Reducing voltage in a circut for the layman.
Let's say I am building something: (This setup is completely imagingary, so I will use strange numbers.) I have a battery box that has 4 AA batteries rated at 1.5V and 2.7amps (according to wikipedia).
Now lets say I want to hook that up to a LED that has a max 4 volt 20mA forward. I know that I need a resister. I know that V = IR. What I can't wrap my head around is this: How do I change that equation so that is tells me how many Ohms resistor do I need? Because according to that equation if I increase the resistance, I increase the voltage. Becuase were talking about the same source. Do I need to use two V=IR equations and solve for one then place that value in the original.
I just can't get it....maybe I have had too many beers since graduating, or using algeba? I need a layman's way of understanding this...(exasperated sigh...)?