What device allows current to flow in only one direction?

What device allows current to flow in only one direction?

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

Answer: (A)

This question tests how a component controls the direction of current flow. A diode is designed to conduct easily in one direction and block in the opposite direction. This happens because of its PN junction: when the anode is made more positive than the cathode (forward bias), the depletion region narrows and charge carriers can cross, allowing current to flow with only a small forward voltage drop (about 0.7 V for silicon). If the polarity is reversed (reverse bias), the depletion region widens and very little current can pass, effectively blocking the current. Resistors, by contrast, don’t prefer any direction—they follow Ohm’s law and will allow current to flow if voltage is applied in either direction. Capacitors conduct briefly when the voltage around them is changing (charging or discharging) but block steady direct current, so they don’t enforce one-way flow. Inductors oppose changes in current and can pass direct current in either direction once the transient settles; they don’t inherently restrict flow to a single direction either.

This question tests how a component controls the direction of current flow. A diode is designed to conduct easily in one direction and block in the opposite direction. This happens because of its PN junction: when the anode is made more positive than the cathode (forward bias), the depletion region narrows and charge carriers can cross, allowing current to flow with only a small forward voltage drop (about 0.7 V for silicon). If the polarity is reversed (reverse bias), the depletion region widens and very little current can pass, effectively blocking the current.

Resistors, by contrast, don’t prefer any direction—they follow Ohm’s law and will allow current to flow if voltage is applied in either direction. Capacitors conduct briefly when the voltage around them is changing (charging or discharging) but block steady direct current, so they don’t enforce one-way flow. Inductors oppose changes in current and can pass direct current in either direction once the transient settles; they don’t inherently restrict flow to a single direction either.