is a leakage of charge from the plates. Wrong. Leakage is a small imperfection in real capacitors. It does not explain why DC is blocked.
• D. The plates have equal but opposite charges. Wrong. This describes the state of a charged capacitor. It is a result of charging, not the reason charge cannot cross the gap.

Exam tip: Remember "a capacitor blocks DC, passes AC." With AC, the plates repeatedly charge and discharge, so current appears to flow in the circuit, but no charge actually crosses the dielectric.