Which component allows current to flow in one direction but blocks it in the other?

• A. Diode
• B. Resistor
• C. Transistor
• D. Capacitor

Answer: (A)

A diode does this. It has a p-n junction that behaves like a one-way gate for charge. When the anode is made more positive than the cathode by about 0.6–0.7 volts (for silicon), carriers can cross the junction and current flows. If the polarity is reversed, the depletion region widens and prevents current, with only a tiny leakage possible. This unidirectional conduction is why diodes are used to rectify AC, protect circuits from reverse spikes, and perform signal demodulation.

Other common components don’t enforce one-way flow in the same simple way: a resistor passes current in either direction according to Ohm’s law; a capacitor blocks steady DC current after charging and only passes AC or changing signals; a transistor controls current flow based on a separate input signal and biasing, rather than acting as a fixed one-way gate.