# What is a short circuit at the electron level?

I'm trying to understand when you can say a circuit is being "shorted".

Obviously you are creating a short circuit when you hook up the two ends of a battery with a wire. But the wire does offer some resistance, even though its extremely little. What if you hook up the two ends of the battery with a 1 ohm resistor? Is it still being shorted?

So where is the line drawn between shorting and well, not shorting? What is actually happening to the electrons in both cases?

Thanks!

• I dislike the saying "what is happening to the electrons". Call me a purist but electrons moving are the side affect of an electric field through a conductor. Yes, you can say, what is happening to the electrons, but why not ask about current? Each electron has thermal energy and relatively random moments, with the electric field applied they have a general drift and as you sum over the very large number of them a current will become apparent. If you were watching a single electron you would learn very little. – Kortuk Aug 19 '11 at 15:39

The name "Short Circuit" comes from an unintentional path for the electrons which is "shorter" than that we intend.

Crudely speaking, the electrons will follow the path of least resistance. Normally we control that flow through our circuit using the components, such as resistors, capacitors, transistors, etc. When an unintentional route between two points is created that is of a lower resistance (thus "shorter") than we want, that is where the electrons will prefer to go. This is what we term a "short circuit".

On an electron level it is no different to any other part of the circuit, but from our point of view it is a bad thing that causes things to happen that we didn't want.

• +1, but not all electrons will follow path of least resistance - only most of them, and there will still be a much lower parallel current in the intended path. – sharptooth Aug 19 '11 at 14:33
• @sharptooth Hence the "crudely speaking". – Majenko Aug 19 '11 at 14:34

The definition of a short-circuit is not clearly defined, but it has to do with a lot less electrical resistance and hence more current than we expect in a given situation.
Much of the electrical current occurs through free electrons in a metal lattice, like in copper wires, where they can move with little resistance. Applying an electric field will move zillions of these electrons, and resistive material is needed to limit this current.
If there's too little resistance to limit the current so that it exceeds what the system can take we talk about a short-circuit. This excessive current may damage certain components unintentionally, or intentionally like in case of a fuse. The fuse is a weak spot in a system that we sacrifice to take the blow, so that the circuit becomes interrupted before other components get damaged.
Sometimes we talk about a short-circuit even when there's no damage at all, like when we bridge (part of) a circuit so that the current flows past it, but that it's limited in another part of the circuit, like the power supply.

At the level of an individual electron a short-circuit doesn't look much different than a normal current path, it will move because of the electric field. It's just the number of them which can flow which makes all the difference.

You are correct that anything we use to carry current does have some resistance due to the physical nature of materials. But in general, the way I understand it, a short circuit means direct connection.

But let's consider a couple cases where we either:

1. have an unintentional spike in current due to an accidental direct connection that is considered unsafe to the rest of the system. The current is flowing in a path that we did not intend (hence "short" like short cut) Or,
2. would like to connect two nodes in our circuit for a functional reason (a switch)

In the first case, we're interested in the health of our system and a short circuit here means that too much current will be drawn or sunk in a place that we find particularly sensitive - so this can simply mean not enough resistance where "enough" is defined by what our system can safely tolerate.

In the second case, we are interested in connecting two different areas of our circuit together with as minimal distortion/signal/information loss as possible. In this case it is in our interest to have as little resistance as possible.

So I believe to answer your question, it is a matter of context and what you are trying to protect or accomplish. At the electron level, it's just current.

A "short" is a generic term that, at the electron level, doesn't make much sense. But our culture interprets it as "something electrical is messed up".

The common use for "short" is when there is electrical conductivity (low resistance) between two parts of a circuit that shouldn't have conductivity. Normally this is from two wires or pieces of metal that are touching that shouldn't touch.

There is no more detailed definition for "short" than that. It's a lot like the term "cancer" in that regard. There are over 200 types of cancer, and just saying "he has cancer" without specifying what kind of cancer is not much more useful than saying "he's sick".

Short-circuit usually means that the resistance is so much lower than expected that it doesn't matter it is nonzero - current in the short circuit path is much higher that in other paths. For example, you short the contacts of the wall outlet with a screwdriver. A screwdriver is not a superconductor but its resistance is so low that current is extremely high and you get a spark, points of contact burn and overcurrent protection hopefully trips.

In terms of electrons short circuit would mean that the dominating flow of electrons will follow the short circuit path and that flow will far exceed the one for which the circuit was intended for.