Craig113 said:
Okay, so you are telling me that the resistance in other devices than resistors is not reasonably constant when applying different voltages? Or even the same voltage and therefore the only formula describing the power correctly is the first.
I don't exactly know what he is saying (though I'm sure he's right) but the resistance over anything that has any degree of inductance, coils, capacitors, etc isn't even constant over time with constant voltage. A capacitor will at first have almost no resistance and rapidly rise to infinite resistance as it charges. A coil will initially have infinite resistance and then rapidly drop to none as it adjusts to the changing current (both cases ideal of course, but even in reallity they by no means at all give constant resistance - that's their whole function). When voltage is dropped a capacitor will (when treated as a black box) have negative resistance, if you drop it to 0 it will still be charged and have a voltage potential between the sides, provide current (briefly) if shorted, etc. So no, not especially constant..
Don't quite understand your second question, but perhaps someone else does.
[EDIT] Ok, so in case you'd like to further specify you're second question, I now (sort of) get why I'm not understanding it. You talk about "behind" the Earth point, there is nothing behind it. If there is, it's not an Earth point - it's assumed to be tied to earth, i.e. +-0 v with enough of it there to suck up any voltage and/or current going into it. This is obviously in an ideal system, but also holds true for most non-broken real systems. The theory will fail a bit if you, say, connect +5v straight to GND. In an ideal system, this is one of those "irresistable force meets immovable object" situations, a +5 ideal provides +5 no matter what and an ideal GND pulls anything to 0. In real life, you have a short and GND is usually "more ideal" then your +5 and the +5 will give first (not so irresistable as the GND is immovable). Provided your circuit is all nice and balanced, the +5 will drop evenly over your resistances (or rather trickily over things with inductance over time, but at a given moment it still holds sort of true) winding up at 0 at GND.
I'm pretty sure that's not the answer you're looking for, but perhaps you can get a better sense of why I'm not sure what you're asking.
[DOUBLE EDIT] Perhaps I just explained to myself what you're saying.. Yes, a GND point will make that part (up until anything with resistance and a +v on the other side somewhere) free of voltage. In reallity, GND isn't ideal but very close to it. It's considered a point that is large and can absorb any amount of voltage/current (such as pounding a stake into the ground and connection to it) large enough that it will not effect it's potential. Now, in low power circuits GND is usually the negative pole on the power supply, the opposite point to that providing the voltage. Thus, anything put out by the +v will by need be possible to absorb by the -. Any point connected to it with a degree of resistance between it an + (if there is none you have the afforementioned problem, one is just putting out and one is just sucking up until one gives - a short) will put all voltage over this point (it's the fastest way back to gnd) and the rest of the circuit past it will be potential-less baring other sources of voltage.
This is, of course, assuming resistors or resistance only devices - a device containing inductance or capacitance will have to have those considered. If you wind up, for instance, with a capacitor between two ground points that was previously charged you also have a short of sorts - it's discharging itself straight into GND. Any component storing charge must be considered differently although in real life they'll probably (in low v/a devices, high is another matter entirely that I have little experience but much respect for) discharge peacefully until the GND is truly +-0v.
I have no idea about what implementation if any this is for, but I should perhaps mention that the current will rise rather quickly when grounding random points in a circuit so it's by no means something one can do willy nelly with sensitive devices. If a point is grounded the current through the remaining circuit will rise accordingly (it's the new gnd, so everything is loaded over the "new" circuit) possibly way over it's limits (resistors glowing redish, capacitors exploding, ICs suddently losing their magic smoke, etc) so if this is application based rather then theoretical be careful.
