You already know what electric current is: electrons flowing through a wire, pushed by a voltage. Once the current starts flowing, it follows a few rules. Learn them, and you can predict what any circuit will do.
It needs a closed loop
Current only flows when there is a complete path from the battery, through the wires and devices, and back again. That path is an electric circuit. Open the loop anywhere — flip a switch, pull out a plug, break a wire — and the flow stops everywhere at once.
In one loop, the current is the same everywhere
Many people think a lamp “uses up” electricity, so less comes out than goes in. That’s not true! Every electron that goes into a lamp comes out again and flows back to the battery.
What the lamp takes is energy, not electrons. So in a single loop, the current is exactly the same at every point: before the lamp, after the lamp and inside the battery.
At a fork, the current splits
Bulbs can be connected in two ways:
- In a row (series): one after another in a single loop. They share the battery’s push, so each one glows more dimly. And if one bulb breaks, the loop is broken — all of them go out.
- Side by side (parallel): each bulb has its own path back to the battery. At every fork the current splits, and each bulb gets the full push. If one breaks, the others keep shining.
Your home is wired side by side. That is why switching off the kitchen lamp doesn’t switch off the fridge.
It warms things up
When electrons squeeze through a material, they bump into its atoms. The atoms jiggle more, so the material gets warmer. The harder the path — the more resistance — the more heat.
This heat can be useful. Kettles, toasters, hair dryers and electric heaters all work this way. But it can also be dangerous: a cable carrying too much current gets hot. That is why homes have fuses and circuit breakers. They cut the current before a cable gets hot enough to start a fire.
It makes magnets
In 1820, the Danish scientist Hans Christian Ørsted noticed something amazing. When current flowed through a wire, a compass needle next to it swung round. Every electric current makes the space around it magnetic.
Wind a wire into a coil, and the magnetic effect grows much stronger. Put an iron nail inside the coil, and you get an electromagnet: a magnet you can switch on and off.
Electromagnets are everywhere: in doorbells, loudspeakers, scrapyard cranes and every electric motor.
It can flow one way, or back and forth
A battery pushes the electrons in one direction only. That is direct current (DC). The current from a wall socket is different: it swings back and forth, 50 times every second in Europe. That is alternating current (AC).
It takes the easiest path
When current can choose between paths, most of it rushes along the easiest one. Usually that’s fine — until an accidental shortcut appears. A bare wire might touch another wire, or water might get into a plug. Then a huge current can race around the shortcut.
This is a short circuit. Wires and batteries get very hot, sparks can fly, and a fire can start.
This rule also explains why birds can sit safely on power lines — and why a person must never become the easiest path to the ground.
Go deeper: The two rules of Kirchhoff for grown-ups and the extra curious
In 1845, Gustav Kirchhoff wrote down two rules that every circuit follows:
- At any junction, current in = current out. Charge doesn’t pile up or disappear.
- Around any loop, the pushes add up. The voltage of the battery is shared out among the devices in the loop. With two identical bulbs in a row on a 6 V battery, each bulb gets 3 V.
That second rule is why bulbs in a row glow dimly: each one gets only part of the push, so a smaller current flows through all of them.
Go deeper: How fast is electricity? for grown-ups and the extra curious
The electrons themselves drift very slowly: in a household cable, only about a tenth of a millimetre per second. But the push that sets them moving is an electric field that travels along the cable at roughly two thirds of the speed of light. That is why a lamp lights up the moment you flip the switch, even though no electron travels from the switch to the lamp that quickly.
Check yourself
Why do the other lights in your home stay on when you switch one lamp off?
Show the answer
The lamps are connected side by side, each with its own path — Homes are wired side by side (in parallel). Each lamp has its own path, so breaking one path leaves the others working.
Homes are wired side by side (in parallel). Each lamp has its own path, so breaking one path leaves the others working.