Untangle a box of fairy lights and you've already bumped into the biggest idea in electric circuits: one path versus many. Let's build a circuit you can break on purpose.
Start herePicture an electric current as a flow — like water running around a ring of pipe. It only moves if the whole ring is joined up. Break the loop anywhere, and the flow stops everywhere on that path.
A circuit is just that loop: a path that starts at a power source, runs out through whatever you want to power, and comes all the way back. The power source here is a battery — think of it as a pump that pushes the flow around. The thing being powered is a bulb, which glows when current passes through it. Joining them up are wires, the pipes the flow travels along.
That single rule — the loop must be complete — is the secret behind almost everything on this page. It explains what a switch does, why one dead fairy light can wreck a whole string, and why your bedroom light keeps shining even when the kitchen light is off. Get the loop, and the rest falls into place.
Below you'll build a glowing circuit board, wire it two different ways, flip a switch, and unscrew a bulb to see the difference for yourself. No equations — just flow and loops.
Here's the rule again, because it does so much work: current can only flow if there is an unbroken path all the way from one end of the battery, through the bulb, and back to the other end. Snip the wire, pull a plug, or leave a tiny gap — and the flow simply stops. There's nowhere for it to go.
That sounds like a problem, but it's actually the most useful trick in the whole subject. A switch is nothing more than a gap you can open and close on command. Close it and the two ends of wire touch, the loop is complete, and the flow runs. Open it and you've made a deliberate break, so everything downstream goes dark. Every light switch on your wall, every power button on every gadget, is doing exactly this: deciding whether the loop is joined up or not.
So a "broken" circuit isn't always an accident. Sometimes you broke it, on purpose, with a switch — and sometimes it's a frayed wire or a burnt-out bulb breaking it for you. Either way, the physics is identical: no complete loop, no flow. Keep that picture in your head and the demo below will make instant sense.
Once you want more than one bulb in a circuit, you have to make a choice that changes everything: do the bulbs share a single loop, or does each bulb get a loop of its own? Those two answers have names you'll use for the rest of your science life.
In a series circuit, all the components sit on one single path, like beads threaded on one string. The current has no choice — it must pass through every bulb in turn before it gets back to the battery. That makes them feel like a team: connected, in order, all on the same line.
In a parallel circuit, the path splits. Each bulb sits on its own branch, with its own complete loop back to the battery. The current divides at the split, sends some down each branch, and rejoins afterwards — so every branch is independent of the others.
That one difference — shared path versus separate paths — decides what happens when something breaks. And the cleanest way to feel it is to build both and start pulling bulbs out. So let's do that.
Pick series or parallel, then play. Tap a bulb on the board (or use the buttons) to "unscrew" it, and flip the switch to open or close the loop. Watch the glowing dots — that's the current flowing. The big question to test: when you unscrew one bulb, which others stay lit?
Pick a wiring style and start breaking things.
Glowing dots = current flowing. A broken bulb shows a snapped filament and a red gap.
Try this on purpose: in series, unscrew any one bulb. The whole string drops dark, because the single shared path now has a gap in it — a broken bulb is exactly like an open switch. Now switch to parallel and unscrew the same bulb. Only that one goes out; the others keep glowing, because each still has its own complete loop back to the battery. That's the entire lesson, right there in your hands.
Now you can answer a question you've lived with your whole life without noticing. Why doesn't switching off the kitchen light also kill the fridge, the TV, and every lamp in the house? Because the lights and sockets in a building are wired in parallel. Each one sits on its own branch with its own loop, so each can be switched on and off — or fail completely — without touching the rest.
Imagine the alternative. If your house were one giant series circuit, every light and appliance would share a single path. Flick off one switch and the whole house would go dark. Worse, the moment one bulb burned out anywhere, everything would die at once, and you'd have to test every single bulb to find the culprit. Parallel wiring quietly saves you from all of that: independent paths mean independent control.
This is also the secret behind the fairy-light puzzle from the very start. Old-style decorative light strings were often wired in series to keep them cheap and simple — which is exactly why one dead bulb could darken the whole string, and why you'd hunt bulb by bulb for the broken one. Newer strings are cleverer, using little tricks so the rest stay lit when one fails. Same physics, smarter wiring.
Its own path. In parallel, every bulb, socket and appliance gets a complete loop of its own back to the supply. Switch off the kitchen light and the fridge, the TV and every other light keep humming along — because none of them shared that broken path.
There's one quiet assumption hiding inside everything so far: that the wires actually carry the current. They do — because of what they're made of. Materials split into two camps, and which camp a material falls into is the difference between a working circuit and a safe one.
Materials that let current flow through them easily. Most metals are good conductors — which is why wires are made of copper, and why the pins on a plug are metal. The current's "pipe" has to be a conductor, or nothing flows.
Materials that block current. Plastic, rubber, glass and dry wood are insulators. That's why wires are wrapped in coloured plastic — the conductor carries the flow safely inside, while the insulator stops it escaping to your hand.
So every wire is really two materials doing two jobs: a conductor down the middle to carry the current, and an insulator around the outside to keep it where it belongs. You'll meet conductors and insulators again in much more detail later — for now, just know that the loop has to be built from conductors, and that insulators are what keep electricity tame.
Everything on this page is about small, gentle circuits: a battery, some wire, a few bulbs. That kind of electricity is safe to build and explore, which is exactly why it's where every scientist starts. The power that comes out of the wall, though, is a completely different animal.
Never experiment with wall sockets or anything that plugs into the mains. The electricity in your home's sockets is far, far stronger than a battery and can give a deadly shock. Keep your experiments to batteries and bulbs, keep electrical things away from water, and if a plug, cable or socket ever looks damaged, tell an adult — don't touch it.
The ideas are the same in both worlds — loops, paths, switches — but only the battery version is yours to play with. Curiosity is brilliant; sockets are off-limits.
Here's a picture almost everyone carries around, and it's wrong: that electricity pours out of the battery, gets used up in the first bulb, and arrives at the later bulbs all worn out — so in a series string, the first bulb should glow brightest. It feels obvious. It's also not how circuits work.
Current is a flow that goes all the way round the loop. Whatever flows out of one side of the battery flows back into the other side — none of it vanishes along the way. In a simple series circuit, the same current passes through every bulb, so identical bulbs glow equally brightly, not brightest-first. What the bulbs actually do is convert the battery's energy into light and heat as the current passes through; the flow itself isn't consumed, it just keeps circulating until the loop is broken.
A close cousin of this myth is the idea that current "leaks out of the open ends" when a circuit is broken — that if you snip a wire, electricity dribbles out of the cut tip. It doesn't. Break the loop and the flow simply stops, everywhere, instantly. There's no complete path, so there's no flow at all — not a trickle, not a leak. An open end is just a dead end, and the current never even starts.
If you only remember one correction, make it this: current is shared around a loop, not spent like pocket money. That single idea quietly fixes half the mistakes people make about electricity.
You've built the intuition — now put it to work. Read each set-up, predict what happens, then check yourself. Trace the loop in your head every time: is there a complete path, and is it shared or separate?
Current is a flow that needs a complete loop. Break it anywhere — with a switch or a snap — and the flow stops on that path.
Series shares one path, so one break kills them all. Parallel gives each bulb its own path, so the rest stay lit.
That's why your house is wired in parallel — and why one good loop is all electricity ever really needs.