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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.

MrAP
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    https://en.wikipedia.org/wiki/Short_circuit – Mario Dec 18 '16 at 20:59
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    A zero ohm connection across one or more circuit entities. – Chu Dec 19 '16 at 00:01
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    @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 06:52
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    Customer to technician: "Don't just stand there saying 'short circuit'! Lengthen it!" – Nick Gammon Dec 19 '16 at 08:33
  • It is when someone connects both white wires to one terminal of a switch, and both black wires to the other. (saw that happen once) –  Dec 19 '16 at 17:16
  • Short circuit is the path a current takes as a shortcut. Why to bother with that motor if I can flow through that copper wire instead? – Crowley Dec 19 '16 at 18:04
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    Downvoted because not even the most basic research effort is shown. – Lightness Races in Orbit Dec 19 '16 at 18:53
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    @LightnessRacesinOrbit The question is not so silly as it may seem. The term "short-circuit" is slightly overloaded in electrical engineering and the concept(s) it conveys are not so obvious if one delves deeper in the theory. It is one of those concepts that many textbooks give almost for granted, but can give some headaches when one begins to nitpick. – LorenzoDonati4Ukraine-OnStrike Jan 06 '17 at 21:56
  • @LorenzoDonati: I didn't say it was a silly question, I said there was no research effort shown, which I maintain to be true. – Lightness Races in Orbit Jan 07 '17 at 01:34
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    @LightnessRacesinOrbit i think that you have not read the question details properly. I have said that i searched many websites but there is discrepancy all around. – MrAP Jan 07 '17 at 06:40
  • That is the problem with short circuits, they don't last long. –  Jun 05 '17 at 19:21

7 Answers7

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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:

schematic

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.

MrAP
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Majenko
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  • If the potentials across the path are the same, the resistance remaining low, then also will it be known as a short circuit and will there be current across in that case? – MrAP Dec 18 '16 at 21:16
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    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
  • What if we set the resistance and potential difference both to 0?Will the charge not flow(I=0/0)? – MrAP Dec 18 '16 at 21:23
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    @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
  • Could you please add a good diagram for short circuits? – MrAP Dec 18 '16 at 22:15
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    @MrAP: Any elecrtical wire is generally considered a short ciruit; draw a straight line and there you have your schematics. –  Dec 18 '16 at 22:51
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    Schematics added. – Majenko Dec 19 '16 at 00:13
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    Deserves a +1 for the 'On Fire' probe alone – Matthew Gordon Dec 19 '16 at 00:20
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    @MrAP: *"What if we set the resistance and potential difference both to 0?"* - That is similar to the question "What happens when two point-particles collide?" _(the PE equation says both get infinite velocity)_. The real answer to both questions is "our equations don't work, but that's fine because that situation is impossible in the real world" – BlueRaja - Danny Pflughoeft Dec 19 '16 at 02:48
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    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 07:34
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    ...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 08:51
  • /agree with user23013 5000A would appear only when the Rint of the battery is 0, which would assume the voltage source is ideal - but an ideal voltage source simply *wouldn't get on fire*; I'd say that mentioning Rint is worthwhile here, since it, in some situations, actually prevent any serious damage from even a complete shorting Rload. –  Dec 19 '16 at 08:51
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    Battery on fire assumes that all conductors can handle 5000A. The PCB/the wires might burn before before the battery itself does. – Lundin Dec 19 '16 at 08:55
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    @Lundin if a conductor could not handle the current, it should be labelled a 'fuse' instead. Similarly, the battery that catches fire should be labelled "incendiary device". Thus, all electronics is illegal. –  Dec 19 '16 at 17:14
  • I am wondering if anyone actually read the last paragraph of the answer. By the comments I am guessing not. – Majenko Dec 19 '16 at 17:15
  • @BlueRaja-DannyPflughoeft: What happens if we set both resistance and potential difference to 0? First task: Imagine a piece of cooper wire is ideal superconductor. Second task: Let it lie on the table. Voila, no current flows. Nasty explanation may be: You are dividing hard zero (real voltage) by soft zero (idealised resistance). – Crowley Dec 19 '16 at 18:14
  • @Majenko Just a nitpick: "Current flows between differences in potential." Well, excluding the corner case of ideal conductors, i.e. "real" short-circuits, where the resistance is actually zero. V = R * I, if R is zero any amount of current can flow and still we have no voltage drop. In other words, ideal conductors don't need a voltage across them to support a current flow. – LorenzoDonati4Ukraine-OnStrike Jan 06 '17 at 21:44
  • @LorenzoDonati Except of course the fact that they don't exist. There is no "ideal" anything. Even superconductors aren't ideal conductors (though they are getting close). – Majenko Jan 06 '17 at 21:45
  • @Majenko Of course ideal things don't exist *physically*. They are useful nonetheless. It's all about the models we use and how well we want to model the physical phenomena. Ideal conductors *do exist as models*, since all those lines we draw in a schematic connecting components *are* ideal conductors, because usually (at least at 1st order approximation) we don't care about the resistances of the PCB tracks or connection wires. – LorenzoDonati4Ukraine-OnStrike Jan 06 '17 at 21:51
  • @LorenzoDonati However, in reality, they do have a resistance, and thus there is a potential difference across them. Approximation for circuit design is one thing. Describing how current flows is another entirely. Besides - if you place an ideal conductor across an ideal voltage source the voltage across it will be 0 regardless of the voltage of the source, so I=V/R = 0/0 = 0. So no current, no voltage, and probably no universe either because you just broke it ;) – Majenko Jan 06 '17 at 21:54
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    @Majenko Of course that's the problem of ideal models! The are all nice and dandy and easy to understand... until you realize that if they existed they would blow up the universe! :-D – LorenzoDonati4Ukraine-OnStrike Jan 06 '17 at 22:02
  • @Majenko, Re "explode" Are you serious? Practically speaking does it happen in real world or would the safety net be activated? – Pacerier Oct 04 '17 at 13:09
  • @Pacerier Yes. Just ask Samsung. – Majenko Oct 04 '17 at 13:09
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This is a Short Circuit:

Johnny Five!

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!

Cort Ammon
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    The robot shown is called "number five" or "Johnny 5". –  Dec 19 '16 at 07:27
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    Can you please explain the picture which you have given? – MrAP Dec 19 '16 at 08:45
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    @MrAP It's main character from "Short Circuit" show. – Mołot Dec 19 '16 at 08:47
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    @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
  • https://www.youtube.com/watch?v=WjeptaI2T8E – Matthew Whited Dec 19 '16 at 18:44
  • +1 for making the joke that I desperately wanted to make but couldn't without commenting or answering frivolously. – Todd Wilcox Dec 20 '16 at 01:14
  • With the wire across the wall socket, I'd expect the circuit breaker to be triggered. After all, that's what it's for... – Jasper Dec 21 '16 at 18:00
  • @Jasper Yes, there is that circuit breaker. I didn't know how deeply I wanted to get into that. If you put a 8AWG wire across a wall socket, you're right that the circuit breaker will trip first. If you put a 20AWG, it may melt before the circuit breaker trips. (see also: arc welding for dummies. And by that, I mean "dummies") – Cort Ammon Dec 21 '16 at 19:09
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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.

MrAP
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Passerby
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    +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 06 '17 at 18:53
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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.

Peter Bennett
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    Unwanted? Only sometimes. – Passerby Dec 18 '16 at 21:09
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    @passerby usually called closed if it's wanted – Jasen Слава Україні 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 09:59
  • @nigel222 an intentional short is sometimes called a 'scotch' - e.g. on a HV a line disconnected for working on it, the scotch is put between ground and HV so the circuit will trip if someone inadvertently switches back on. – Pete Kirkham Dec 19 '16 at 13:12
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    @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. –  Dec 19 '16 at 14:06
  • @PeteKirkham Not to be confused with [Scotchlok connectors](http://solutions.3m.com/wps/portal/3M/en_US/EMDCI/Home/Products/ProductCatalog/~/Electrical-Commercial-Industrial-Products-Catalog/Electrical-Wire-Connectors-and-Tools/Insulation-Displacement-Connectors-and-Tools/3M-Scotchlok-IDC-Connectors-for-Electrical-Applications/?N=5430181+3294857497&rt=r3). – Andrew Morton Dec 19 '16 at 19:20
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    @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
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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.

supercat
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0

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.

dim
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Tom Bridgeville
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    The inductance of a lightbulb's filament is negligible: http://electronics.stackexchange.com/questions/41587/why-are-lightbulbs-considered-resistive-load. Lightbulbs actually have a very high startup current, it just lasts for a fraction of the cycle. – Bryan Boettcher Dec 19 '16 at 15:07
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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. 

Suraj Khutale
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