What causes a light-emitting diode (LED) to emit light?
The correct answer is Forward current. A light-emitting diode (LED) emits light when forward current flows through it - that is, when the anode is positive relative to the cathode and current flows in the forward direction. LEDs are semiconductor devices that convert electrical energy directly into light through a process called electroluminescence. When electrons recombine with electron holes in the semiconductor material, they release energy in the form of photons (light). This only occurs with forward bias - reverse current doesn't produce light and can damage the LED. For amateur radio operators, LEDs are commonly used as indicator lights, status displays, and visual feedback in equipment. Understanding that LEDs require forward current helps when troubleshooting non-functioning indicators and when designing circuits with LED displays.
Exam Tip
LEDs emit light with forward current only. Think 'F'orward current = 'F'orward light. Reverse current blocks light and can damage the LED. LEDs are diodes, so they only work in one direction.
Memory Aid
"LED = Light with Forward current. Think 'L'ED needs 'F'orward current to 'L'ight. Forward bias = forward light."
Real-World Application
In your transceiver, the power indicator LED lights up when you turn on the radio because forward current flows from the power supply through the LED. If you accidentally reverse the LED connections, no light appears because reverse current is blocked. You need to connect the anode (positive) to the positive supply and the cathode (negative, marked with a stripe) to ground or through a current-limiting resistor for the LED to work properly.
Key Concepts
Why Other Options Are Wrong
Option B (Reverse current): Incorrect. Reverse current does not cause LEDs to emit light. LEDs are designed to block reverse current, and excessive reverse voltage can damage the LED. Light emission only occurs with forward bias.
Option C (Capacitively-coupled RF signal): Incorrect. While RF signals might cause some effects, LEDs require DC forward current to emit light. Capacitive coupling doesn't provide the direct current needed for electroluminescence.
Option D (Inductively-coupled RF signal): Incorrect. Inductive coupling doesn't provide the forward DC current required for LED operation. LEDs need direct forward current, not RF signals.
题目解析
The correct answer is Forward current. A light-emitting diode (LED) emits light when forward current flows through it - that is, when the anode is positive relative to the cathode and current flows in the forward direction. LEDs are semiconductor devices that convert electrical energy directly into light through a process called electroluminescence. When electrons recombine with electron holes in the semiconductor material, they release energy in the form of photons (light). This only occurs with forward bias - reverse current doesn't produce light and can damage the LED. For amateur radio operators, LEDs are commonly used as indicator lights, status displays, and visual feedback in equipment. Understanding that LEDs require forward current helps when troubleshooting non-functioning indicators and when designing circuits with LED displays.
考试技巧
LEDs emit light with forward current only. Think 'F'orward current = 'F'orward light. Reverse current blocks light and can damage the LED. LEDs are diodes, so they only work in one direction.
记忆口诀
LED = Light with Forward current. Think 'L'ED needs 'F'orward current to 'L'ight. Forward bias = forward light.
实际应用示例
In your transceiver, the power indicator LED lights up when you turn on the radio because forward current flows from the power supply through the LED. If you accidentally reverse the LED connections, no light appears because reverse current is blocked. You need to connect the anode (positive) to the positive supply and the cathode (negative, marked with a stripe) to ground or through a current-limiting resistor for the LED to work properly.
错误选项分析
Option B (Reverse current): Incorrect. Reverse current does not cause LEDs to emit light. LEDs are designed to block reverse current, and excessive reverse voltage can damage the LED. Light emission only occurs with forward bias. Option C (Capacitively-coupled RF signal): Incorrect. While RF signals might cause some effects, LEDs require DC forward current to emit light. Capacitive coupling doesn't provide the direct current needed for electroluminescence. Option D (Inductively-coupled RF signal): Incorrect. Inductive coupling doesn't provide the forward DC current required for LED operation. LEDs need direct forward current, not RF signals.
知识点
LEDs, Forward current, Electroluminescence, Light emission
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.