# Why the wire burns in short current with flow of current?

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1. Oct 12, 2015

### 123kid

In short circuit it is said that there is no resistance. So the current produce heat and wire is burnt. But as the rise in temperature increase the resistance then in short circuit heat must rise the resistance. How can we say there is no resistance in short circuit?

2. Oct 12, 2015

### f95toli

There is no resistance in an ideal short circuit. In a real world situation even a good conductor such as a copper wire will have some (small) resistance, which in turn means that there will be Joule heating W=R*I^2 when a current flows through it.

3. Oct 12, 2015

### CWatters

What f95toli said. Only an ideal short circuit has no resistance.

You need to look at the whole situation and understand the difference between real world and ideal case. Consider a battery with two wires connected to it. If the ends of the wires are connected together then we say that there is a "short circuit" but in reality there might be THREE resistors in such a circuit...

1) Where the wires are connected together. Is it a good electrical contact? Are they just touching? Is the metal oxidised? etc
2) In the wires themselves. Copper has some resistance.
3) In the battery. A battery is not an ideal voltage source. Most have some internal resistance even if its a few milli ohms.

The current that flows in the so called short circuit will depend on the sum of all these resistances.

PS. Super conductors have no resistance BUT only below a critical current. If you used a super conductor to short circuit a power supply it's likely that the critical current would be exceeded and the super conductor would suddenly have some resistance again.

4. Oct 12, 2015

### Staff: Mentor

The numbers here can be fairly striking. One meter of 12-gauge copper wire has a resistance of about $5\times{10}^{-3}$ ohms. If we were to connect the two terminals of a typical 12-volt lead-acid auto battery (which would be a very stupid dangerous thing to do - we're calculating here, not planning an experiment!) with that length of wire we'd have a pretty decent approximation of an an ideal short circuit.

The current through the wire will be 2.4 kiloamps until something burns or explodes (or both). The power dissipated in the wire will be about 30 kilowatts. The wire only has a total volume of about 3 cc and a mass of about 25 grams, so we're talking enough power to raise the temperature of the wire by 3000 degrees celsius (that's well above the boiling point of molten copper) in one second if something doesn't break or burn during that second. Of course something does break or burn... but it happens very quickly and it is exciting, not in a good way.

Last edited: Oct 12, 2015