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An LED (light-emitting diode) lights only when current goes the right way: the long leg (anode) towards + and the short leg (cathode, next to the flat edge of the rim) towards −. Backwards, it simply does not light. It also needs a resistor in series, or too much current flows and it can burn out. To choose the resistor, take the LED’s own voltage away from the supply and divide by the current: R = (supply V − LED V) ÷ current.
✓ You passed this lesson’s test.
Long leg plus, resistor always — “an LED works one way only and needs a resistor to limit its current”
Use it for: Building a first LED circuit from a battery kit with an adult, and reading LED circuit plans.
Check: 0.02 A × 150 Ω = 3 V, and 3 + 2 = 5 V ✓
▶ Watch it animatedWork it out in your head or on paper, type the answer and press Check. Stuck? Press “Show me how”.
Right: 0 · Tried: 0
20 questions. Check each answer, then go to the next one. Get 16 or more right to pass.
🏅 Sign in with Google to earn the 💡 LED Lighter badge when you pass — and a certificate when you finish all 10 lessons of Part 2 · Components and Ohm’s law. You can still take the test without signing in.
Why does it work? When should you use it? Ask in English, मराठी or हिंदी.
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