In Canada anyway and i assume around most of the world, two wires come from the electrical company to your house. The hot and the neutral. ( there is also another hot 180 degrees out of phase with the first one but forget about that for now)What i am wondering is simple, why don't they just send the hot wire and you connect it to the ground (after passing through some resistance obviously). This would complete the circuit fine and save them so much money in installing the wire.
There is such thing as single wire power distribution systems that use the ground (earth) as the return. They are often used in big rural places where the costs of wire are high like Australia...
But even in these systems typically there are two (or more) wires coming into the house. This is because the long distribution wire is run at very high voltages to keep the current low. These voltages would be unsafe and unuseful to bring into a home, so they are stepped down with a transformer to a usable voltage and sent into the home over a normal two (or more) wire connection.
More locally, you could connect one side of a light bulb to the hot wire in your house and the other side to a rod driven into the earth and, depending on the soil condition (moist and dense is better) and how close the nearest other earth connection is, it is possible that light would turn on. This is not so good for a few reasons though, among them...
- The brightness of the light will depend on the ever changing soil conditions.
- If the soil has a high resistance (likely), there will be a large voltage drop across it, which represents wasted power.
- Because of the voltage drop across the soil, it is possible you could get shocked if you stood barefoot in the wrong place (thing about the voltage different between your two feet, or between your feet and some other object and "ground" potential).
As a direct answer to your question: Just connecting a conductor in a different potential to ground does not "complete the circuit fine", unfortunately. In multiple locations, the current carrying capacity of the soil is very bad, as there are no guaranteed specifications for the electrical characteristics. It can be 100% bedrock, for example. Bedrock not being a good conductor (1mS/meter = 1kOhm/meter)
Other things: Over here we have a 3-phase system, not 2 (implied in your 180degree secondary phase) I'm not quite sure it would save them that much money, even if it worked. I believe most of the cost would be in clearing the path, putting up poles.
-Source: EE student, designed mock-up wiring for houses. Not a grid designer, though. If you have anything to correct me on, feel free to!
Most houses in the UK (at the very least) have protection devices called RCDs (residual current devices) - these are small inline devices, usually located at your fuse board, that monitor the current down the neutral wire and live wire. If that current difference is unequal by more than about 30 mA, it trips to feed the that line.
It trips the line on the basis that if there is 30 mA more current in the live wire than the neutral wire then that extra current is dangerously going to earth through your body or through your cat, dog or child.
If you don't have a neutral wire and relied on ground then you lose that protection - you would fry because the only thing that can limit the current is the fuse (rated in amps and not milli amps).
In the US they are called GFCIs (ground fault current interruptors). See this question and answer on RCDs. This one too and this also. All related to safety in homes and the need to have a seperate earth and neutral wire.
You're proposing a one-wire power feed, with the user providing the return-current path through their own ground connection? A user cannot provide a good ground with low-enough resistance. It'd be a good way to heat up the earth, rather than provide power to your appliances.
Power transmission with excellent conductors like copper and aluminum at high voltage minimizes losses. But even so, much power is dissipated as heat in these conductors. Ground resistance is far poorer - compare copper resistivity with ground resistivity (lower is better):
Copper: 17 nano-ohm-metres.
earth: 10 -1000 ohm-metres.
To put it another way: earth is about 1,000,000,000 times poorer than copper for conducting electricity.
The copper-wire feed from outside into your house, appliance inside your house, and ground connection would all be in series, so the highest resistance of these three consumes most power and dissipates most heat.