Light one LED: a schematic puzzle
The first circuit everyone builds, and the first one everyone gets backwards.
Current leaves the supply, passes through the resistor, then through the LED to ground. The LED only sets its own voltage, around 2 V; it is the resistor that decides how much current flows: (9 V − 2 V) ÷ 470 Ω ≈ 15 mA. Think of the resistor as the tap and the LED as the lamp: without the tap, the lamp takes everything the pipe can give.
470 Ω resistor: Sets the current. The LED drops about 2 V, so the resistor sees 7 V and lets through 7 ÷ 470 ≈ 15 mA, a comfortable brightness that the LED survives for years.
Red LED: Turns current into light, but only in one direction: anode toward the supply, cathode (the flat side, the shorter leg) toward ground.