Why Does Electricity Need to Return to the Positive Terminal?

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Ive read a few things about electricity since i haven't brushed up on it in a while, and I realize that a generator pushes the electrons and each one hits off the other one producing an electrical current but only when it returns to the positive terminal, why does it have to return to the positve terminal, i don't remember being explained this and I am just curious.

Also what are the most common forms of generators used in electrical circuits? and what is the generator for the human bodies electrical currents to take place?
 

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electrons are attracted to net positive charges, that's why the electron flow is toward the positive terminal. electron flow is in the opposite direction of conventional current flow, but i think that is an artifact of network theory. some people, military i think, learned current flow in the same direction as electron flow.

in the body, cells pump ions such as sodium and potassium through their membranes, producing a net charge across the membrane(polarization). when say a nerve cell opens up these channels and depolarizes, it starts a chain reaction by depolarizing the cell adjacent to it. this traveling depolarization wave is probably the current you're thinking about. you can also observe muscle activity by measuring electric fields on the skin surface (myogram). for details, you could check a physiology textbook like Guyton's Physiology. i don't trust my memory that well, anyway.
 
can u explain to me how a very simple battery works because I don't understand how the battery starts the electron flow. I mean doesn't something have to instigate the battery to produce an electron flow?

also why are metals so much better at conducting electricity?

and is it better to use atoms that have more orbits of electrons since those electrons on the outside of the atom are easier to be bumped off and help continue the current?
 
To answer your question as precisely as possible, a battery is stored charge between two terminals; a positive and negative. Quite frankly, the resistance that closes the circuit produces the electron flow since the electrons want to flow towards the positive terminal and produce a current flowing into the positive terminal (passive sign convention). Its simply an application of Ohms law and understanding KVL and KCL along with other preliminary circuit models.

As for your question about why metals are better at conducting, you probably remember something called Bohr's Model which is a diagram of the nucleus with concentric energy levels with various designations of states in each. The topmost energy level known as the valence band has a bandgap with the conduction band which is the next highest energy level that an electron can 'escape' to via thermal excitation or photon absorption. In short, the bandgap Eg is very small and minimal excitation is required for electrons to jump into the conduction band of a crystal, perhaps an alloy of some sort, where free electrons mobility increases and now current can be produced. The larger the bandgap, the more insulator like the crystal behaves since a lot of energy will be required to have electrons jump into the conduction band.

The closer you are to the nucleus, the strong force gets stronger and stronger. The more electron energy levels you have filled, the smaller the bandgap should be.
This is my understanding. Correct me if anything needs clarification.
 
so how do u tell what the bandgap is for a specific atom?