Short circuits

The bulb is the squeeze

Picture our simple loop: a battery, two wires and a bulb. The bulb has resistance. It squeezes the flow, so only a little current goes around the loop, and the bulb glows. In a normal circuit, the part doing the work (the bulb, a motor, a heater) is called the load, and it is what limits the current.

The shortcut

Now connect the two battery wires straight to each other, skipping the bulb. The current now has a path with almost no resistance in the way: just a bit of wire. It takes that shortcut. Almost none goes through the bulb any more, so the bulb goes dark.

Ohm's law: less resistance, way more current

Ohm's law says I = V ÷ R. Keep the voltage the same and make the resistance tiny, and the current becomes huge. With a short, the current is limited only by the small resistance of the wires and the battery's own internal resistance, so it can be many times the normal current.

That current heats things up. The heating in a wire grows with the square of the current (P = I² × R), so ten times the current means about a hundred times the heat in the same wire. The sparks you see are a tiny electric arc and bits of molten metal at the moment the wires touch.

Fuses and breakers

In a house, that heat can melt the plastic insulation on wires and start a fire. So every circuit is protected. A fuse has a thin metal link that melts open when the current goes above its rating. A circuit breaker does the same job with a switch that trips, and you can reset it. Either way, the power is cut the instant the current spikes.

Try it yourself

Without touching anything inside: find your home's electrical panel and look at the breakers. Each one has a number on it, like 15 or 20. That's the current, in amps, above which it trips. Many small electronics and car fuse boxes have fuses with their rating printed on them too.

Good to know

• A short circuit is an unintended, very low-resistance path that bypasses the load.
• The current is then limited only by the small resistance of the wires and the source's internal resistance, so it can be many times the normal current (Ohm's law, I = V ÷ R).
• The heating comes from P = I² × R, in the wires and inside the battery.
• Sparks are a tiny arc and bits of molten metal at the moment of contact.
• Even small batteries can be dangerous when shorted: lithium cells can overheat, swell, vent or catch fire, and a 9 V battery shorted by keys or coins in a pocket can burn. Don't try this at home.
• Fuses melt and breakers trip when the current goes above their rating; breakers get their own episode later.
• Never short or probe wall wiring. Leave wall wiring to a licensed electrician.