In the Arduino Start Kit, the first circuit is very basic: 5V power, 220 ohm resistor, and an LED with a forward voltage of 2.2V. However, here's two things I can't reconcile:
The booklet says that current can be calculated via Ohm's law: I = V/R, I = 5V / 220 ohms, I = .023 amps.
However, I know that, when starting from just the power supply and LED, calculating the resistance required uses Ohm's law as well: R = V/I. This time, though, you need to subtract the voltage drop across the LED (forward voltage) from the power voltage, so we subtract 2.2V from 5V giving us 2.8V. We then use 2.8V as the voltage in the R = V/I calculation (and then provide the LED's desired ideal current as I, etc.)
How do I reconcile these two things? Based on what I know in #2, shouldn't the circuit's current be calculated using the voltage across the resistor, 2.8V, instead of the 5V power supply as is used in #1 above?