# Why does a short circuit occur? [duplicate]

For example, if you have a battery and you connect both ends with a short circuit with zero or low resistance, why does it short circuit? What causes this internally? What are really happening with the charges that cause this "explosion"?

edit: I specifically want to know why connecting two ends of the battery with a wire with (hypothetically) no resistance would short it.

• Charges don't cause "explosions". Rapid energy release in matter does. If we are shorting out a battery, then most of the power of the battery will be dissipated inside the battery as heat, the temperature rises rapidly and then either the electrolyte starts boiling or there is an additional release of chemical energy e.g. because the battery catches fire (there is an additional oxidation reaction with the oxygen in air). Jun 6 at 21:10
• Could you explain why this happens when we connect two ends of the battery with a non-resistant wire? @FlatterMann ? Jun 6 at 21:39
• The battery has an internal resistance. That's why the heat is dissipated inside the battery and not in the low resistance wire. It's simply P=R*I^2. Jun 6 at 21:40
• So how does that cause a short circuit? I do not understand. That is my question. Jun 6 at 22:24
• Please look at the second and last of my figures here: chemistry.stackexchange.com/a/137102/79678. In normal operation, a voltaic cell utilizes a spontaneous energy releasing reaction to generate useful electrical energy. So it converts the chemical energy of the reaction to useful electrical energy. Shorting the electrodes results in the chemical reactions going as fast as possible and the resulting energy release heats the chemical mixture, possibly violently so. Essentially the same thing would happen if you just chopped up the cell: the reactions take place and heat is released.
– Ed V
Jun 6 at 22:58

A "short circuit" generically just refers to electricity taking a more optimal path than you may have desired it take. That is, there is an accidental bridge in the circuit such that electricity "flows" along a different path than intended, or the electric potential difference is sufficiently high that it is able to jump over an air gap between parts of the circuit.

The case of a low resistance wire connecting the ends of a battery isn't really a short circuit, but it's similar enough that we lump it in with the category.

Any destructive event caused by this is simply because the resulting current exceeds the operational parameters for the system. This results in blown fuses and other circuit protectors if available, but at worst will result in rapid and extreme overheating of the circuit, which can result in circuit elements exploding and catching fire.

• Could you explain the part about the electrical potential difference? I'm mostly curious of the circuit case, where you attach both ends of a battery with a non-resisting wire. Jun 6 at 21:38
• On a battery there are positive and negative terminals. Assuming the battery is charged, there is an electrical potential difference between these two terminals. This is what incites the "flow" of electricity through the circuit. Jun 6 at 21:41
• So how does it cause a short circuit if no resistance is present in the circuit? Jun 6 at 21:59
• The battery itself has some resistance. Jun 6 at 22:21
• Yes but why does it short circuit? I am asking that question. You are stating something else. Jun 6 at 22:23

A short circuit of a battery is the formation of a low resistance path between the terminals of a battery.
With a short circuit present the resistance of the whole circuit is dominated by the internal resistance of the battery.
That being so the current in the circuit is of the order of emf of battery ÷ internal resistance which can be very high if the internal resistance is very low.
This can lead to a very large generation of heat within the battery which is usually enclosed in a sealed container so potentially you have all the components of a (smallish) bomb, rapid generation of heat in a sealed container.

• So to be clear, you are saying that (except maybe in superconductors, which still use batteries), $V = IR$ (say R is very small and is the internal resistance of the battery), and $I = \frac{V}{R}$ and a large current causes heat (I have not learned about this) and that is what causes this? Jun 6 at 22:29
• Yes, it is the heating within the battery which can cause a problem if the total resistance of the circuit is low. For a NiMH AA battery the internal resistance is between $30\rm \,m\Omega$ and $100\rm \,m\Omega$ and for a lead-acid battery $\approx \rm m\Omega$. Jun 6 at 22:40
• And the resistance is proportional to the heat? Or is it the current that is proportional to the heat generated, which is caused by lower resistance? Jun 6 at 22:47
• @user129393192 Heat is actually dissipated power, and the power dissapated in a device is actually equal to $IV$. If the load is resistive (and that's a reasonable model for a battery's internal resistance), we can combine that with $I=\frac V R$ to get a power dissapation of $\frac{V^2}{R}$. You can see how the power dissapated (and thus heat) grows rapidly as R gets small, and as Farcher pointed out, the internal resistences of batteries tend to be quite small indeed. Jun 7 at 2:48

A short circuit means that an essentially zero electrical resistance material is placed between two locations where a potential difference (voltage) is present prior to introducing the material bridging between the two locations.

The ramifications of a short circuit can vary widely. It can result in a violent event if the amount of current available from the battery to flow through the short circuit is extremely high (as in the case of a car battery), or in a non-event for a low capacity battery, such as short circuiting a AAA alkaline battery.

Hope this helps.

• Exactly. That is what a short circuit is. My question is: what actually is physically happening when you introduce that material between the two locations that causes this "event"? Jun 6 at 22:01
• @user129393192 all the charge from one terminal of the battery discharges into the other terminal? is that the sort of answer you were looking for? Jun 7 at 5:19
• I was just wondering about why the two ends cannot be at the same Voltage and what this means physically for them to be. Jun 7 at 5:31
• @user129393192 see definition here: en.m.wikipedia.org/wiki/Short_circuit Jun 7 at 9:26
• The two ends of a battery are at a different voltage because of the chemistry of the battery. Putting a very low resistance wire there will actually reduce the voltage difference between the ends significantly (something you don't usually model, but in high current cases is important). It doesn't affect the chemistry of the battery wanting the voltage to be at a specific level and driving the current to try to get it back there, which causes a lot of power and heat to build up. Jun 7 at 22:38

Take this simple image of an electric circuit:

Nothing special about it - it's just a battery with voltage $$V$$ connected to a resistor of resistance $$R$$. Ohm's law tells you that the current in the circuit is $$I = V/R$$.

Now consider the short circuit:

What's the current in the circuit? Ohm's law is still $$I = V/R$$, but since $$R = 0$$, the current is infinite, and you get an "explosion".

In practice, there's always some amount of resistance in the wire and also the internal resistance of the battery, so you don't actually get an infinite current. But you still get a very large current, and by $$H = I^2 R$$, you generate a lot of heat that is obviously dangerous and can do things like melt the wire.

• but why do you get an explosion? Why does the battery just not drain very quickly? (or "infinitely quickly" for an "infinite current") Jun 7 at 7:04
• @ilkkachu depends on what you mean by "explosion". One could describe the large current as an explosion of current. Jun 7 at 7:15
• you said 'the current is infinite, and you get an "explosion".', so perhaps you should explain better what you mean with the word. Jun 7 at 7:27
• Which is why I put it in inverted commas ... (keeping in mind that OP also uses the word 'explosion'). Jun 7 at 7:45

A short circuit occurs when an electrical current, consisting of moving electrons, deviates from its intended path due to factors such as loose wires or damaged insulation. Instead of following the regular circuitry, the current takes an unintended shortcut. The absence of resistance in the shorter path allows for a greater flow of electrons through the short circuit.

• Right. My question is not about what the short circuit is. It is why the "short circuit" is dangerous and causes bad things to occur. Why do the greater flow of electrons (current) cause bad things? Jun 6 at 22:30
• Imagine a dam full of water emptying through a single pipe that has some resistance. It would take a while but everything would be in control. Now imagine the same dam emptying all of it water at one time (No resistance) because the dam suddenly disappears. There would be destruction somewhere.. Jun 6 at 22:36

A short circuit is dangerous because it releases a massive amount of energy within a short period of time possibly causing irreversible damage to battery and/or possible injury to personnel handling it.

Rapid energy release can damage the battery by changing its chemical composition, releasing gas or heat damage .etc because the battery is designed to withstand only a limited amount of current which is suitable/optimal for its intended usage/purpose. When short-circuiting what you are essentially doing is overusing it over its intended safety limits/rated current. Unless there is intermediate protective circuitry battery is going to be physically damaged by massive amount of energy release inside battery probably breaking down its usable chemical composition.

Think of it as this way: A battery is designed so most of its power (energy transfer/time) to be released outside of the battery. Then what happens when you are releasing almost all power inside the battery by short-circuiting it? The power released in components of the circuit will be proportional to the resistance of the components squared. So because the wire has very low resistance almost all power will be released inside the battery.

Why do explosives so damaging? Because a large amount of energy is released within a short amount of time. That is what happens inside a battery during a short-circuit. It damages materials.