Ok I think the best thing for me to do is to start off new here and forget everything I asked.
You know I want to have a LED light up and I will be useing a 9. Volt Battery.
The LED needs 2 Volts at 40 mA to light up. And I know the 9. Volt Battery will be to mutch for it.
So to findout how mutch Resistance I will need I will use Ohms Law.
E over I = R
E = the Voltage 9 Volts and the I = Current that will be the Load 40 mA
So 9 Divided by .040 will be my Resistance YES or NO??
Just tell me if I have this mutch right if I don't just tell me NO and I think I will get it.
almost.
as blueroom said,
"9V
So 9V - 2V (LED forward voltage) = 7V
7V / 0.04 A (or 40ma) = 175 ohms
You need a 175 or 180 (a more common value) resistor."
your battery, assuming its at a set voltage (it declines over time as the battery wears out, but ignore this for now) will provide whatever current is needed through the load to satisfy ohms law. of course this current has an upper limit. so if you put a 1 ohm resistor on there, with your LED (2v forward), then
9V - 2V = 7V
7V / 1Ohm = 7 Amps
which is, of course, way too much current, your battery wont do this much, itll just heat up and bad things will happen.
but say you had 100ohms
then 7V / 100Ohm = .07A = 70mA, which is more reasonable. whatever current is needed will be supplied by ohms law.
now lets say you've been running this LED for a while, and your battery is run down a bit, say its down to 7.7V out of the original 9V (this is with that 175Ohm resistor blueroom suggested):
7.7V - 2V (LED forward voltage) = 5.7V
5.7V / 175Ohm = .032A = 32mA
your LED will only be getting 32mA instead of the maximum of 40mA, so it will be dimmer.
by the way, its probably a good idea to run your LED at less than 40mA (the maximum) so as to make it last longer; if you need the extra brightness get one thats rated for, say, 50mA and run it at 40mA, or something like that.
ive had 9Vs running at maximum before, and the best they can do is about 500mA, or .5A, and this isnt for very long and they get fairly warm. possibly up to 1A in spikes, i dont remember too well.