I've read the basics of modern domestic electrical systems, but I have a few questions about the very basic premise that I don't think was really explained.
The analogy used in the book compares electric delivery to water delivery – it comes into the house under pressure, and exits under no pressure. Anyway, the electricity leaves the house through the neutral wire, which is zero voltage, or "no pressure" as the book says.
Does this mean that if you have a circuit with just one outlet, and nothing is plugged in, wouldn't you still get a shock from touching the neutral? The voltage wasn't used – this is what I don't understand. If nothing is plugged in, how is the flow suddenly under zero pressure, or no voltage?
Is there a better analogy than water? Doesn't water leave the house either through gravity or… more pressure, i.e. other water pushing it towards gravity?
Oh one more thing: if electricity coming into the house is a constant loop, how does the meter work? Does it take a "resting" state and subtract the surges? The water analogy fails here, because if you don't use any water, no new water comes in (or is that incorrect?) With electricity, if it's always flowing (even if not used), how does the meter know how much you're "using"?
I'm on the brink of getting it, but I'm not quite there yet!
Best Answer
The water pressure/flow analogy is, unfortunately, a common but very imperfect one. If we insist on using it, voltage is something like pressure and current is something like flow rate.
The key concept you're missing is that you need a circuit, not a single wire.
Voltage, like pressure in the pipe, is always measured relative to some reference point. If you were perfectly insulated otherwise, you could hold the hot wire all day and not do yourself any harm, as birds on high-tension wires demonstrate on a regular basis. In the electrical analogy, this is like a capped pipe, or a connection into a tank where the outside pressure is the same as that coming from the pipe. You can extend that pipe or enlarge the tank, but as long as the counter-pressure from the cap or surroundings equals the pressure coming in, no current flows. (I'm oversimplifying like mad, but that's inherent in the faulty model.)
Only when there's somewhere for the water, or electrons, to go will you get current. In a properly designed circuit, that's the neutral, which in the water analogy we can almost think of almost as a drain pipe. If we were talking about DC, you could think of the return/neutral path as a drain pipe; to make that idea work with AC we have to remember that the whole system is sealed, current flows both direction, and we get our current from water sloshing back and forth in the loop of pipes rather than any one drop of water flowing all the way around the loop.
It's current, not voltage, that does the actual work. But voltage ("pressure") is what causes current to flow.
(Of course if you poke a hole in the pipe and let current flow in another direction -- not just stopping at you but through you to a ground, for example -- the voltage will happily do its "work" on you, which is why it's dangerous. Ditto for short circuits, where the "work" goes into heating up the wire and potentially starting a fire.)
Your electric meter measures the current, not voltage. Until the circuit is complete and there's a path from hot to neutral (or, in the bad case, to ground), no current flows and the meter just sits there. When current is flowing, the meter uses a bit of the energy from that current to advance itself; a gentle current spins the meter slowly, a strong current spins it more quickly. In an AC system, the meter is set up to measure both directions of the "sloshing", or to act like a ratchet so it ignores one direction, and again you get a measurement proportional to how much current is being used.
As I keep grumbling, this analogy really is not a very good one, because it leads to misunderstandings like yours. I've forced it a bit farther to answer your questions... but really, the best answer is to stop thinking of electricity by analogy and start thinking of it as its own thing, and just learn how it behaves.