# What exactly is a short circuit?

In my book there is no explanation of the phrase short circuit but at many places the author has used it. I had googled it. Some explain it as the flow of charge along a high potential difference while others explain it as the flow of charge along a low resistance path. What exactly is a short circuit? An explanation along with a diagram would be very helpful.

• en.wikipedia.org/wiki/Short_circuit – Mario Dec 18 '16 at 20:59
• A zero ohm connection across one or more circuit entities. – Chu Dec 19 '16 at 0:01
• @Chu In theory yes. In reality usually it's just really close to zero resistance. Altough it would be fun to see a true and ideal zero resistance short circuit. :) – Num Lock Dec 19 '16 at 6:52
• Customer to technician: "Don't just stand there saying 'short circuit'! Lengthen it!" – Nick Gammon Dec 19 '16 at 8:33
• Downvoted because not even the most basic research effort is shown. – Lightness Races in Orbit Dec 19 '16 at 18:53

In simple and practical terms, a short circuit is an unwanted or unintentional path that current can take which bypasses the routes you actually want it to take.

This is normally a low resistance path between two points of differing potential.

For instance:

simulate this circuit – Schematic created using CircuitLab

In the left simple LED circuit, just over 6 mA is flowing round the circuit. Create a short circuit, represented by a very low resistance (no wire is a perfect 0 Ω conductor) and 5000 A wants to try to flow through it. That's bad news for the battery. The battery could well explode. What is certain, though, the internal resistance of the battery will limit the current that can exist and a large voltage drop will be seen at the terminals of the battery causing the whole circuit to stop functioning.

• If there is no difference in potential then no current can flow. Current flows between differences in potential. – Majenko Dec 18 '16 at 21:20
• @MrAP 0/0 is an undefined value. In fact in this case it can be any value. Looking at Ohms law the other way, for a '0' ohm resistor, there will be 0V across it regardless of the current: $V=IR=I\times0=0$ holds true for any value of I. – Tom Carpenter Dec 18 '16 at 21:28
• Deserves a +1 for the 'On Fire' probe alone – Matthew Gordon Dec 19 '16 at 0:20
• I think you should just add the internal resistance of the battery to the circuit. Keeping 6.38mA on the intended path doesn't seem realistic. – user23013 Dec 19 '16 at 7:34
• ...could lead to being misunderstood as an experiment instruction: "Add LED and 470 Ohm resistor, done. Add 1mOhm Resistor, done. Set battery on fire, done. Add battery (on fire), done." – rackandboneman Dec 19 '16 at 8:51

This is a Short Circuit:

Okay, that's what I really wanted to put down. Let's see if we can answer your question with the rest of the space.

A short circuit is a connection between two elements which you did not intend to connect. In most cases, this behavior is highly unexpected and has a tendency to cause your circuit to behave improperly.

One of the most common short circuits is a wire which connects two points that are being driven to a fixed potential between them (such as the two prongs of a 120V wall socket, which are being driven by the generator at the power company to remain 120V AC apart). In these cases, the result is usually spectacular, and involves secondary effects. For example, if you put a wire across a 120V wall socket, you will find out very quickly that that wire did not have a 0ohm resistance, like an ideal wire, but rather had a very small resistance (0.001ohm or similar), and is now permitting a huge amount of current to flow through it... at least until it heats up and melts!

The important thing about the short circuits is that they always involve some unintended facet of the circuit. You may have a computer power supply that generates a 5V signal using transistors. Now we both know that transistors are imperfect, and there's some secondary effects, but power supply manufacturers go to great lengths to minimize these effects so that you and I can just say "oh, this wire delivers 5V!" When you use a wire to connect this to a ground (0V), we create a short. If you were modeling this power supply as a perfect 5V ideal voltage generator, you'd find the equations just don't work. You cannot connect a 0V line and a 5V line with an ideal wire, because a wire can only be at one voltage. We can no longer model the power supply as an ideal voltage source.

What will happen is the transistors in the power supply will start to limit the current. Typically this limit is very high, and may be high enough to start overheating the transistors. This can do all sorts of nasty things (like melt solder), and the result can easily be the failure of the power supply!

• The robot shown is called "number five" or "Johnny 5". – JWRM22 Dec 19 '16 at 7:27
• Can you please explain the picture which you have given? – MrAP Dec 19 '16 at 8:45
• @MrAP It's main character from "Short Circuit" show. – Mołot Dec 19 '16 at 8:47
• @mrap short circuit is a set of 19 80s movies with the main character being a government kill bot that gained sentience and escapes. Funny movies. – Passerby Dec 19 '16 at 17:45
• youtube.com/watch?v=WjeptaI2T8E – Matthew Whited Dec 19 '16 at 18:44

A short circuit is a specific type of parallel circuit where on connection between any two given nodes of the circuit has a significantly low resistance compared to the other two.

In this case, as the voltage should be equal at the two nodes, the resistance of each parallel circuit path will divide the total current through them. Since one path has a very very low resistance, the bulk of the current will go through that path. This often deprives the other path of any significant current for most practical use.

i.e. A 100 ohm motor in parallel with a 0.01 ohm resistor will be a short circuit. A 0.01 resistor in parallel with a 0.01 resistor will not be considered a short circuit, as the two resistances are fairly similar. Note a wire is just an imperfect conductor with some resistance typically in the single or lower digit range.

• +1. As another example, consider a bias circuit which using 100MOhm resistors - even something at 10kOhm could be considered a short circuit. – Tom Carpenter Dec 18 '16 at 21:30
• @Passerby, why "In this case, as the voltage should be equal at the two nodes"? – MrAP Jan 6 '17 at 18:53

A short circuit is simply an unwanted connection between two points in a circuit. This unwanted connection will usually cause the overall circuit to misbehave in some way.

• Unwanted? Only sometimes. – Passerby Dec 18 '16 at 21:09
• @passerby usually called closed if it's wanted – Jasen Dec 18 '16 at 22:18
• Interesting observation, can a short circuit be a wanted connection? I was once trying (ill-advisedly) to diagnose a nonfunctioning starter motor with a little multimeter. I bridged the corroded connector between battery and vehicle wiring with my meter probe (i.e. made the missing connection). The motor current vaporised the meter probe. – nigel222 Dec 19 '16 at 9:59
• @nigel222, yes, google "crowbar circuit" - a method that in an emergency actively sacrifices some cheap parts (a fuse) by deliberately applying a hard short to a power rail in order to protect more valuable circuitry. – Wossname Dec 19 '16 at 14:06
• @AndrewMorton or anything else which starts 'scotch' - I only found out about them because there's a large box with 'scotches' written on it on a platform at Glasgow Central railway station, and I didn't think it likely was a whisky cabinet. – Pete Kirkham Dec 20 '16 at 12:00

A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit. This results in excessive current flow in the power source through the 'short,' and may even cause the power source to be destroyed. If a fuse is in the supply circuit, it will do its job and blow out, opening the circuit and stopping the current flow.

A short circuit may be in a direct- or alternating-current (DC or AC) circuit. If it is a battery that is shorted, the battery will be discharged very quickly and will heat up due to the high current flow.

Short circuits can produce very high temperatures due to the high power dissipation in the circuit. If a charged, high-voltage capacitor is short circuited by a thin wire, the resulting huge current and power dissipation will cause the wire to actually explode.

• Re "cause the power source to be destroyed" Why? – Pacerier Oct 4 '17 at 13:20

If you're talking about your house, you'll notice that your breakers are rated at, for example, 10 Amps, 20 Amps and 100 Amps for the main circuit breakers.

Anything you plug in to say the 10 Amp circuit should draw no more than 10 Amps or naturally it trips the breaker. If you look on the device you are plugging in it will probably tell you how much current it draws, for example, a radio may draw 2.5 amps. The radio is a load, and electrical potentials (120 Volts or sometimes designated as 117 volts, or 110 volts) at the socket deliver current through a load, that is through a resistor which limits current.

Forget for a moment that AC resistance has to be calculated by taking into account capacitance, inductance AND resistance. AC resistance is called impedance, but just assume that the radio's load is all resistive. It draws 2.5 Amps, but now if you unplugged that radio and stuck one single piece of wire into the outlet thus connecting the hot and neutral together, the current would shoot up to infinity (or perhaps 1,000s of amps) because there's an almost immeasurable resistance in that little piece of wire. The breaker trips due to this high current.
The little piece of wire is a direct short circuit which you don't ever want to do. And, suppose a component inside the radio loses its resistive quality and turns into a piece of wire, there you go, you have a short circuit inside the radio and the 10 Amp breaker trips. One unique property of a light bulb is that you can consider it a "piece of wire" (the filament) but it's in a coil (look at a filament closely, it's not straight but is coiled) which provides inductance which limits current, similar to a resistor, but also different from a resistor, and also the filament is inside a vacuum which provides no oxygen, so the filament "burns" but lasts a long time.

A short circuit is typically a path from a supply to its return, whose resistance is small relative to effective series resistance of the supply and the wires connecting it.

If a supply behaves as a resistance in series with a constant voltage, then the amount of power drawn from the supply will increase as the load resistance goes down, but but the fraction of the supply power which reaches the load will increase as the load resistance goes up. These effects will balance out when the load resistance matches the supply-side resistance.

If the load resistance is small relative to the supply-side resistance, a large amount of power will be drawn from the supply, but very little of it will actually make it to the load. A common trait of short circuits is that the amount of power that is usefully employed by a small circuit will generally be far below the amount of power that could be harvested if the load resistance was higher.

## protected by W5VO♦Dec 19 '16 at 22:29

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).