Before we dive into series circuits we need to consider an interesting question involving the direction of current flow. Does it flow from positive to negative or from negative to positive? For that matter, does it even make a difference as far as our analyses will be concerned?
Benjamin Franklin (pictured in Figure 3.2.1 ) began experimenting with the phenomenon of electricity in 1746. In 1752 he performed his famous kite experiment proving that lightning is a form of electricity by capturing charge from storm clouds in a leyden jar (an early form of an electrical charge storage device)1. At this time the modern concept of an atomic model with electrons and protons did not exist and electricity was conceived of as a sort of fluid. Franklin surmised that the “electrical flow” moved from positive to negative. This idea was accepted and became the conventional view. Today we call this idea conventional current flow. In this model, current flows from a more positive voltage to a less positive voltage. We know now that the electron is the charge carrier in metals and the electrons travel in the reverse direction. Essentially, Franklin guessed wrong. Electrons move from a lower potential to a higher potential. We call this model electron flow. For most work, engineers and technicians use conventional flow, although in some cases, such as the explanation of semiconductors, electron flow is easier to visualize for some people. In short, conventional flow exists for historical reasons, and it is the model used for most analyses, including this text.
Figure 3.2.1 : Benjamin Franklin: Technically incorrect but it doesn't really matter.
You might think the current direction would make a big difference in an analysis; after all, it certainly makes a big difference if you drive a car in the wrong direction. It turns out that both forms will achieve the desired results, we just have to be consistent with the usage.
To better understand this, consider that the movement of a net negative charge in one direction can be thought of as a movement of a net positive charge in the other direction. That is, the movement of an electron creates a “hole” where it used to be and that hole is net positive. This is illustrated in Figure 3.2.2 . Here we start at the top with a tube of identical marbles, all pushed to the right. In each step below it we move a marble to the left, mimicking the flow of electrons in a circuit. When we reach the bottom, each marble has been pushed left by one place. We can also arrive at the bottom drawing by simply taking the right-most marble in the top drawing and inserting it to the extreme left by jumping over the other three marbles. Here's the important bit: instead of imagining the marbles moving left, we can also think in terms of “negative space” and imagine the empty slot moving to the right. That's hole flow. The two views are functionally identical as they lead to the same outcome.
Figure 3.2.2 : Electron versus hole flow.
1It is worth noting that Franklin's kite was not struck by lightning. If it had been, he likely would have been killed. The hemp string that was used for the kite was sufficiently wet from rain that it was possible to transfer charge from the atmosphere to the leyden jar, and subsequently to a metal key which would emit a spark.