Complete Circuits
Open, closed and short: what makes a circuit work, and the one mistake that makes batteries hot.
The question
Why does a flashlight need a switch, and why do batteries get hot if you connect the ends with a wire?
What you’ll discover: what makes a complete circuit, what open and short circuits are, and why one of them is dangerous.
In simple terms
BeginnerA road with no gaps
Every working circuit has the same parts:
- A source of energy, like a battery.
- A load, the thing that uses the energy, like an LED, motor or buzzer.
- Conductors (wires) that carry the current there and back again.
- Often a switch, to open or close the path.
Current only flows when there’s a complete loop from one end of the battery, through the load, and back to the other end. That’s a closed circuit. Make a gap anywhere, and it becomes an open circuit: the flow stops everywhere at once. A switch is simply a safe, controlled gap.
The dangerous one: a short circuit
A short circuit is a path that lets current skip the load and go straight from one end of the battery to the other, usually through a bare wire. With almost no resistance in the way, a large current flows. The wire and the battery heat up quickly, the battery is damaged, and some batteries can leak, swell or even catch fire.
Try it on screen first
On the Virtual Workbench, open and close the switch, then set the resistor to “none” to see what an unprotected LED would face. It’s a safe way to see why resistors matter.
Go deeper
IntermediateSeries circuits
When parts are connected one after another in a single loop, they’re in series. The same current flows through every part, because there’s only one path. The battery’s voltage is shared among the parts: in the circuit above, some of the 4.5 V is “used” by the LED and the rest by the resistor.
A common mistake is thinking the LED “uses up” current, so less comes back to the battery. It doesn’t: the current is the same all the way around. What the LED takes is energy.
AdvancedKirchhoff’s laws
Two rules from Gustav Kirchhoff (1845) describe every circuit:
- Current law (KCL): the current flowing into any junction equals the current flowing out. Charge doesn’t pile up or vanish.
- Voltage law (KVL): around any closed loop, the voltage rises (from sources) equal the voltage drops (across loads). In our LED circuit: 4.5 V = VLED + Vresistor.
In a short circuit, the loop’s total resistance is just the wire plus the battery’s own internal resistance, which is why the current can be so large and why the battery itself heats up.
Quick check
What three things does every working circuit need?
A source (like a battery), a load (like an LED) and a complete path of conductors back to the source.
In a series circuit, is the current bigger before the LED than after it?
No. The current is the same everywhere in a series loop. The LED takes energy, not current.
Why is a short circuit dangerous?
Almost nothing limits the current, so the wire and battery get hot quickly and the battery can be damaged, leak or catch fire.
Words to know
- Closed circuit
- A complete loop, so current flows.
- Open circuit
- A loop with a gap, so no current flows.
- Short circuit
- An accidental low-resistance path that skips the load. Dangerous.
- Series
- Parts connected one after another in a single path.
Learn more
- Science World: Build a Circuit: a simple classroom circuit activity
- Alloprof (Québec): Electricity and electrical circuits: a plain-language explainer for students
- Next: Electrical safety
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.