eli5: What happens within an electric short circuit?


I know what a short circuit is and how to make that happen. But what actually happens? Why does low resistance in a circuit causes the conductive material to become so hot?

In: 7

The heating effect of the current is given by the power developed in the resistance.

That’s `P=V²/R`

i.e, the voltage squared, divided by a very small resistance.

Lots of power, lots of heat.

The ideal assumption we would use in studying a first or second year circuits class is that wires have 0 resistance. That’s usually a close enough approximations for simple circuits. In real life, wires do have resistance. Let’s say a 20 foot piece of wiring in your walls has 4Ω of resistance. In a short circuit scenario, there is 120 volts across 4Ω which means it tries to draw 30 amps of current.

That would pop most breakers, but there are 30A breakers. If no other devices on that circuit are drawing any current, then potentially, the breaker would allow that 30 amps to flow through that 4Ω wire. If you remember, power is current squared times resistance P=I²R so this wire is drawing 3600 watts. A resistor dissipates pretty much all it’s energy in heat. (If the heat is high enough, you also get light) so basically, you’ve created a 3.6 kW space heater in your walls.

Eli5 it goes BANG very quickly (and if you are lucky there is a fuse in the circuit to stop a fire igniting.)

Resistance resists current. If the current is being pushed by the same electromotive force (AKA EMF, or Voltage), then more resistance means less current, and less resistance means more current. Most of the power sources that we use can apply the same EMF over a very wide range of currents*, so low resistance->large current->the energy has to go somewhere, it turns into heat.

*Batteries do this because the chemical reaction in the battery stops when the terminals in the battery have a certain Voltage difference, and starts up again if the difference drops. Wall power does this because the electrical grid is very very big, so you would have to draw an enormous amount of current to cause any noticeable change.

Every circuit has a “load” which usually is the thing doing work. It can be a motor, a lightbulb, or even just simple heater coil.

Loads are specifically designed to have a certain amount of resistance. A 1200W room heater will have thicker wire than a 600W room heater. The 600W heater has more resistance to choke down the current so it doesn’t overheat. A 1200W heater has less resistance to allow more amps through the device.

For instance, the standard outlet in a US home is 15A and 120V, so it has a breaker that will trip if you give it the full 15 x 120 = 1800W. Allowing for voltage spikes and appliances that draw a hair more amps than they should, the most powerful 120V microwave will typically be 1500W as a safety margin.

If you somehow bypass the full circuit (by going around the load) then, there is almost zero resistance, and huge volumes of amp-current will flow without any resistance. Hopefully the breaker will instantly trip, but if it doesn’t, the wire will get so hot it glows and then it will melt.