To get back to the original question ---
Here's a physicist's view of circuit theory. It might help you understand things better - or not!
I see you've already learnt about atoms and electrons and that current is a flow of electrons. That's a reasonable place to start but here are a few things that might have been left out.
1) In a metal, one of more electrons from each atom are cast adrift to move freely through the metal. The remaining positively charged ions stay put - unless the metal melts!
What the electrons can't easily do is move right out of the metal. That's because unlike charges attract. Electrons can hop from one positive ion to another but pulling one right out is much harder. In an insulator, all electrons are tightly bound to their atoms.
2) Charge is conserved. It cannot be created or destroyed. If you put some extra electrons into a metal they cannot simply disappear.
3) Like charges repel. If you push an extra electron into a piece of metal, all the others feel its presence and they move over to make room. The information about the new electron's arrival travels at the speed of light so for our purposes it takes no time at all. Less obviously, if you pull an electron out of a piece of metal the others sense its loss and they move to fill the gap.
4) Current is a movement of charge, which you probably knew already. If you could watch the electrons in a wire carrying a modest current of one amp, you would count about 6,000,000,000,000,000,000 electrons going past each second.
5) To make the electrons move you need a force; an electromotive force (emf). This force is commonly - and wrongly - called voltage. The distinction isn't important right now so voltage will do.
6) You should now be able to see that you need an unbroken loop of wire (a circuit) and some volts to get an electric current.
DC circuits are the easiest to understand so we'll start with a battery, some wire and a bulb. The voltage is provided by chemical reactions inside the battery. When you complete the circuit, electrons are pushed out of the negative terminal. All the other electrons in the circuit feel this push so they all try to move at once - but they can only do this if there is somewhere for them to go.
There is.
For every electron that comes out of the battery, one will be sucked into the positive terminal. Charge is conserved. Batteries don't make electrons; they only push them around.
When electrons move through metal they keep bumping into the fixed ions. When this happens the electrons lose energy and the ions get knocked about. The wobbling movement of the metal ions is what we call heat. This conversion of electrical energy into heat is something you don't want in a wire but it's exactly what you want in a bulb. The bulb filament is very, very long and very, very thin. There will be many collisions as the electrons are forced through and thus lots of heat.
Now lets try an AC circuit. Although there are three phases out in the street, your circuit starts and ends at one of the three secondary windings on the substation transformer. This is a complete, unbroken circuit from the neutral end, through the winding, into the house in the live wire, through your own wiring and back through the neutral wire to the transformer. The voltage is supplied by an alternating magnetic field inside the transformer winding. The only real difference between this and the DC circuit is that the current keeps changing direction. This alternating current will heat up a bulb filament just as effectively as DC.
So where does the three phase bit come in?
Yours is not the only house on the substation. There will be more houses on the same phase (same transformer winding) as yours and the electrons in their wiring will all move in time with your own. There will also be houses on the other two phases. The clever bit about three phase supplies is that the currents in the phases are not all moving the same way at the same time. While electrons in one winding are returning through the neutral wire to the substation, electrons in the other two phases are going the other way. In fact, if all three windings are pushing equal currents through the three live wires in the street, the neutral currents will add up to zero!
Your neutral current doesn't have to go all the way back to the substation; it gets sucked up by your neighbours' houses which are on different phases to yours. The advantage of this arrangement is that you can use a much smaller neutral wire out in the street.
To really understand three phase circuits you will, at some point, need to learn about phasor diagrams. You might have seen some of these already. The voltage - or current - in each phase is represented by a line which has both length and direction. The length is proportional to the voltage (or current) and the direction is its phase angle. If two phasors are drawn diametrically opposite to each other, it means that the two voltages (currents) are 180° out of phase. For currents, while one is going one way the other is doing the exact opposite. They add up to zero. Now let's draw three lines 120° apart. Maybe you can intuitively see that these also add up to zero!
For voltages you can do something else. If each line represents 230 volts from phase to neutral (end to centre), the distance between any two ends gives you the voltage from phase to phase. Neat or what!
