This was an interesting thread to read.
In every basic electricity / electronics class I have taught I have always been amazed how much time we would spend getting across the idea that electricity has potential (yes, like a pressurized pipe) and that it will flow from the highest potential (negative potential pole) to the lowest potential (positive potential pole) if a path is provided. In advanced electronics, when we start talking about semiconductors we introduce "hole flow". A Model A owner does not need to care about hole flow. (BTW, very nice analogy Mike V.)
So, in our positive grounded cars. (positive terminal connected to the chassis)
The starter in our cars use one wire and one wire only. It is the big honkin' wire that comes from the battery and goes the the switch on the top of the starter. When you push the starter button, electrons flow from the battery, through the big wire, through the now closed switch, through the starter windings and into the frame of the engine. From there they travel into the chassis of the vehicle and beck to the battery through the cable attached from the chassis to the other battery terminal.
The yellow wire has nothing to do with the starter circuit at all. If you disconnected the yellow wire the engine would still turn over, it just would not start.
The yellow wire, as has been pointed out, supplies power to the rest of the car, including ignition, lights, horn and the generator.
The ignition, lights and horn will all complete their electrical path by using the chassis of the car to let the electron get back to the grounded side of the battery. So, when you push the horn button the electrons will flow through the big honkin' starter wire just to get to the yellow wire, through the horn switch, though the windings of the motor in the horn and then to the chassis back to the battery.
And that leaves us with the generator. But wait! It doesn't "use power", it makes it.
We don't want electrons flowing from the battery through the generator and then to ground. We want the generator to put electrons back into our battery. Is it pushing the electrons against the flow you ask? (come on.... ask... you know you want to.
)
Well... no... and yes. It depends on the point of view.
This is where it gets tricky...
Electrical potential always wants to equalize. So....our battery wants to get its extra electrons on the negative pole over to the positive pole where there is a lack of electrons. If you lay a wrench across the terminals (not recommended unless you like heat, smoke, fire and possible explosion) the electrons will scurry over to the positive side until there is the same amount on both sides. The potential energy has been equalized and your battery is dead. (or in other parts of the world your battery is flat) In any circuit where there is a difference in potential, electrons will flow in the direction needed to equalize.
So, if the generator, when it is not running, where just hooked directly to the battery it would behave like a motor. That is why we have the cutout relay in there, to prevent electrons from flowing through the generator like a motor.
But, when running,
from the point of view of the generator, its output post has lots of electrons and is the negative side of the circuit and the negative post on the battery is the positive because the generator has more electrons than the battery. (assuming the generator is functioning properly and is adjusted with a voltage higher than the battery) So, electrons are still flowing from the highest potential (negative post of the generator) to the lowest potential (negative post of the battery) and the circuit is complete from the positive post of the battery through the frame to the frame of the generator.
So, it isn't really pushing electrons against the flow. It is a completely different flow that just happens to be using the same wire.
Well, there's the bell. Give this AMPle time to sink in and it won't seem so reVOLTing. And remember - resistance is futile if less than 0.01 ohms.
All you have to know is that the current flows when it has a complete path from one terminal to the other, and when it flows it can be converted to do work, either as light, heat, or torque, as in turning the stater armature.
