In order for the Source-and-Destination scheme to work, both would have to have a huge reservoir of electrons in order to sustain a continuous flow! Using the marble-and-tube analogy, the marble source and marble destination buckets would have to be large reservoirs to contain enough marble capacity for a "flow" of marbles to be sustained.
The answer to this paradox is found in the concept of a circuit: a never-ending looped pathway for electrons. If we take a wire, or many wires joined end-to-end, and loop it around so that it forms a continuous pathway ( a circle), we have the means to support a uniform flow of electrons without having to resort to huge reservoirs.
Each electron advancing clockwise in this circuit pushes on the one in front of it, which pushes on the one in front of it, and so on, and so on, just like a hula-hoop filled with marbles. Now, we have the capability of supporting a continuous flow of electrons indefinitely without the need for reservoirs. All we need to maintain this flow is a continuous means of motivation for those electrons, which we'll address in the next section of this chapter.
It must be realized that continuity is just as important in a circuit as it is in a straight piece of wire. Just as in the example with the straight piece of wire between the electron Source and Destination, any break in this circuit will prevent electrons from flowing through it
An important principle to realize here is that it doesn't matter where the break occurs. Any discontinuity in the circuit will prevent electron flow throughout the entire circuit.
A combination of batteries and conductors with other components is called an electric circuit or circuit. The word circuit implies that you return to your starting point and this is an important property of electric circuits. They must contain a closed loop before charge can flow.
The simplest possible circuit is a battery with a single conductor. Now how do we form a closed loop with these two components? The battery has two terminals (connection points). One is called the positive terminal and one the negative. When we describe charge flowing we consider charges moving from the positive terminal of the battery around the conductor and back into the battery at the negative terminal. As much charge flows out of the positive terminal as flows into the negative terminal so there is no build up of charge in the battery. The battery does work on the charges causing them to move round the circuit.
Now I keep talking about batteries and circuits but we need to draw these things to help keep the ideas clear in our minds. To do this we need to agree on how to draw things so that other people can understand what we are doing. We need a convention.