Short-Circuiting
Have your heard the term “short-circuit” before? Usually in our “normal lives,” the phrase is a
bad thing and it often means a piece of electronic equipment, like a computer or cellular
telephone, is not working right. While using the term in this way makes sense (more on that
later), short-circuiting can actually be a good thing. Consider the circuit below:
From the video with Mr. Liechty, you probably know that the bulb in this circuit will be turned on, but
not as brightly as it would be if the resistor wasn’t there.
Suppose now that we added a wire to the circuit like this:
Imagine the current flowing counterclockwise like Mr. Liechty was showing. When it comes to the
intersection that is circled in red, it has to two choices: it can go through the resistor like before or it can
go through the wire we added. Because there is no resistance in it, however, all of the current will flow
through the wire we have added. We call this a “short-circuit” because we have given the current a
“shorter” path to follow around the resistor (the path is physically longer, but it’s shorter in terms of
resistance: it’s easier for the current to flow). Therefore, the circuit acts like the resistor isn’t there.
In other words, it acts just like this circuit:
Thus, the bulb will be brighter than before. By adding the wire, we have built a circuit that acts like the
resistor wasn’t there in the first place.
Now of course if that were the only point, it would be a bit pointless. If we wanted the light to burn
brightly, we could just build the circuit without the resistor in the first place. But imagine if instead of
just adding a wire, we actually added a wire with a switch in it, like this:
With the switch open, current can’t flow through the wire we’ve added and it’s as if it isn’t there at all.
Therefore the circuit will act like the first circuit above and the bulb will be dim. On the other hand,
closing the switch will be like adding the wire and so the circuit will act like the second circuit above (and
so like the third circuit). The bulb will shine brighter. Thus we have added a switch that controls
whether the bulb is bright or dimmer!
For the final point, I will explain why “short-circuit” is a bad thing in our normal lives (despite the fact
that it is useful for designing circuits). First of all, everything electronic in our lives is made of circuits.
Sometimes if a circuit inside gets damaged, two wires can be jammed together when they weren’t
supposed to be. This damage changes the circuit by adding a new path for the electricity (just like we
have been doing) but, when it’s not done on purpose, it can stop the circuit from working properly and
the device might break. Thus while short-circuiting can be very useful for electrical engineers, it can be
very frustrating for other people.