I learned about stuff like ohm's law in physics class so I figured I would apply it. I hooked up the LED and one 100 ohm resistor and tried to calculate things like current through a 9 v battery and then tried to calculate voltage drop across the 100 ohm resistor.

As you may be thinking to yourself right now, that was pointless. When I figured out the current and then plugged it back in to V=IR, it said the voltage drop across the 100 ohm resistor was all 9 volts. The LED still lit so I clearly did something wrong. But what?

Do LED's carry a resistance? Is they do, I think that's where I went wrong. I would need to use the 100 ohm resistor + the resistance of the LED to get the correct current, and thus the correct voltage drop across the 100 ohm resistor. Only problem is, I can't find any resistance reading on any of the packaging for the LED. So I don't know.

One more thing. The relationship between current and resistance confuses me. The voltage is a constant of the battery you put in, but adding more or less resistors changes the current? Let's say I have 1 100ohm resistor hooked up to a 9v. the current is 90 mA. But if I add another 100 ohm resistor, it changes to 45 mA. Why does this change the current, aside from the obvious mathematical answer? Why would adding resistors change the flow of electrons?

I have a 5V LED rated at 80 mA. Will it be brighter at 80 mA, than say, 50 mA? So then what effect would adding or subtracting voltage do?

I realize this is already a sprawling post, so I'll stop it there. Thanks for the help guys!