A loop the
charge can run.
with a battery and a bulb.
Electricity is charge on the move.
Inside every wire there are tiny particles called electrons — far too small to see, billions upon billions of them, already there in the metal. Normally they jostle about with no particular direction. An electric current is what you get when they all start to drift the same way at once. That is all a current is: charge, moving.
To make them move you need a push, and a battery (a cell) provides it. The push is called voltage, measured in volts. And you need a path with no gaps — a complete loop of conducting material — for the charge to travel all the way round and back. Break the loop anywhere and the flow stops at once, the way water stops when you pinch a hose.
Current — how much charge flows past a point each second, measured in amps (A). Voltage — the push driving it, measured in volts (V). Resistance — how much the path fights the flow, measured in ohms (Ω). Hold those three; the rest of the lesson hangs off them.
A single bolt of lightning carries a current of around 30,000 amps — tens of thousands of times more than anything in your house — and heats the air it passes through to about 30,000°C, roughly five times hotter than the surface of the Sun. The thunder is that air exploding outward.
Drawn in symbols, read like a map.
Nobody draws the real wires and bulbs every time. Instead, every part has a symbol — a tiny agreed picture — and a circuit is drawn with straight lines for the wires. Once you know a handful of symbols, you can read any circuit diagram in the world.
Put a few of those together and you have a circuit diagram. Here is a simple series circuit — one single loop — drawn the way you'd sketch it in a book. Trace it with your finger: out of the cell's long line, through the switch, on to the lamp, and back to the short line of the cell.
The symbols you've just learned are agreed worldwide. An engineer in Tokyo, São Paulo or Longstanton reads the very same diagram and builds the very same circuit — electricity is one of the few truly universal languages, with no translation needed.
Series in a line, parallel in branches.
There are two ways to join more than one thing into a circuit, and they behave very differently.
In series
- Everything sits on one single loop, end to end.
- The same current flows through every part.
- Add another lamp and they share the voltage — so each one grows dimmer.
- Break the loop anywhere — one bulb blows — and everything goes out. Old fairy lights worked this way, which is why one dead bulb killed the whole string.
In parallel
- Each part sits on its own branch across the cell.
- Each branch gets the full voltage, so lamps stay at full brightness.
- One branch breaks and the others keep working.
- This is how your house is wired — switch off one lamp and the rest of the room stays lit.
When the very first electric streetlights and homes were wired in the 1880s, two giants fought over how to do it — Thomas Edison backed one system and Nikola Tesla another. The argument was so fierce it became known as the "war of the currents". Tesla's side won; it is why the sockets in your wall are the kind they are.
Conductors, insulators, and the push-flow rule.
Some materials let charge move through them freely — we call them conductors. Metals are the best: copper, gold, aluminium. Others hold their electrons tight and let almost nothing through — these are insulators: plastic, rubber, glass, dry wood. That is why a wire is copper on the inside and plastic on the outside — the metal carries the charge, the plastic keeps it (and you) safe.
The push (voltage), the flow (current) and the fight (resistance) are linked by one short rule: voltage = current × resistance, written V = I × R. More push gives more current. More resistance gives less. It is the most-used equation in all of electronics, and you now know it.
Resistance has a use, not just a cost. Push current through a thin, high-resistance wire and it heats up — that glowing wire is how an old bulb, a toaster and an electric heater all work. The resistance turns electrical energy into heat and light on purpose.
Cool some metals down to nearly −270°C and their resistance vanishes completely — current can flow round a loop forever with no push at all. These are superconductors, and they're how the magnets in a hospital MRI scanner and the Large Hadron Collider are made strong enough to do their work.
Name the symbols.
Here is the same series circuit, with four parts numbered. Pick a label below, then place it in the matching Pin slot. These are the symbols you met earlier. Two of the labels are not parts of this circuit at all — read carefully before you place them.
Match each label to its pin
The unit of resistance, the ohm, is named after Georg Ohm — but when he first published his law in 1827, it was so coldly received that he resigned his teaching post in disappointment. It took years for the rest of science to catch up and realise he'd been right all along.
Ohm's law — find the voltage.
Fresh one. A current of 3 A flows through a 4 Ω resistor. What is the voltage across it?
What a current actually is.
Break the loop.
Conductor or insulator?
Why a house is wired in parallel.
The unit of current.
A single amp is a huge number of electrons — about six billion billion of them flowing past each second. The charge on one electron is so tiny that it takes that staggering crowd of them, every second, to add up to the gentle current lighting a small torch.
Worth watching.
A short film to watch alongside today's lesson.
The words from today.
- Current
- The flow of electric charge round a circuit, measured in amps (A).
- Voltage
- The electrical push that drives a current, measured in volts (V).
- Resistance
- How much a component fights the flow of current, measured in ohms (Ω).
- Series circuit
- A circuit where everything sits on one single loop, so the same current passes through every part.
- Parallel circuit
- A circuit where each component sits on its own branch across the cell, each getting the full voltage.
- Conductor / insulator
- A material that lets charge flow easily (copper) / a material that lets almost none through (rubber).
You can read a circuit now.
You learned that a current is charge on the move, and that it needs a complete loop and a push to flow. You met the symbols and traced a circuit diagram. You know the difference between series and parallel, between a conductor and an insulator, and you have Ohm's law — V = I × R — in your hand. Find a battery, a bulb and some wire, and close the loop yourself. Florence, this is physics.
Your own body runs on tiny electric currents. Every heartbeat is triggered by a pulse of charge sweeping across the heart muscle, and every thought is electrons shifting between nerve cells. An ECG at the doctor's is a voltmeter reading the circuit that is you.
Video recommendations are factual reference only — see each source's own copyright owner.