The Tollbooth Analogy

The effect of adding resistors is quite different if added in parallel compared to adding them in series. Adding more resistors in series means that there is more overall resistance; yet adding more resistors in parallel means that there is less overall resistance. The fact that one can add more resistors in parallel and produce less resistance is quite bothersome to many. An analogy may help to clarify the reason behind this initially bothersome truth.

The flow of charge through the wires of a circuit can be compared to the flow of cars along a tollway system in a very crowded metropolitan area. The main sources of resistance on a tollway system are the tollbooths. Stopping cars and forcing them to pay a toll at a tollbooth not only slows the cars down, but in a highly trafficked area, will also cause a bottleneck with a backup for miles. The rate at which cars flow past a point on that tollway system is reduced significantly by the presence of a tollbooth. Clearly, tollbooths are the main resistor to car flow.

Now suppose that in an effort to increase the flow rate the Tollway Authority decides to add two more tollbooths at a particular toll station where the bottleneck is troublesome to travelers. They consider two possible means of connecting their tollbooths - in series versus in parallel. If adding the tollbooths (i.e., resistors) in series, they would add them in a manner that every car flowing along the highway would have to stop at each tollbooth in consecutive fashion. With only one pathway through the tollbooths, each car would have to stop and pay a toll at each booth. Instead of paying 60 cents one time at one booth, they would now have to pay 20 cents three times at each of the three tollbooths. Quite obviously, adding tollbooths in series would have the overall effect of increasing the total amount of resistance and decreasing the overall car flow rate (i.e., current).

The other means of adding the two additional tollbooths at this particular toll station would be to add the tollbooths in parallel fashion. Each tollbooth could be placed in a separate branch. Cars flowing along the tollway would stop at only one of the three booths. There would be three possible pathways for cars to flow through the toll station and each car would chose only one of the pathways. Quite obviously, adding tollbooths in parallel would have the overall effect of decreasing the total amount of resistance and increasing the overall car flow rate (i.e., current) along the tollway. Just as is the case for adding more electrical resistors in parallel, adding more tollbooths in parallel branches creates less overall resistance. By allowing for more pathways (i.e., branches) by which charge and cars can flow through the bottleneck areas, the flow rate can be increased.

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Information taken directly from http://www.physicsclassroom.com/class/circuits/Lesson-4/Two-Types-of-Connections