zachtheterrible said:
I know that this is probably totally simple, but I have yet to understand the concept of ground, -, +, etc.
I keep on hearing reference to ground, and 0 potential. I have always thought that ground just means the negative side of a battery (if u have a 9 volt battery, ground = -9v, not 0v). The only other ground that I know of is tapping a transformer in the middle to achieve 0 potential. I am totally confused :?
thanx 4 explaining
Zach, there is no such thing as absolute 0V.. all voltages are with respect to some other point. If I have a 9V battery I can call the - terminal ground or 9V_common or anything for that matter. If you have a 9V battery, ground = -9V only whith respect to the + terminal. it may not be so with respect to my wall outlet "ground" and may not be so with respect to my conductive wristwatch band.
Ground is usually the term given to a circuit node that currents return to for analysis purposes (this is easiest to see with the single 9V battery and calling the - terminal ground)
Lets call the -9V terminal ground go ahead make it = 0v. Now the positive terminal = +9V wrt -terminal (0V). If I measure the - terminal with respect to itself, I get 0V by my choice of what I call ground. Had I called the + terminal ground (0V) then when I measure the - terminal I would read -9V with respect to what I called ground (0V)
You can choose any node in your circuit to be the reference point from which all voltages will be measured with respect to. But some choices are easier to deal with than others. It would not make much sense to assign a 0V (ground) to a node that moves around depending on circuit conditions.
How would you label your circuit if you had a stack of 9V batteries connected in series? Which - terminal would you call ground(0V)? You could call any of them ground and all analysis proceeds sticking with that label. If the number of batteries is even, a common choice would be the center-most node for symmetry.
Some argue that the only true ground is earth-ground but this too is just a reference. If I build circuits in an airplane, the airplane frame makes more sense as ground than does the earth (which I cant even get to anyways)
Oh, and what would the martian designers have to say?
The best way to keep it straight is to remember that ALL voltages (All potentials) have no meaning unless they are with respect to some node which is either stated clearly on a schematic or implied.